Integration and Challenges of Lignocellulosic Materials into Bio-Based Construction Systems

Abstract

The construction sector is responsible for substantial energy consumption, greenhouse gas emissions, and resource depletion, driving the search for sustainable alternatives to conventional petroleum-based insulation materials. Lignocellulosic biomass, comprising cellulose, hemicellulose, and lignin, offers a renewable resource for the development of bio-based foams with potential application in construction systems. This review provides a comprehensive analysis of bio-based foams tailored to building applications, positioning recent scientific advances against the technical properties of commercial synthetic insulation foams. Key performance parameters, including density, thermal conductivity, compressive strength, dimensional stability, water vapour diffusion resistance, and fire behaviour, are critically examined. Developments in lignocellulosic-based foams are discussed, highlighting processing strategies such as crosslinking, chemical modification, and hybrid reinforcement to enhance mechanical, thermal, and fire performance. The reported results demonstrate that lignin-based polyurethane and phenolic foams can achieve competitive compressive strength and thermal insulation, while cellulose-based aerogels and foams exhibit ultra-low density and promising conductivity values. However, challenges related to moisture sensitivity, fire classification, process scalability, standardisation, and market integration remain significant. Overall, lignocellulosic foams represent a promising pathway toward decarbonised, circular construction systems, provided that technical optimisation and regulatory alignment are successfully achieved.

1. Introduction

1.1. Construction Sector—Spending, Pollution, and Impact

The transition to a bioeconomy is essential for reducing the environmental and social impacts associated with the current economic model, particularly regarding the intensive consumption of materials [1]. The construction sector is responsible for 36% of global energy consumption, 38% of annual greenhouse gas (GHG) emissions, and 40% of global energy demand [1,2,3].

This sector is also characterised by an extremely intensive use of resources. In 2018, 40% of the global use of materials whose renewal depends on long geological processes was associated with construction and infrastructure [2]. In addition, construction is the largest generator of waste in the European Union (EU) and also one of the main consumers of water [1]. This, together with the estimated need for around 300 million new homes by 2030 and the growing demand for denser, larger buildings, puts great pressure on resources, increasing the global environmental impact [4].

There are also social and urban impacts, such as the increased heat island effect, habitat degradation, and large volumes of waste from both construction and demolition of 63.74 kg/m² and 1615 kg/m², respectively [5]. It is estimated that recycling waste materials commonly used in construction, such as concrete and brick, can generate a savings of approximately 45 million dollars, reflecting the economic potential of circularity strategies [5].

The accumulated environmental, economic, and social impacts reveal an urgent need for sustainable alternatives. The construction sector faces increasing risks associated with climate change, such as increased carbon taxes and disruptions in supply processes. On the other hand, it also has great potential to reduce emissions, resource consumption, and waste generation through circularity, dematerialisation, and recycling strategies [2,4].

In this context, there has been a substantial increase in demand for renewable, biodegradable, and low environmental impact materials. Thus, bio-based natural materials, including those rich in lignin, cellulose, and hemicellulose, emerge as promising alternatives to high-energy and high-carbon-intensity building materials, contributing decisively to decarbonisation, circularity, and reducing pressure on non-renewable resources [3,5,6].

1.2. Lignocellulosic Materials

Lignocellulosic biomass comprises three essential biopolymers: cellulose, hemicellulose, and lignin. Cellulose, the most abundant biopolymer in nature [7,8], has a wide range of commercial applications. Its primary industrial sources are wood and various plant fibres, including cotton, jute, and sisal [9]. Beyond terrestrial plants, cellulose is also produced by certain acetic acid-generating bacteria and occurs in algae, fungi, and even some animals, such as urochordates, the only known animal group capable of synthesising cellulose [9,10,11]. Structurally, cellulose is a homopolysaccharide composed of linear D-anhydroglucopyranose units (AGUs) linked by β-(1→4) glycosidic bonds between C(1) and C(4). This β-linkage constrains each glucose ring into a low-energy chair conformation, contributing to the formation of cellulose’s highly ordered and complex supramolecular structure [12,13,14].

Hemicelluloses, the second most abundant structural component of wood, are heteropolysaccharides composed of a mixture of sugar with an average degree of polymerisation (DP) of 100–200. Their backbone and side-chain structures are built from a mixture of monosaccharides, including hexoses (mannose, galactose, and glucose), pentoses (xylose and arabinose), and hexuronic acids (D-glucopyranosyluronic acid and D-galactopyranosyluronic acid) [9,15].

Lignin is a complex, three-dimensional, amorphous phenolic polymer composed predominantly of aromatic structures derived from phenylpropane units [9,15,16]. Its structure is formed through the enzymatic dehydrogenation of three primary monolignol precursors, p-coumaryl, coniferyl, and sinapyl alcohol, and the subsequent recombination of their phenoxy radicals. These reactions generate the three characteristic phenylpropane units: p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) [9,17].

Lignocellulosic materials have shown promise as alternatives to conventional building materials, especially in the field of thermal insulation. In addition to being of renewable origin, they have a temporary carbon storage capacity, low embodied energy, and significant potential to reduce carbon dioxide emissions throughout the lifespan of buildings, either by improving thermal performance or by capturing biogenic carbon during the plant growth phase [1,2].

The use of second-generation lignocellulosic biomass, such as agricultural and forestry residues, stands out particularly, as they have a smaller environmental footprint and can replace energy- and carbon-intensive materials such as synthetic foams, plastics, steel, cement, and concrete [2,5,6]. In addition to their lower environmental impact, the abundance, recyclability, biodegradability, and relatively low cost of residual biomass [18] have driven the development of functional foams and biocomposites. These materials, explored for thermal insulation and building systems applications, are derived from lignocellulosic feedstocks such as sugarcane [19,20], wheat straw [21], pinewood [22], hemp shives [23,24,25,26], sunflower cake [27], and kenaf, sisal, and flax fibres, used in synergy with polylactide acid (PLA) for reinforced hybrid composites [28,29,30].

However, the efficient valorisation of lignocellulose is subject to significant technical challenges. The rigid, heterogeneous, and complex structure of the biomass implies difficulties in the processing and extraction of most commonly occurring components: lignin, cellulose, hemicellulose, and phenolic compounds, such as tannins. The complexity of refining processes is conditioned by the release of various sugars during degradation, while the inherent heterogeneity of the raw material gives rise to differences at the structural and compositional levels, depending on origin and environmental conditions [2,31]. That said, to maximise the economic and environmental performance of these materials, it is important to optimise several stages through its lifecycle, from the valorisation processes in a biorefinery concept to recycling of end-products [2,31]. Thus, the transition to a bioeconomy based on lignocellulosic waste must be evaluated simultaneously in the environmental, economic, and social dimensions [2].

The high thermal performance of lignocellulosic materials is due to their low conductivity, resulting from porosity, low density, and cellular structure, which contributes to the reduction of energy consumption in heating and cooling, thus reinforcing the role of these materials as viable solutions for the decarbonisation of building envelopes [1,3]. The fact that they are non-toxic, non-allergenic, and biodegradable promotes even healthier environments and reduces the generation of persistent waste [3].

In summary, lignocellulosic materials offer a robust set of advantages, making them essential candidates for more efficient construction aligned with global decarbonisation goals. Their effective use represents a strategic opportunity to reduce dependence on fossil fuels and mitigate the environmental impacts of the sector, without compromising the technical performance of construction systems [2,3,5].

This review article distinguishes itself by offering a comprehensive overview of lignocellulosic foams tailored for the construction sector. It covers scientific developments in the field and identifies the relevant technical properties of insulation foams—primarily commercial synthetic ones—used in construction, positioning the results of the scientific literature accordingly. Additionally, it examines market opportunities, the current state of standardisation and certification, and the key challenges that integrating bio-based foams into building systems may encounter. As a result, this review offers a holistic perspective that extends beyond a typical review of scientific literature.

Beyond the introduction, this review article comprises five additional sections. The section titled Requirements for foams in construction applications identifies the relevant properties of foams for building use. Additionally, it provides a brief survey of existing foams on the market and their performance regarding the identified properties, aiming to position research outcomes related to lignocellulosic foams within the scope of these products. The section Current developments of bio-based foams addresses the following: (1) the main developments regarding lignocellulosic foams; (2) market studies on insulation products and lignocellulosic materials, to highlight opportunities for lignocellulosic foams; commercial lignocellulosic-based raw materials with potential for lignocellulosic foam production; commercial cellulosic foams that, despite not being specifically developed for construction, are noted by developers for their potential as insulation materials; and (3) current certifications and challenges to the standardisation of lignocellulosic foams, identifying opportunities and possible future directions. The section Challenges of the integration of bio-based foams in building systems examines the most relevant challenges facing bio-based materials, aiming to position lignocellulosic foams within this context. The section Future research highlights research directions. Finally, the section Conclusions summarises the main findings.

The writing of a review focused on the integration and challenges of lignocellulosic materials into bio-based construction systems is motivated by a critical convergence between the urgent need for decarbonisation in the construction sector and the growing interest in renewable, low-environmental impact materials. Despite their potential, widespread adoption is hindered mainly by fragmented research and a lack of standardised performance data. This review article aims to bridge the gap between science and construction engineering by providing a comprehensive synthesis of current knowledge, systemic challenges, and the technical/standardisation/certification requirements for scaling up these solutions in modern building practices.

2. Requirements for Foams in Construction Applications

The importance of thermal insulation materials has increased over recent decades, resulting in a corresponding growth in the variety of available products, their specific properties, and their intended uses [32]. As a result, selecting insulation materials for construction projects requires the consideration of several key factors: (1) evaluation of fundamental material properties, (2) identification of relevant regulatory standards, and (3) determination of specific installation and operational requirements [33,34].

While low thermal conductivity remains the main performance criterion for thermal insulation, other factors such as compressive strength and reaction to fire may be decisive depending on the application [32,34]. Additionally, density, dimensional stability, and water vapour diffusion resistance are important criteria [34,35]. Although most commercially available insulation materials meet a wide range of these requirements, no single insulation material satisfies all performance criteria completely.

The following provides a brief overview of the most important technical features of insulation foams, emphasising their relevance in the construction sector. This is accompanied by a concise list of foams available on the market, together with their applicability and, where available, their performance parameters (

Table 1. Synthetic-based insulation materials on the market.

Product	Foam	Application	Insulation Type	Reference
UTHERM Roof L, UTHERM Roof BGM, UTHERM Roof M	PIR	Roof	Rigid	[36]
UTHERM Wall A	PIR	Ventilated facade	Rigid	[36]
UTHERM Floor K	PIR	Floor	Rigid	[36]
UTHERM Premium LE	PIR	Floor	Rigid	[36]
PIR F ALK, PIR ALK, PIR BV	PIR	Metallic deck-type roofs	Rigid	[37,38,39]
PIR AD	PIR	Metallic deck-type roofs	Rigid	[40]
PIR AF	PIR	Ventilated facades and external walls	Rigid	[41]
PIR 7C	PIR	Cavity wall thermal insulation.	Rigid	[42]
DANOPREN 500	XPS	Industrial flooring or car park roofing	Rigid	[43]
DANOPREN FS	XPS	ETICS	Rigid	[44]
DANOPREN PR	XPS	Flat roofs, residential and commercial overloaded floors, and air chambers	Rigid	[45]
SOPRA XPS SL	XPS	Roof	Rigid	[46]
SOPRA XPS CB	XPS	ETICS	Rigid	[46]
SOPRA XPS 500	XPS	Parking garages and industrial buildings with heavy traffic	Rigid	[46]
SAFE R—SR/PR	Phenolic	Ventilated, warm, or hybrid sloped roofs	Rigid	[47]
SAFE R—SR/CW	Phenolic	Partial fill cavity walls	Rigid	[48]
SAFE R—SR/UF	Phenolic	Floors	Rigid	[49]
Kooltherm K107	Phenolic	Pitched roof	Rigid	[50]
Kooltherm K5	Phenolic	External insulation for masonry walls	Rigid	[51]
Kooltherm K103	Phenolic	Floor	Rigid	[52]

and Table 2).

2.1. Density

The density of thermal insulation foams is critical at several levels. Regarding compressive strength, the relationship between density and compressive strength in foams is described by the Gibson–Ashby model [53]. This model shows that the mechanical behaviour of cellular materials can be predicted primarily from the properties of the solid material forming the cell walls (the ‘parent’ or ‘solid’ material), the relative density of the cellular structure, and the cell geometry (open-cell vs. closed-cell, bending vs. stretching).

Parameters such as cell size, porosity, cell density, and wall thickness collectively determine the volume fraction of gas within the material. For instance, larger cells and higher porosity increase the void fraction, leading to lower densities, whereas smaller cells and a higher number of cells per unit volume result in a more compact structure [54,55]. Similarly, increases in cell wall and edge thickness contribute to a higher solid fraction, thereby increasing density [56]. Morphological modifications induced by fillers or processing conditions can further alter these structural parameters, indirectly affecting density through changes in the cellular architecture [57,58]. In this way, density can be interpreted as a direct macroscopic consequence of the underlying foam morphology.

Density is a key factor in material handling and the overall weight of the insulation system. High-density foams can increase the complexity of the application process for professionals. Conversely, foam density directly determines the weight of the insulation system and, consequently, the loads transmitted to the supporting structure. Lower-density foams allow lighter and more efficient solutions, provided they maintain sufficient mechanical resistance to meet service requirements. The synthetic foams on the market have low density, between 30 and 40 kg/m³ (Table 2), i.e., they are light.

Table 2. Technical properties of synthetic-based insulations on the market. The obtained performance data are accompanied by the standards used in the calculation, provided this information is available from the manufacturer.

Product	Fire Classification (EN 13501-1)	Thermal Conductivity (W/mK)	Density (kg/m3)	Compressive Strength (100% Deformation, EN 826 [59], kPa)	Vapour Diffusion (μ)
UTHERM Roof L, UTHERM Roof BGM, UTHERM Roof M	F	0.022–0.027	32	≥150	50–100
UTHERM Wall A	N/A	0.022	30	≥150	50–100
UTHERM Floor K	N/A	0.022	30	≥150	50–100
UTHERM Premium LE	E	0.020–0.021	30	≥150	50–100
PIR F ALK, PIR ALK, PIR BV	E–F	0.023–0.028 (EN 12667 [60])	32	≥175	N/A
PIR AD	E	0.030 (EN 12667)	N/A	≥400	N/A
PIR AF	N/A	0.023 (EN 12667)	32	≥175	N/A
PIR 7C	E–F	0.023 (EN 12667)	N/A	≥175	N/A
DANOPREN 500	E	0.034 (EN 12667)	38	500	80 (EN 12086 [61])
DANOPREN FS	E	0.035 (EN 12667)	32	≥200	≥80 (EN 12086)
DANOPREN PR	E	0.034 (EN 12667)	32	≥200	≥80 (EN 12086)
SOPRA XPS SL	E	0.033–0.035 (EN 12667)	30–40	300	150
SOPRA XPS CB	E	0.033–0.035 (EN 12667)	30–40	250	≥80
SOPRA XPS 500	E	0.033–0.035 (EN 12667)	30–40	500	150
SR/PR	C	0.021–0.023 (EN 12667)	N/A	100	N/A
SAFE R—SR/CW	D	0.020–0.021 (EN 12667)	N/A	100	N/A
SAFE R—SR/UF	D	0.020–0.021 (EN 12667)	N/A	100	N/A
Kooltherm K107	F	0.019 (EN 12667)	N/A	100	N/A
Kooltherm K5	C	0.021–0.020 (EN 12667)	N/A	100	N/A
Kooltherm K103	C	0.019 (EN 12667)	N/A	120	N/A

Table 2. Technical properties of synthetic-based insulations on the market. The obtained performance data are accompanied by the standards used in the calculation, provided this information is available from the manufacturer.

Product	Fire Classification (EN 13501-1)	Thermal Conductivity (W/mK)	Density (kg/m3)	Compressive Strength (100% Deformation, EN 826 [59], kPa)	Vapour Diffusion (μ)
UTHERM Roof L, UTHERM Roof BGM, UTHERM Roof M	F	0.022–0.027	32	≥150	50–100
UTHERM Wall A	N/A	0.022	30	≥150	50–100
UTHERM Floor K	N/A	0.022	30	≥150	50–100
UTHERM Premium LE	E	0.020–0.021	30	≥150	50–100
PIR F ALK, PIR ALK, PIR BV	E–F	0.023–0.028 (EN 12667 [60])	32	≥175	N/A
PIR AD	E	0.030 (EN 12667)	N/A	≥400	N/A
PIR AF	N/A	0.023 (EN 12667)	32	≥175	N/A
PIR 7C	E–F	0.023 (EN 12667)	N/A	≥175	N/A
DANOPREN 500	E	0.034 (EN 12667)	38	500	80 (EN 12086 [61])
DANOPREN FS	E	0.035 (EN 12667)	32	≥200	≥80 (EN 12086)
DANOPREN PR	E	0.034 (EN 12667)	32	≥200	≥80 (EN 12086)
SOPRA XPS SL	E	0.033–0.035 (EN 12667)	30–40	300	150
SOPRA XPS CB	E	0.033–0.035 (EN 12667)	30–40	250	≥80
SOPRA XPS 500	E	0.033–0.035 (EN 12667)	30–40	500	150
SR/PR	C	0.021–0.023 (EN 12667)	N/A	100	N/A
SAFE R—SR/CW	D	0.020–0.021 (EN 12667)	N/A	100	N/A
SAFE R—SR/UF	D	0.020–0.021 (EN 12667)	N/A	100	N/A
Kooltherm K107	F	0.019 (EN 12667)	N/A	100	N/A
Kooltherm K5	C	0.021–0.020 (EN 12667)	N/A	100	N/A
Kooltherm K103	C	0.019 (EN 12667)	N/A	120	N/A

2.2. Thermal Conductivity

Thermal conductivity is a fundamental parameter in evaluating the performance of foams used in thermal insulation systems, as it quantifies a material’s ability to conduct heat. This parameter establishes a direct relationship between the foam’s microscopic structure and its macroscopic thermal behaviour.

Thermal insulation foams are characterised by low thermal conductivity values, primarily due to the high-volume fraction of gas trapped within the cells [62]. Air or low-conductivity gases present in the cellular cavities limit heat transfer mechanisms—namely conduction and convection—significantly contributing to the material’s insulating efficiency. Additionally, cell geometry and size, as well as the ratio of open to closed cells, directly influence the foam’s overall thermal conductivity [62,63], although the governing trends vary between foam systems. In polyurethane foams, small, closed cells with narrow size distributions are particularly effective in limiting heat transfer [64]. In phenolic foams, thermal conductivity is influenced by a combination of porosity, cell density, and solid-phase properties, with optimised cellular structures leading to improved insulation performance [65]. In contrast, polystyrene foams exhibit a more complex relationship, where cell size can reduce thermal conductivity at lower densities due to decreased radiative heat transfer, while having a limited influence at higher densities [66]. This demonstrates that the morphology–thermal property relationship is system-dependent and influenced by the dominant heat transfer mechanisms. The degree of cell openness further illustrates differences between foam types. Closed-cell structures are typically associated with improved thermal insulation in PU foams [64]. However, in polystyrene-based systems, an increase in open-cell content, often related to processing conditions, can also reduce thermal conductivity, indicating a different governing mechanism [67]. The thermal performance of a construction system is often expressed by thermal resistance, defined as the ratio of the thickness of the insulating material to its thermal conductivity [37]. Thus, using foams with low thermal conductivity enables high thermal resistance with reduced thickness, optimising available space and reducing the system’s dead weight. This characteristic is particularly relevant for energy-efficient buildings and lightweight construction solutions. In summary, thermal conductivity plays a central role in determining the efficacy of thermal insulation foams, directly influencing energy performance, required material thickness, the weight of the construction system, its long-term sustainability, and cost [68]. Thermal conductivity varies among commercially available foams: phenolic foams exhibit values between 0.019 and 0.023 W/mK, polyisocyanurate-based foams range from 0.020 to 0.03 W/mK, and polystyrene-based foams fall within 0.033–0.035 W/mK (Table 2).

2.3. Dimensional Stability

Dimensional stability is a crucial requirement for the performance of foams used in thermal insulation systems, as it refers to the material’s ability to maintain its original dimensions when subjected to variations in temperature, humidity, and mechanical stress over time [35].

The dimensional stability of thermal insulation foams depends on several factors, including cellular structure, density, base polymer composition, and the type of gas trapped within the cells [69]. Foams with a homogeneous cellular structure and a predominance of closed cells tend to exhibit lower sensitivity to hygrothermal variations [70], demonstrating greater dimensional stability. A lack of dimensional stability can significantly compromise both thermal efficacy and the integrity of the constructive system. Excessive dimensional changes—such as shrinkage, expansion, or permanent deformation—can lead to discontinuities in the insulation, including the formation of cracks, gaps, or thermal bridges [71]. These discontinuities facilitate heat transfer and reduce the overall thermal resistance of the building element, partially negating the benefits provided by the insulating material [71]. Additionally, dimensional stability is a relevant criterion within regulatory and normative frameworks, being the subject of standardised tests that define allowable limits for dimensional variation under specific temperature and humidity conditions [72,73]. Compliance with these requirements ensures the initial thermal performance and its preservation under real service conditions over time. In brief, dimensional stability is a critical parameter in the selection and application of thermal insulation foams, as it guarantees insulation continuity, the durability of construction systems, and the maintenance of energy performance throughout the building’s service life.

2.4. Water Vapour Diffusion Resistance

Thermal insulation foams with high water vapour diffusion resistance reduce the risk of interstitial condensation within construction layers, thereby preventing associated issues such as material degradation, the growth of microorganisms, and loss of thermal performance [34,70]. Foams with a predominantly closed-cell structure, such as polyurethane (PUR), polyisocyanurate (PIR), or extruded polystyrene (XPS), generally exhibit high values of water vapour diffusion resistance (Table 2), functioning simultaneously as thermal insulation and vapour barriers [32]. In contrast, foams with a higher percentage of open cells show lower diffusion resistance, allowing greater vapour permeability [74]. The presence of moisture, resulting from water vapour diffusion, within thermal insulation foams can increase thermal conductivity, reducing insulation efficiency, as well as inducing dimensional changes [75] and loss of mechanical properties. Thus, water vapour diffusion resistance contributes not only to condensation control but also to the preservation of thermal performance and dimensional stability over time.

Water vapour diffusion resistance is an essential parameter in evaluating thermal insulation foams, influencing the hygrothermal behaviour of building envelopes, the prevention of construction pathologies, material durability, and the preservation of the building’s energy performance. The water vapour resistance of products on the market varies between 50 and 100 for polyisocyanurate foams and 80 and 150 for polystyrene foams (Table 2).

2.5. Compressive Strength

Compressive strength is a fundamental mechanical property of foams used in thermal insulation systems, as it indicates the material’s ability to withstand compressive loads without excessive deformation or significant performance loss [76]. This parameter is particularly important in applications where the insulation is exposed to permanent or variable mechanical actions. From a functional perspective, adequate compressive strength ensures the maintenance of the insulation’s effective thickness, which is essential for preserving the designed thermal resistance. Excessive deformation due to crushing reduces the thickness of the insulating layer, leading to increased thermal transmittance and decreased energy efficiency of the construction elements. In addition to immediate compressive strength, long-term compressive strength is critical, especially when the insulation is subjected to permanent dead loads. For this reason, technical standards set specific criteria for evaluating compressive strength and behaviour under prolonged loading conditions [77].

Mechanical behaviour of foams, including compressive strength, is dependent on cell size, cell density, and structural uniformity. In general, smaller cells are associated with improved mechanical resistance, as they reduce the distance between cell walls and enable a more efficient distribution of applied stresses [65,78]. This results in higher compressive strength and resistance to deformation, as observed in both polyurethane and phenolic foams. Similarly, in polystyrene-based foams, an increase in cell size can reduce specific mechanical properties, although in some cases, the effect may be limited depending on the material structure and cell configuration [55]. These observations indicate that, while the beneficial effect of smaller cells is generally consistent, its magnitude depends on the foam type and microstructural context. Morphological uniformity is another common factor influencing mechanical performance. More homogeneous and well-defined cellular structures promote uniform stress distribution, leading to improved mechanical properties, as reported for both PU and phenolic foams [65,79]. In contrast, irregular and heterogeneous morphologies, such as those observed in tannin-based foams, lead to stress concentration and increased brittleness, facilitating cell collapse under load [80]. Given the importance of morphological structure to mechanical performance, compositional and processing modifications have been studied by the scientific community. For phenolic foams, for example, research has focused on the use of nanotubes, nanofibres, and cork, among other additives, to enhance their mechanical properties [50,51,52]. For example, in phenolic foams, the incorporation of cellulose fibres refines the morphology by decreasing cell size and increasing cell density, which is associated with improved properties [57]. In PU foams, the formation process itself, particularly the role of pre-dispersed air bubbles as pore precursors, plays a decisive role in defining the final morphology [64].

Overall, compressive strength is a decisive parameter in selecting thermal insulation foams, ensuring dimensional stability under load, maintenance of thermal performance, durability of the construction system, and the functional safety of insulation solutions over time.

For commercially available products, compressive strength values are aligned with their final application (Table 1 and Table 2). Foams used in roofs and walls—such as ETICS, cavity wall insulation, and ventilated facades—typically exhibit compressive strength values between 150 kPa and 200 kPa, with some products exceeding this range. Conversely, for flooring or roofing that are subject to high compression, such as industrial floors, parking areas, or trafficable roofs, the market offers foams with compressive strength in the order of 500 kPa.

2.6. Reaction to Fire

Thermal insulation foams with high fire resistance retard flame spread, extending evacuation time and reducing the risk of structural collapse during fire events. Furthermore, they act as barriers that limit heat transfer and the spread of flames between compartments, contributing to fire containment at the point of origin. Another important aspect is the reduction of smoke and toxic gas emissions. Untreated foams may release hazardous substances, whereas materials with incorporated flame retardants significantly mitigate these risks [81,82]. Additionally, fire resistance preserves the structural integrity of construction elements by delaying the heating of components such as steel and concrete, thereby prolonging their stability during fire situations. Compliance with safety standards, including the Euroclasses for reaction to fire, is another key factor in the use of insulation foams in architectural and civil engineering projects [83]. In summary, fire behaviour in thermal insulation foams is an essential requirement for contemporary buildings, promoting safe, efficient, and regulated construction solutions. Apart from phenolic foams, which are known for their high fire performance, commercially available products are generally classified as Class E—one of the lowest ratings in the Euroclass system. Nevertheless, numerous studies have sought to enhance the fire performance of these foams (e.g., [84,85,86]).

This section demonstrates that for lignocellulosic foams to achieve the same applications as synthetic foams, they must prioritise low density and low thermal conductivity—particularly when compared to phenolic foams. Additionally, they require high water vapour diffusion resistance (unless protected by a vapour barrier), medium to high compressive strength, and a fire reaction classification at least equal to Class E.

3. Current Developments of Bio-Based Foams

This section presents studies that use bio-based raw materials to produce foams, with a specific focus on those that use lignocellulose as raw material. It intends to address current developments in this field while also providing a very brief benchmarking of these foams’ performance against those currently available on the market and used in the construction sector.

3.1. Sustainable Feedstocks

The construction sector is being transformed by the introduction of more sustainable and bio-based materials, reducing environmental impact and increasing resource efficiency [87]. Materials like bio-composites, natural fibres, and biopolymers are derived from renewable and sustainable sources, reducing reliance on petrochemical-based alternatives [88].

Developments in bio-based foams have explored a wide range of raw materials and foaming methods, achieving encouraging results [89,90]. These foams reveal strong potential for use as thermal and acoustic insulation materials, reaching low thermal conductivity values, similar to those of commercial petroleum-based foams. This potential is particularly attributed to their low cost and good insulating properties. There has been a significant amount of research and development in the field of bio-based foams, as their performance is approaching that of petroleum-based foams. Bio-based foams are typically composed of biomass and can be based on soybean, corn, starch, or others. Soybean-based foams show superior thermal insulation capability, high specific strength, and excellent energy-absorption capacity for shock, vibration, and sound [91], and a fire-retardant property [92]. Corn-based foams are an alternative, prepared with corn polyols and corn-derived additives. They have closed-cell structures with reduced cell sizes and higher mass densities reaching enhanced mechanical properties [93]. Fire-retardant properties were also enhanced using dimethyl methyl phosphonate (DMMP) [94]. Starch is also a low-cost, biodegradable, and nontoxic bio-based material that can be used to produce bio-based foams. Starch-based foams are typically derived from natural sources, e.g., potato, corn, cassava, and oca starch. Duan et al. applied a two-step extrusion process to reinforce starch-based foams and achieve a density and compressive strength similar to expanded polystyrene (EPS) foam [94]. Tannin is also being used to produce rigid foams obtained by copolymerising the tannic extract with furfuryl alcohol in an acidic medium [88,95]. Previous studies have proved the ability of these foams to match synthetic foams based on the positive characteristics of thermal insulation, low density, and reduced cost [96,97] and also acoustic behaviour [98]. The main advantage of this foam is its fire behaviour, with low emission of smoke and harmful gases [98]. Li et al. also reached an excellent thermal conductivity of 0.0239 W/m·K [99]. This foam is very promising, and Rodrigues et al. [80] investigated its possible use in sandwich panels and achieved excellent performances comparable to conventional foams, such as PUR foams. However, the drawback of this solution is related to water absorption, which shows higher results when compared with PUR. This can affect the thermal performance of the foam. Although displaying low swelling, this property can increase the conductivity [80].

Within the category of sustainable feedstocks, we also find lignocellulosic materials, which will be explored in detail in the next section.

Table 3. Technical properties of bio-based insulations.

Type of Foam	Density (kg/m3)	Thermal Conductivity (W/(m·K))	Compressive Stress at 10% Deformation (kPa)	Reference
Calcium silicate foam		0.045–0.065		[100]
Urea-formaldehyde resin in situ foam (UF)	10	0.035–0.040		[100]
Melamine foam	8–11	0.035	4–20	[100]
Phenolic foam	40–160	0.021–0.04	120	[101]
Soy-based foam	54.9–98.8	0.025–0.027	61–137	[91]
Soy-based rigid polyurethane foam	28.9–32.4	0,022	148–229	[92]
Sorbitol and corn-based rigid polyurethane foam	66–103	0,3537–0.3643	1015–396	[93]
Starch-based foam	19.94–32.53		75–125.1	[102]
Polyethylene foam (PE)	50–110	0.033		[103]
Tannin-based foams	82–122	0.026	200–350	[98]
Tannin-based sandwich			160	[80]

presents the performance regarding density, thermal conductivity, and compressive strength for bio-based foams. In terms of density, there is a wide range of values, and it is possible to observe foams with a lightness similar to those on the market. Regarding thermal conductivity, the results are also encouraging, as mentioned above. For several foams, the values are close to the PIR and phenolic foams on the market, while others are comparable to XPS foams (Table 2). Regarding compressive strength, the performance range is broad and includes values similar to the XPS foams on the market, albeit at the cost of a higher density—as is the case with the tannin-based foam developed by Chen et al. [98]. Among the foams presented in Table 3, the one developed by Petrovic et al. [92] exhibits the best balance among density, thermal conductivity, and compressive strength as compared to those on the market.

3.2. Lignocellulosic Feedstocks

Lignocellulosic resources can be classified as native lignocellulose, found in untreated biomass such as wood, grasses, and agricultural residues, or as processed lignocellulose, produced after chemical fractionation. Among industrial treatments, the kraft process is the most widely used method for delignification [104]. In this process, wood chips are cooked in an alkaline solution containing sodium hydroxide and sodium sulphide, which dissolve lignin and part of the hemicellulose. The resulting kraft pulp may be unbleached (70–85 wt% cellulose, 5–10 wt% lignin, 10–20 wt% hemicellulose) or bleached (90–98 wt% cellulose, <1 wt% lignin, 2–8 wt% hemicellulose), depending on the degree of purification [105]. These compositional differences significantly affect fibre reactivity, bonding ability, and final foam properties.

The availability of purified cellulose fibres and nanofibrillated cellulose (NFC) from lignocellulosic biomass has enabled the development of lightweight, highly porous foams with adjustable mechanical and thermal properties for use in construction, packaging, environmental remediation, and acoustic insulation.

3.2.1. Cellulose

Cellulose foams produced without chemical crosslinking or surface functionalisation rely primarily on physical entanglement, hydrogen bonding, and controlled pore formation to achieve structural integrity. Ultra-lightweight cellulose foams fabricated using a NaOH/urea dissolution–regeneration system, combined with mechanical frothing and freeze-drying, exhibit densities of approximately 0.04 g·cm⁻³ and highly porous microstructures. The transition in morphology from a fibril-interconnecting sheet-like-network to a close-grained sheet-like-network occurs in regenerated cellulose (RC) or cellulose-based hydrogels with the addition of Polyethylene Glycol (PEG). This structural change, often described as a transition from a 3D fibrillar network to a denser, compacted, or “close-grained” network, is due to PEG acting as a crosslinker, filling the spaces between the cellulose fibres and facilitating a denser structure [106]. During dissolution and regeneration, cellulose transitions from crystalline cellulose I to cellulose II. Li et al. found that despite their extremely low density, the resulting foams exhibit characteristic J-shaped compressive stress–strain behaviour, indicating elastic recovery and structural stability. Wet foam bubbles initially measuring 20–50 µm expand to over 100 µm after freeze-drying, contributing to high porosity and reduced thermal conductivity [107]. In general, cellulose foams prepared by mechanical foaming, surfactant-assisted foaming, or freeze-drying exhibit densities ranging from 0.010–0.03 g·cm⁻³ for ultra-lightweight systems to 20–35 kg·m⁻³ for insulation materials, with porosities often exceeding 98% and pore sizes between <70 nm and 500 µm. The suppression of gaseous heat conduction through mesoporous structures (<70 nm) contributes to thermal insulation via the Knudsen effect [103,108,109].

Fibre morphology has a decisive influence on mechanical performance and air permeability. Pevec et al. performed comparative analysis of foams prepared from short eucalyptus fibres and long spruce fibres and showed that short fibres produce denser foams (~28 kg·m⁻³) with a higher tensile strength and elastic modulus, while long fibres yield lower-density foams (~19 kg·m⁻³) with greater thickness, ductility, porosity, and approximately 30% higher air permeability. Blended fibre systems provide intermediate, balanced performance, highlighting the importance of fibre aspect ratio and flexibility in network formation [110].

Overall, improving the global performance of cellulose foams depends critically on chemical crosslinking and related chemical modification strategies, which enable the fabrication of mechanically robust and durable foam structures. In non-crosslinked systems, the porous architecture is mainly stabilised by hydrogen bonding and physical entanglements; these interactions are highly sensitive to moisture, resulting in dimensional instability, pore collapse, and significant deterioration of mechanical properties under humid conditions [111,112]. Covalent crosslinks strengthen the three-dimensional network, greatly enhancing compressive strength, structural integrity, and resistance to moisture-induced deformation [105]. Therefore, crosslinking and chemical functionalisation or reinforcement are often essential for scaling cellulose foams from laboratory materials to technically viable, competitive insulation systems [113,114].

To overcome intrinsic limitations such as moisture sensitivity, limited compressive strength, or flammability, various modification strategies have been introduced in cellulose foams. Wet-strength enhancement through polyamide epichlorohydrin (PAE) crosslinking significantly improves water stability [18]. Bleached kraft pulp foams crosslinked at 120 °C for 1 h maintain structural integrity after prolonged immersion in water (up to 60 days), whereas non-crosslinked foams disintegrate upon stirring. This strategy improves durability without fundamentally altering foam density. A bio-inspired borate crosslinking approach introduces sodium tetraborate during foam formation, establishing coordination bonds between cellulose chains [115]. Borate-modified foams exhibit densities as low as 13.3–16.4 mg·cm⁻³ and compressive strengths up to 74.1 kPa—approximately 28 times higher than unmodified counterparts. In addition to mechanical reinforcement, boron incorporation (~3.45 wt%) confers self-extinguishing flame behaviour and maintains low thermal conductivity (~0.045 W·m⁻¹·K⁻¹). Mechanical performance can also be enhanced through the formation of organic–inorganic hybrid networks within microfibrillated cellulose foams (MFC) [116]. At a density of 32.9 mg·cm⁻³, these reinforced foams achieve compressive moduli of 451.3 kPa and yield strengths of 25.1 kPa, surpassing many surfactant-foamed MFC systems. Importantly, recyclability and biodegradability are preserved, and life-cycle assessment demonstrates reduced carbon emissions compared to petroleum-based foams.

NFC aerogels represent an advanced class of physically structured cellulose foams [22]. After chemical purification and ultrasonication, nanofibrils with diameters of 10–40 nm are obtained. Freeze-drying following rapid freezing yields homogeneous open-pore aerogels. Thermal stability increases significantly, with the degradation onset rising from 260 °C in raw wood to 340 °C in purified cellulose. Optimal NFC concentrations (1.00 wt%) achieve minimal thermal conductivity (25.5 mW·m⁻¹·K⁻¹), while higher concentrations produce denser morphologies and higher conductivity. These aerogels provide superior thermal insulation compared to conventional polyurethane foam and glass wool, while maintaining cyclic compressive resilience.

Surface functionalisation also enables environmental applications. Ultra-lightweight foams made from bagasse fibres and NFC have been modified with a PDMAEMA-co-PHA(Poly(2-(dimethylamino)ethyl methacrylate)-co-polyhydroxyalkanoate) copolymer to introduce pH-responsive wettability [20]. The modified foams maintain low density (0.0232 g·cm⁻³) and high porosity (>90%) while doubling compressive stress (from 55.75 to 106.65 kPa). Adsorption capacities range from 20 to 60 g·g⁻¹, with about 80% efficiency retained after five cycles due to reversible protonation of tertiary amine groups. This approach enables controlled oil–water separation and reusable environmental remediation. In addition to direct cellulose network modification, cellulose-derived polyols have been explored as reactive intermediates to produce polyurethane (PUR) foams. Szpiłyk et al. demonstrated that biodegradable PUR foams synthesised from cellulose-based polyols exhibit competitive density and water uptake, along with improved thermal stability [117]. Complementary work reported in European Polymer Journal further confirms that bio-based polyols derived from cellulose can be effectively integrated into PUR formulations, enabling partial replacement of petrochemical components while maintaining suitable mechanical strength, dimensional stability, and thermal performance for insulation applications [118]. Together, these studies illustrate that both direct crosslinking of cellulose networks and indirect valorisation routes via cellulose-derived intermediates represent viable pathways toward high-performance, sustainable foam materials.

3.2.2. Lignin

Lignin is commonly incorporated into polyurethane foams (flexible and rigid) and recognised as a promising strategy to reduce dependence on petroleum-based polyols and improve sustainability. Lignin, an abundant aromatic biopolymer rich in hydroxyl groups, can react with isocyanates and partially replace conventional polyols in PU formulations [119]. Due to its rigid aromatic structure and high carbon content, lignin can enhance the thermal stability, stiffness, and flame resistance of developing foams [120]. However, its structural heterogeneity, limited solubility, and relatively low reactivity often affect cell morphology, mechanical performance, and processability, requiring chemical modification or formulation optimisation to achieve homogeneous foam structures and desired properties.

• Flexible Polyurethane Foams incorporating lignin

Gondaliya and Nejad replaced 20% of petrochemical polyol with 15 different unmodified lignins derived from hardwood, softwood, and bagasse sources, including kraft and organosolv types [121]. Lignin addition generally increased foam density while maintaining values suitable for carpet underlay and insulation. Mechanical properties, including compression force deflection, compressive modulus, tensile strength, tear resistance, support factor, and thermal stability, consistently improved, while elongation at break decreased. Lignins with higher hydroxyl content produced denser foams and greater compression resistance. Organosolv lignin showed superior substitution efficiency, while kraft lignin produced denser foams with enhanced mechanical strength, highlighting the impact of lignin origin and extraction process. Zhu et al. incorporated lignin derived from enzymatic hydrolysis residues at levels up to 15% of the polyol fraction [122]. At low loadings (2.5–7.5%), foam density decreased compared to the control, while 15% substitution caused a substantial density increase (117 kg m⁻³). The compressive modulus increased nearly tenfold at the highest lignin content. Although compressive strength remained stable up to 10% substitution, it increased significantly at 15%. A higher lignin content resulted in less homogeneous cellular structures and slightly reduced thermal stability. Carriço et al. incorporated up to 40% kraft lignin into flexible polyurethane foams formulated with castor oil and crude glycerol [123]. Density decreased slightly up to 15% lignin, then increased linearly at higher loadings. Mechanical performance met commercial standards up to 17.5% substitution. At 40%, density nearly doubled relative to the control, compressive strength increased, and thermal stability declined modestly. To address lignin’s intrinsic reactivity limitations, chemical modification strategies have been widely used. Wang et al. converted phenolic hydroxyl groups into aliphatic hydroxyl functionalities with PEG2000, enhancing lignin’s reactivity toward isocyanates and enabling flexible PUR foam production [120]. Modified lignin enabled partial polyol substitution and produced lighter foams with improved compressive resistance and flexibility. Increasing the lignin content shifted the thermal degradation onset to lower temperatures but increased the maximum degradation temperature. Alkaline-treated lignin outperformed untreated lignin in cushioning and insulation performance. Liu et al. functionalised four lignin types via oxypropylation (10–25 wt%) [124]. Substitution levels above 25% caused gelation during blending. The resulting foams had densities of 0.08–0.09 g cm⁻³ and compressive moduli of 0.3–0.82 MPa, slightly lower than the control, but all lignin-based foams exceeded 100 kPa compressive strength. High glass transition temperatures (120–160 °C), 100% shape fixity, and 80–96% shape recovery enabled use in shape-memory systems. Liang et al. synthesised lignin-based biopolyols and achieved substitution levels up to 100% [125]. Fully bio-based foams showed biodegradability nearly four times higher than petroleum-based references. Maximum compressive stress occurred at 60% substitution, with values four times higher than those of conventional foams. Density and thermal conductivity increased with lignin content, while water uptake decreased. Zhang et al. demonstrated that blending lignin with propylene carbonate reduces brittleness in lignin-rich PUR foams [126]. Foams with 80% substitution showed closed-cell morphology, increased compressive strength, improved moisture resistance, and competitive thermal insulation performance. Flame resistance decreased slightly as the lignin content increased, while compressive strength improved.

Overall, modified lignin offers clear advantages over unmodified lignin in PUR foams due to its higher reactivity and improved compatibility with the polymer matrix. Native lignin typically presents limited accessibility of hydroxyl groups and structural heterogeneity, leading to poor dispersion and irregular foam morphology. Chemical modifications such as oxypropylation or phenolation increase hydroxyl functionality and reactivity toward isocyanates, promoting better incorporation into the PUR network and more controlled crosslinking. As a result, modified lignin-based foams generally exhibit improved mechanical performance, enhanced thermal stability, and more homogeneous cellular structures compared to foams produced with unmodified lignin.

• Rigid Polyurethane Foams Incorporating Lignin

In rigid PUR foams, incorporating lignin (either directly or through modified polyols) has led to significant improvements in thermal and fire performance. Haridevan et al. improved lignin dispersion by pre-mixing kraft lignin in glycerol and aromatic polyether polyols (3 wt%) before foaming. This approach reduced thermal conductivity by 30%, slightly increased density and compressive strength, decreased cell size, enhanced flame resistance, and raised the onset of degradation temperature from 314 to 330 °C. Henry et al. demonstrated that rigid PUR foams with 0–100% substitution of petrochemical polyols by unmodified softwood kraft lignin met ASTM Type I insulation requirements [127]. A fully lignin-based foam reduced burn length by 127%. Density and thermal conductivity increased with the lignin content, while compressive strength peaked at 50% substitution. Further systematic evaluation of 19 lignins confirmed that source, hydroxyl content, pH, and impurities strongly influence foam performance [128]. Enzymatic hydrolysis of lignin from corn stover showed the most favourable performance profile. Oxypropylated lignins enabled even broader property tuning. Cateto et al. achieved comparable or superior performance to reference foams at substitution levels up to 100% [129]. Modified lignin reduced density, cell size, and thermal conductivity, although the compressive modulus initially declined before increasing at higher contents. Additional modification strategies—including hydroxymethylation and aromatic polyol synthesis—consistently demonstrated improved compressive strength, reduced thermal conductivity, and enhanced thermal stability [130,131,132]. Lignocellulosic foams combining cellulose fibres and lignosulfonates achieved low densities, high compressive modulus, and very low thermal conductivity, with enhanced flame resistance provided by inorganic nanocoatings [133].

• Phenolic Foams Incorporating Lignin

Early investigations demonstrated that lignin could function both as a phenol substitute and as a structural modifier within phenolic foams. Del Saz-Orozco et al. systematically evaluated lignin nanoparticle-reinforced phenolic foams (LRPFs) containing 1.5–8.5 wt% lignin nanoparticles and compared them with unreinforced phenolic foams (PFs) [134]. Increasing the lignin nanoparticle content led to a consistent reduction in foam density and cell size, which the authors attributed to the surfactant-like behaviour of lignin nanoparticles during bubble nucleation and stabilisation. Statistical modelling (ANOVA-based predictive models) further confirmed that the optimal mechanical performance was achieved at approximately 8.5 wt% lignin nanoparticles, particularly within the density range of 120–160 kg·m⁻³, which is relevant for insulation and structural applications. Within this density range, LRPFs outperformed neat PFs in both compressive modulus and strength. For example, at 120 kg·m⁻³, the compressive modulus increased from 14.70 MPa (PF) to 18.86 MPa (LRPF), corresponding to 128% of the unreinforced value. Similarly, compressive strength increased from 0.474 MPa to 0.824 MPa, representing 174% of the reference foam. Notably, improvements in modulus were observed at densities below 155 kg·m⁻³, while strength enhancements were particularly pronounced below 145 kg·m⁻³. These gains are comparable to, or in some cases superior to, those reported for phenolic foams reinforced with non-biodegradable fibres such as aramid or glass fibres, highlighting the efficiency of lignin nanoparticles as bio-based reinforcing agents.

Subsequent studies focused on direct phenol substitution using technical lignin. Li et al. developed bio-phenol–formaldehyde foams incorporating kraft lignin with up to 50% replacement of petroleum-derived phenol and formaldehyde [135]. Foam properties were strongly dependent on formulation, particularly on the blowing agent (hexane) content and catalyst concentration. At low hexane levels, 50% phenol substitution resulted in the lowest thermal conductivity, whereas at higher hexane contents, the same substitution level led to increased thermal conductivity. Nevertheless, the optimised foams exhibited low densities (20–80 kg m⁻³), predominantly closed-cell morphology, compressive strengths up to 1.01 MPa, thermal conductivity between 0.03 and 0.048 W m⁻¹ K⁻¹, and flame retardancy with LOI values of 32–33%, confirming their suitability for insulation applications. D’Souza et al. systematically evaluated kraft lignin substitution levels from 10% to 50% in phenolic foams [136]. Increasing the lignin content enhanced the water absorption capacity (up to 2050%), absorption kinetics, and biodegradability (50–68%), although compressive strength declined beyond 30% substitution. Density showed non-monotonic behaviour, increasing up to 30% lignin and then decreasing at higher substitution levels. The open-cell content and water uptake increased progressively with lignin addition. These studies indicate that lignin incorporation enables multifunctional phenolic foams, but optimal substitution requires balancing mechanical strength, insulation performance, and sustainability parameters such as degradability.

In conclusion, incorporating lignin into phenolic foams offers a promising way to enhance sustainability while maintaining or improving material performance. With its aromatic structure and phenolic functionalities, lignin can partially replace phenol in resin formulations, reducing the fossil-based content and environmental impact. Properly optimised systems show improved thermal stability, increased char formation, and competitive flame-retardant properties. However, challenges related to lignin heterogeneity, reactivity, and compatibility must be carefully addressed to ensure uniform cell morphology and consistent mechanical properties. Overall, lignin-modified phenolic foams represent a viable pathway toward more sustainable, high-performance insulation materials.

• Lignin Nanocellulose Composite Foams

Nanocellulose, particularly in the form of cellulose nanofibrils (CNFs), serves as a versatile nanofibrillar scaffold for the fabrication of lightweight, highly porous lignocellulosic foams. Its high surface area and ability to form interconnected networks enable tunable pore structures and mesoporosity, which contribute to low thermal conductivity (~0.020–0.040 W·m⁻¹·K⁻¹) via the Knudsen effect and enhanced phonon scattering [137,138]. Beyond thermal insulation, CNFs act as reinforcing and nucleating agents, improving pore uniformity, mechanical properties, and foam stability. However, their intrinsic hydrophilicity can lead to increased moisture uptake, compromising dimensional stability and thermal performance, which motivates surface modification strategies such as hydrophobisation or incorporation into polymeric composites. For instance, polyvinyl alcohol (PVA) blended with lignin-containing nanocellulose (LCN) forms stable hydrogen-bonded networks that enhance structural integrity and thermal stability, although additional components may be required to improve flame retardancy or tensile performance at higher loadings [139]. Processing techniques such as freeze-drying and ice-templating allow precise pore control but are limited in scalability, whereas chemical crosslinking strategies using glycerol–succinic anhydride (GSA) or citric acid (CA) effectively produce mechanically robust foams with high porosity (up to 99.5%), low density (~13–20 mg·cm⁻³), and multifunctional properties including sound absorption, antioxidant activity, and improved thermal insulation [140,141]. Recent studies have shown that CNFs can also act as self-reinforcing agents, reducing or eliminating the need for hazardous chemical crosslinkers. In tannin-based foams, CNFs at very low loadings (0.1 wt%) form highly entangled nanonetworks that increase mechanical strength by up to 30%, reduce foam density by ~25%, and raise thermal degradation temperatures by 30–50 °C while decreasing mass loss under fire conditions [142]. Similarly, combining CNFs with lignin and green crosslinkers such as citric acid produces highly porous, mechanically robust foams with uniform pore structures, reduced shrinkage, and significantly higher compressive modulus and strength compared to CNF alone. These foams also exhibit enhanced thermal insulation, acoustic damping, and antioxidant activity, demonstrating multifunctional performance suitable for sustainable, bio-based applications [143]. Moreover, the incorporation of lignin-containing nanocellulose fibrils (LCNFs) into polymeric foams such as rigid polyurethane further improves thermal and mechanical performance. LCNFs act as nucleating agents, producing finer, more heterogeneous cell structures and slightly increasing foam density, which enhances compressive strength and modulus while reducing thermal conductivity. This approach allows for lower isocyanate usage, resulting in more sustainable and cost-effective lightweight foams with tailored thermal and mechanical properties [144]. Overall, the integration of nanocellulose into composite systems and chemically crosslinked networks represents a promising strategy for developing next-generation lignocellulosic foams with controlled porosity, low thermal conductivity, and multifunctional properties.

Table 4. Technical properties of lignocellulosic foams.

Type of Foam	Density (kg/m3)	Thermal Conductivity (W/(m·K))	Compressive Stress at 10% Deformation (kPa) *	Reference
Cellulose/poly(ethylene glycol) composites	40.0–120	-	70.0 (at 30% compression)	[106]
Cellulose biocomposite foams	-	-	90–2660 (maximum stress)	[145]
SDS/cellulose/NaOH/urea blend foams	25.0–150.0	-	0.31–1.41	[107]
Cellulose nanofibril foam	5.6–60	-	4.0–40.0	[103]
Cellulose fibre-based foams	19.0–28.0	-	2.8–10	[110]
Cellulose foams from organosolv pulps	20	-	13.3–22.4	[108]
Cellulose-based bio-foam	12.6–16.8	0.05	13.9–221.2	[18]
Cellulose pulp/borate cross-linking foams	13.3–16.4	0.045	74.1	[115]
Cellulose-based/MFC foam	32.9	0.040	25.1	[116]
Nanofibrillated cellulose aerogel	8.1–20.3	0.025	-	[22]
PDMAEMA-co-PHA cellulose-based foams	98.84–0.0175	-	55.75–106.65	[20]
Lignin PUR flexible foams	58.0–95.0	-	4.0–14.0 (at 50% compression)	[121]
Bio-based polyurethane (BPU) foams	60.0–117.0	-	0.61–59.87	[122]
Kraft lignin, castor oil and residual glycerol PUR foams	50.0–120.0	-	10.0–65.0	[123]
PEG2000 grafted-alkali lignin PUR foam	120.0–344.0	-	54.6–201.0 (at 50% compression)	[120]
Lignin-based polyol PUR foams	80.0–90.0	-	-	[124]
Lignin-based polyurethane foam	145.0–192.0	0.0375–0.0432	-	[125]
Polyol lignin and PEG polyurethane foams	38.0–75.0	-	50.0–226.0	[126]
Rigid lignin-based PUR foam	18.0–54.0	0.009–0.033	125.0–270.0 (at 13% compression)	[127,129,146]
Lignin poliol/rigid PUR and PIR Foam	28.0–445.0	0.09 0–0.012	40.0–265.0 (at 13% compression)	[128]
Oxypropylated lignin rigid PUR foam	44.0–130.0	0.03348–0.4919	150.0–830.0	[130,131]
Lignin-based polyols with ethylene carbonate PUR foams	27.0–29.0	0.025–0.025	214.0–244.0	[132]
Close-cell kraft lignin/bio-phenol formaldehyde foams	20.0–80.0	0.030–0.048	30.0–1010	[135]
Open cell kraft lignin-based phenol formaldehyde foams	40.0–54.0	-	4.62–7.40	[136]
Cellulose nanofibrils/xyloglucan bio-based aerogels	23.0–32.0	-	5.84	[147]
Hydrogels of hemicelluloses with poly(vinyl alcohol phosphate) and chitin nanowhiskers	-	-	1000–2000	[148]
Konjac glucomannan/shellac bilayer composite film	-	-	35,000–56,000	[149]

* For compressive strength values obtained at a strain other than 10%, the percentage value is indicated in parentheses.

resumes the main technical properties for foams produced from cellulose and lignin origins. Particularly interesting is the compressive strength data, which are found to be much higher for lignin-derived products than for cellulose-derived products. Scientific developments in the field of cellulosic foams show that it is possible to obtain lightweight foams, even lighter than the synthetic ones available on the market (Table 2 and Table 4); however, their mechanical performance, specifically compressive strength, is below the level required for the applications listed in Table 1. Regarding thermal conductivity, the values are slightly higher than those recorded for XPS foams, which are the ones with the highest values on the market. In the scope of lignin-based foams, the density range is wide, and the results obtained are mixed. There are foams with low densities and values very close to those on the market but also high-density foams (greater than 50 kg/m³). Regarding thermal conductivity, the results are promising, with several foams reaching values within the range observed for synthetic foams on the market (Table 2). Concerning compressive strength, the range of values is broad, with a predominance of values lower than those recorded for foams on the market (Table 2). The foam developed by Avérous et al. [132] presents the best balance between the three variables; that is, it is a lightweight foam with low thermal conductivity and a compressive strength that resembles that of XPS foams.

3.2.3. Hemicellulose

In addition to cellulose and lignin, hemicelluloses constitute an increasingly important fraction of lignocellulosic biomass for the development of lightweight porous materials. Due to their branched molecular architecture, high density of hydroxyl groups, and inherent film-forming ability, hemicelluloses such as xylan, arabinoxylan, and glucomannan (including konjac-derived glucomannan) have been successfully processed into low-density foams and aerogels, often in combination with reinforcing polymers or nanostructured cellulose [150].

Hemicellulose-based systems are frequently blended with poly(vinyl alcohol) (PVA), agarose, or reinforced with cellulose nanofibrils (CNF) or cellulose nanocrystals (CNC). These materials can yield ultralight foams and aerogels with densities in the range of 0.02–0.03 g·cm⁻³, high porosity, and good elastic recovery. For example, CNF/xylan aerogels have reported apparent densities of approximately 23–32 kg·m⁻³, reflecting the reinforcing role of nanocellulose in stabilising the pore architecture and improving mechanical resilience [147]. PVA-based networks have demonstrated compressive strengths in the range of 2–3 MPa under large deformation conditions, depending on the crosslink density and solid content. These results indicate that hemicellulose-containing networks, when appropriately crosslinked and reinforced, can significantly mitigate the brittleness typically observed in highly porous bio-based aerogels [148]. Hemicellulose-based foams have also been engineered for thermal energy storage applications. Deng et al. developed konjac glucomannan (KGM) foams integrated with bilayer phase change microcapsules (PCM), producing materials with high latent heat storage capacity and effective thermal insulation performance [149]. The reported melting enthalpy reached approximately 149 J·g⁻¹, while the bilayer microcapsule design effectively suppressed paraffin leakage, maintaining structural integrity during repeated thermal cycling. These results highlight the multifunctional potential of hemicellulose-based foams for combined insulation and heat-storage systems. Processing strategies for hemicellulose aerogels typically include freeze-drying, supercritical drying, and nanocellulose reinforcement to preserve hierarchical porosity and reduce shrinkage. A broader biorefinery-oriented review emphasises that hemicelluloses such as arabinoxylan and xylan can be integrated into aerogel fabrication routes alongside cellulose nanofibrils, enabling tailored pore structures and multifunctional performance [150]. However, high-performance drying methods (particularly supercritical CO₂ drying and lyophilisation) remain energy-intensive and costly, posing scale-up challenges.

3.3. Market Tendencies

This section summarises the insulation foam, lignocellulosic materials, and lignocellulosic fibre markets, including their future outlooks, growth drivers, and market constraints, to highlight opportunities for lignocellulosic foams. It then presents bio-based products on the market for direct use in construction and products developed by research institutions and companies with commercial potential (lignocellulosic-based raw materials with potential applications in insulation foams, and lignocellulosic-based foams that, although already commercially available, are not yet produced on a large scale).

3.3.1. Market Studies

Valued at USD 30.33 billion in 2025, the insulation foam sector (polystyrene, PUR and PIR, polyolefin, elastomeric, phenolic, and others) is projected to rise to USD 31.81 billion by 2026. Looking further ahead, the market is anticipated to hit USD 40.39 billion by 2031, sustained by a compound annual growth rate (CAGR) of 4.89% during the 2026–2031 period [136]. Dominating the landscape in 2025, PUR and PIR foams commanded a 40.74% market share, with the building and construction industry emerging as the primary end-user, accounting for 36.72% of total demand [151]. Key drivers include building energy codes such as the International Energy Conservation Code (IECC) 2024, which establish minimum energy efficiency standards for both new constructions and renovations, alongside sustainable building certifications like LEED (Leadership in Energy and Environmental Design) [151]. To comply with these increasingly stringent regulations, builders are adopting high-density polyurethane, polyisocyanurate, and structural insulated panels (SIPs). Renovation activities are also on the rise, fuelled by incentives that reward energy efficiency upgrades in existing structures. Foam solutions are preferred due to their high thermal resistance at lower thicknesses, meeting the performance limits required by the code. Additional drivers include the rapid expansion of the e-commerce cold chain and the growth of off-site modular construction utilising foam-insulated panels [151]. As constraints, we have the market volatile fossil raw material prices. Additionally, fire safety regulations impose stricter limits on flame spread and smoke density, particularly for high-occupancy structures. As a result, manufacturers are reformulating products with advanced flame retardants, which increases the final cost. Finally, recyclability mandates and limited foam recovery programmes are beginning to require these materials to be recyclable at the end of their life cycle; however, the necessary technology and infrastructure remain non-existent [151].

It is precisely in addressing the obstacles presented by synthetic-based foams in conjunction with the rising demand for sustainable materials, awareness of environmental impact and government initiatives promoting renewable resources [152] that lignocellulosic-based foams can play a decisive role. Although market studies specifically for lignocellulosic foams are non-existent, studies focusing on lignocellulosic materials and fibres highlight the construction sector’s growing interest in products of lignocellulosic origin.

The lignocellulosic materials market is poised to attain a value of USD 28.57 billion by 2030, maintaining a consistent CAGR of 7.2% throughout the 2023–2030 forecast window. When segmented by application, the pulp and paper industry stands as the primary consumer; however, the construction sector also ranks as a significant end-user of these materials [152]. Regarding the market constraints, technical hurdles in the extraction of lignocellulosic materials, requiring niche equipment and skilled labour, act as a barrier to entry. The lack of uniformity in both biomass composition and processing techniques creates quality gaps that stifle market growth [152]. Also, despite their potential, bio-based lignocellulosic inputs struggle to gain ground against long-standing industry standards. The need for manufacturers to overhaul existing processes introduces financial and logistical burdens that can stall adoption. Furthermore, the lack of consumer familiarity with these sustainable options often leads to a preference for traditional, well-known products. Finally, there is a lack of economic analysis for bio-insulation products; specifically, Life Cycle Cost (LCC) analyses for lignocellulosic foams are currently non-existent. However, the main conclusions of studies addressing bio-based insulation costs point to market constraints, as these products tend to be more expensive than well-established insulation materials on the market [153,154].

The global lignocellulosic fibre industry is anticipated to expand at an annual growth rate of 7% through 2031 [155]. Historical data from 2022 show that bioplastics and packaging were the dominant sectors; however, insulation has also established itself as a major functional application for these fibres [155].

3.3.2. Bio-Based Materials on the Market

When analysing commercial products, we can categorise them into three groups: (1) bio-based materials, some of them lignocellulosic-based, for direct use in construction, which are not foams, (2) lignocellulosic-based raw materials with potential applications in foam production, and (3) lignocellulosic-based foams.

Bio-based materials on the market.

There are bio-based insulations on the market produced from raw materials such as wood fibres, recycled waste wood chips, newspapers, and industrial hemp fibres, etc., (Table 5). In addition to the bio-based raw material, these insulations might include other components, with specific functions, such as binders made from recycled materials. Ekolution^® Hemp Fiber Insulation IB30 (Malmö, Sweden) uses recycled polyethylene as its binder. To address the limitations of lignocellulosic materials, the formulations on the market rely on additives aimed at increasing resistance to water, moisture, and fire [156,157]. For example, paraffin acts as a hydrophobic agent, PUR resin serves as a binder and increases moisture resistance, and ammonium phosphate functions as a fire retardant. However, in cases of prolonged failures in the airtightness of construction systems incorporating these insulations, the additives may not remain sufficiently effective in the long term. The insulations on the market are rigid, semi-rigid, and flexible in nature and can be applied in interior walls, pitched roofs, floors, exterior walls in ETICS systems, and cross-laminated timber (CLT) walls. These applications are similar to those for synthetic insulations on the market (Table 1).

The analysis of the various technical properties of commercially available bio-based insulators and their comparison with foams in Table 2 reveals challenges regarding energy efficiency, density, and compressive strength—the latter depending on the intended application. Concerning thermal conductivity, bio-based insulation available on the market shows values in the range of 0.036–0.049 W/m K (Table 6). Although these thermal conductivity values are slightly higher than those of commercially available polystyrene-based foams, they are much higher than those of PIR and phenolic foams. Density variation is broad, with values between 30 and 70 kg/m³ for flexible and semi-rigid insulation and between 110 and 240 kg/m³ for rigid insulation. Challenges also arise at this stage, as the high densities of rigid panels—well above those recorded for rigid PIR and XPS panels—require robust structural elements to support the weight. For example, application in an ETICS system becomes demanding for professionals, fastening systems, and the supporting structure due to the high weight. Case studies reveal further difficulties associated with handling these materials: wood fibre is somewhat heavier than other materials and, while it can be cut, it cannot be shaved or planed as easily as, say, polystyrene [158]. Despite the high density values, the compressive strength values are like those obtained for commercially available PIR and XPS insulation, except for those whose application involves high loads, such as DANOPREN 500. The water vapour diffusion resistance (0.48–5) is significantly lower than that of the foams in Table 2, making these insulations quite permeable and breathable, and suitable for pitched roof or wall structures where a breathable system is required. In cases where these panels are used in a deck system, where high vapour diffusion resistance is necessary, a vapour barrier must be applied beneath the insulation to protect it from internal moisture. In these systems, the top layer, usually the waterproofing membrane, has high vapour diffusion resistance, preventing the drying or passage of accumulated internal moisture to the exterior. In the absence of a vapour barrier, moisture will accumulate in the insulation, which is undesirable as water reduces the thermal insulation properties of the material [159]. Regarding fire resistance, bio-based insulation exhibits performance similar to synthetic insulation, with a Class E rating. Results obtained by adding fire retardants are insufficient to achieve superior performance.

Table 5. Bio-based insulation materials on the market.

Product	Components	Application	Insulation Type	Reference
Gutex Thermoroom	Recycled waste wood chips, PUR resin (4%)	Interior insulation system	Rigid	[160]
Gutex Multiplex Top	Recycled waste wood chips, PUR resin (4%), Paraffin (1.5%)	Pitched roofs and timber frame walls	Rigid	[161]
Gutex Multitherm	Recycled waste wood chips, PUR resin (4%), Paraffin (1%)	Externally on timber frame and CLT walls and roofs	Rigid	[162]
Gutex 5in1	Wood fibre	Universal application including approval for ETICS	Rigid	[163]
Gutex Omnitherm	Untreated fir and spruce wood, PUR resin (4%) paraffin (1%)		Rigid	[164]
Gutex Thermoflex	Untreated fir and spruce wood, ammonium salts (6%), textile binding fibres (5%)	Roof, ceiling, inside wall, outside wall	Semi-rigid	[165]
MULTITHERM 110	Wood fibres, PMDI gluing, paraffin	Universally for roofs and walls	Rigid	[166]
WALL 140	Wood fibres, PMDI gluing, paraffin	ETICS, exterior masonry and solid wood walls	Rigid	[167]
Steico Universal Dry	Wood fibre, polyurethane resin, paraffin wax	ETICS, pitch roofs, walls, and floors	Rigid	[168]
Steico Special Dry	Wood fibre, polyurethane resin, paraffin wax	ETICS, pitch roofs, walls, and floors	Rigid	[169]
Steico Therm Dry	wood fibre, polyurethane resin, paraffin wax	Pitch roofs, walls, and floors	Rigid	[170]
Ekolution® Hemp Fiber Insulation IB30	Industrial hemp fibres (92%), recycled binders (5%, polypropylene and recycled polyethylene), fire retardant (mineral salt- ammonium phosphate, 3%)	Above-ground structural applications, including exterior and interior walls, roofs, and intermediate floors	Semi-rigid	[171]
Ekovilla slab	Paper, newspaper, cardboard, flame retardant (boric acid, 0.1%), polyester staple fibre, additives–78.3% recycled materials	Roofs, floors and walls	Semi-rigid	[172]
VestaEco Flex	Lignocellulosic fibres, cellulose fibres, BICO fibres	Between-rafter insulation and as an insulating infill of walls and slabs with a timber frame structure	Flexible	[173]
VestaEco Therm	Lignocellulosic fibres, PMDI resin	Insulation of walls and roofs of houses with timber frame structure	Rigid	[174]
VestaEco Wall	Lignocellulosic fibres, cellulose fibres, BICO fibres	Masonry walls	Rigid	[175]
Gramitherm® grass fibre	Grass fibre (70%, +/−5%), recycled jute fibre (20%, +/−5%) and synthetic binder (10%, +/−2%)	Insulation of walls, floors, ceilings, attics and roofs	Semi-rigid	[176]
IndiBreathe Flex	Industrial hemp and recycled jute fibres	Between rafters, joists, and within cavities in roofs, walls, ceilings, and floors	Flexible	[177]
Pavatherm	Wood fibres	Ventilated facade on a timber frame, ventilated pitched roof, ETICS	Rigid	[178,179]
Isolair Multi	Wood fibres	Ventilated facade, ventilated pitched roof, ETICS	Rigid	[179,180]
Pavawall® GF XL	Wood fibres	ETICS	Rigid	[179,181]
Pavaflex® Confort 36	Wood fibres	Internal ceilings and walls	Semi-rigid	[179,182]

Table 5. Bio-based insulation materials on the market.

Product	Components	Application	Insulation Type	Reference
Gutex Thermoroom	Recycled waste wood chips, PUR resin (4%)	Interior insulation system	Rigid	[160]
Gutex Multiplex Top	Recycled waste wood chips, PUR resin (4%), Paraffin (1.5%)	Pitched roofs and timber frame walls	Rigid	[161]
Gutex Multitherm	Recycled waste wood chips, PUR resin (4%), Paraffin (1%)	Externally on timber frame and CLT walls and roofs	Rigid	[162]
Gutex 5in1	Wood fibre	Universal application including approval for ETICS	Rigid	[163]
Gutex Omnitherm	Untreated fir and spruce wood, PUR resin (4%) paraffin (1%)		Rigid	[164]
Gutex Thermoflex	Untreated fir and spruce wood, ammonium salts (6%), textile binding fibres (5%)	Roof, ceiling, inside wall, outside wall	Semi-rigid	[165]
MULTITHERM 110	Wood fibres, PMDI gluing, paraffin	Universally for roofs and walls	Rigid	[166]
WALL 140	Wood fibres, PMDI gluing, paraffin	ETICS, exterior masonry and solid wood walls	Rigid	[167]
Steico Universal Dry	Wood fibre, polyurethane resin, paraffin wax	ETICS, pitch roofs, walls, and floors	Rigid	[168]
Steico Special Dry	Wood fibre, polyurethane resin, paraffin wax	ETICS, pitch roofs, walls, and floors	Rigid	[169]
Steico Therm Dry	wood fibre, polyurethane resin, paraffin wax	Pitch roofs, walls, and floors	Rigid	[170]
Ekolution® Hemp Fiber Insulation IB30	Industrial hemp fibres (92%), recycled binders (5%, polypropylene and recycled polyethylene), fire retardant (mineral salt- ammonium phosphate, 3%)	Above-ground structural applications, including exterior and interior walls, roofs, and intermediate floors	Semi-rigid	[171]
Ekovilla slab	Paper, newspaper, cardboard, flame retardant (boric acid, 0.1%), polyester staple fibre, additives–78.3% recycled materials	Roofs, floors and walls	Semi-rigid	[172]
VestaEco Flex	Lignocellulosic fibres, cellulose fibres, BICO fibres	Between-rafter insulation and as an insulating infill of walls and slabs with a timber frame structure	Flexible	[173]
VestaEco Therm	Lignocellulosic fibres, PMDI resin	Insulation of walls and roofs of houses with timber frame structure	Rigid	[174]
VestaEco Wall	Lignocellulosic fibres, cellulose fibres, BICO fibres	Masonry walls	Rigid	[175]
Gramitherm® grass fibre	Grass fibre (70%, +/−5%), recycled jute fibre (20%, +/−5%) and synthetic binder (10%, +/−2%)	Insulation of walls, floors, ceilings, attics and roofs	Semi-rigid	[176]
IndiBreathe Flex	Industrial hemp and recycled jute fibres	Between rafters, joists, and within cavities in roofs, walls, ceilings, and floors	Flexible	[177]
Pavatherm	Wood fibres	Ventilated facade on a timber frame, ventilated pitched roof, ETICS	Rigid	[178,179]
Isolair Multi	Wood fibres	Ventilated facade, ventilated pitched roof, ETICS	Rigid	[179,180]
Pavawall® GF XL	Wood fibres	ETICS	Rigid	[179,181]
Pavaflex® Confort 36	Wood fibres	Internal ceilings and walls	Semi-rigid	[179,182]

Table 6. Technical properties of bio-based insulations on the market. Most of the products are according to EN 13171.

Product	Fire Classification (EN 13501-1)	Thermal Conductivity (W/mK)	Density (kg/m3)	Compressive Strength (kPa)	Vapour Diffusion (μ)
Gutex Thermoroom	E	0.039–0.40	130–150	≥50	3
Gutex Multiplex Top	E	0.045	220	≥200	3
Gutex Multitherm	E	0.04	140	≥100	4
Gutex 5in1	E	0.04	N/A	≥100	4
Gutex Omnitherm	E	0.04	140	≥100	4
Gutex Thermoflex	E	0.036	50	N/A	2
MULTITHERM 110	E	0.038	110	≥50	3
WALL 140	E	0.04	140	≥100	3
Steico Universal Dry **	E	0.043–0.045	180–210	≥200	3
Steico Special Dry **	E	0.04	140	≥100	3
Steico Therm Dry **	E	0.037	110	≥50	3
Ekolution® Hemp Fiber Insulation IB30 N/A		0.038–0.041	30	N/A	N/A
Ekovilla slab	E	0.039	32–42	N/A	N/A
Vesta Eco Flex	E	0.037	70	N/A	2
Vesta Eco Therm	E	0.049	240	150	5
Vesta Eco Wall	E	0.039	110	30	3
Gramitherm® grass fibre ***	E	0.041	40	N/A	2
IndiBreathe Flex	E	0.039–0.42	35	N/A	0.48
Pavatherm	E *	0.038	110	50	3
Isolair Multi	E	0.041–0.044	145–200	100–200	3–4
Pavawall® GF XL	E	0.040–0.044	130–190	70–200	3
Pavawall® Smart	E	N/A	115	N/A	3
Pavaflex® Confort 36	E *	0.038	50	N/A	2

* According to EN 13238 [183]; ** EN 13171 [184] and EN 14964 [185]; *** ETA N° 21/0260 [186] and EN 12086 (vapour diffusion).

Table 6. Technical properties of bio-based insulations on the market. Most of the products are according to EN 13171.

Product	Fire Classification (EN 13501-1)	Thermal Conductivity (W/mK)	Density (kg/m3)	Compressive Strength (kPa)	Vapour Diffusion (μ)
Gutex Thermoroom	E	0.039–0.40	130–150	≥50	3
Gutex Multiplex Top	E	0.045	220	≥200	3
Gutex Multitherm	E	0.04	140	≥100	4
Gutex 5in1	E	0.04	N/A	≥100	4
Gutex Omnitherm	E	0.04	140	≥100	4
Gutex Thermoflex	E	0.036	50	N/A	2
MULTITHERM 110	E	0.038	110	≥50	3
WALL 140	E	0.04	140	≥100	3
Steico Universal Dry **	E	0.043–0.045	180–210	≥200	3
Steico Special Dry **	E	0.04	140	≥100	3
Steico Therm Dry **	E	0.037	110	≥50	3
Ekolution® Hemp Fiber Insulation IB30 N/A		0.038–0.041	30	N/A	N/A
Ekovilla slab	E	0.039	32–42	N/A	N/A
Vesta Eco Flex	E	0.037	70	N/A	2
Vesta Eco Therm	E	0.049	240	150	5
Vesta Eco Wall	E	0.039	110	30	3
Gramitherm® grass fibre ***	E	0.041	40	N/A	2
IndiBreathe Flex	E	0.039–0.42	35	N/A	0.48
Pavatherm	E *	0.038	110	50	3
Isolair Multi	E	0.041–0.044	145–200	100–200	3–4
Pavawall® GF XL	E	0.040–0.044	130–190	70–200	3
Pavawall® Smart	E	N/A	115	N/A	3
Pavaflex® Confort 36	E *	0.038	50	N/A	2

* According to EN 13238 [183]; ** EN 13171 [184] and EN 14964 [185]; *** ETA N° 21/0260 [186] and EN 12086 (vapour diffusion).

Lignocellulosic-based raw materials on the market.

Lineo^®, a bio-based polymer derived from lignin and produced by Stora Enso, serves as a sustainable substitute for fossil-based materials in various industries. Its versatility spans multiple applications, from replacing phenol in plywood resins to the development of biodegradable, bio-based polymers [187]. The commercial availability of this raw material is particularly significant, as numerous academic studies have already begun exploring lignin’s potential as a primary component in the production of insulation foams, as described in Section 3 (e.g., [188,189]). UPM Biochemicals manufactures fully bio-based lignin, offering a portfolio of specialised products tailored for industries such as composites, rubber, adhesives, resins, and polymer foams [190]. A notable example is their UPM BioPiva™ technology, which facilitates the substitution of toxic, petroleum-derived phenol in phenolic resins. This represents a significant breakthrough, aligning with extensive literature that emphasises lignin’s viability in the production of phenolic insulation foams (e.g., [191,192]). Borregaard offers a product line of lignin and lignosulfonates. Eco-friendly lignin-derived biopolymers exhibit a diverse array of functional properties, serving as agents for antioxidation, corrosion inhibition, and emulsion stabilisation, as well as providing UV protection, dispersing capabilities, and complexing features. Furthermore, they act asphenolic building blocks and facilitate crystal growth control and binding [193]. Within the insulation foam sector, these lignins and lignosulfonates are particularly valuable when converted into bio-based polyols, which function as sustainable precursors for polyurethane foam synthesis. LEAF’s portfolio includes bio-polyols derived from vegetable oils, ensuring a 100% green base for polyurethane formulations. These polyols feature low viscosity and superior technical performance in foams, adhesives, elastomers, and coatings [194].

Lignocellulosic-based foams.

Among the companies dedicated to producing lignocellulosic-based foams, the products currently available are primarily intended for packaging applications, although manufacturers emphasise their potential as insulation materials. Regarding large-scale production and commercialisation of these foams, available data highlight their promise, yet the technology remains in the testing phase. The products on the market are discussed below.

Stora Enso offers Papira^® and Fibrease^® foams, which are produced using raw materials sourced from sustainably managed forests. Fibrease^® is an eco-friendly, flexible, and low-density wood foam developed to substitute fossil fuel-based polymers in the packaging sector, providing both thermal insulation and impact protection. This material is currently being scaled for commercial availability. Similarly, Papira^® is a lightweight cellulose-based foam engineered for cushioning applications within the packaging industry [195]. Designed for circularity, it provides a biodegradable solution for various end-users and has been in pilot-stage production since 2021. Through a partnership with Novapor, this pilot phase aims to optimise large-scale manufacturing processes. Notably, both Fibrease^® and Papira^® are compatible with existing paper recycling streams, allowing them to be disposed of in standard fibre collection bins without requiring disassembly. A collaborative effort between the Research Institutes of Sweden (RISE) and Apple resulted in the development of a cellulose-based foam designed for cushioning in the packaging industry [196]. With a notably low density of 24 kg/m³, the foam’s structural and mechanical characteristics indicate a high potential to displace fossil-derived alternatives like synthetic polymer foams. These applications span across packaging, thermal insulation, and lightweight composite manufacturing. To expedite commercialisation, both organisations are actively seeking partners to expand upon this research and adapt the material for diverse industrial uses. Innofibre, an institution specialising in advanced cellulosic materials, has developed foams composed entirely of cellulose fibres. Consequently, these materials are 100% bio-based, fully recyclable, and biodegradable at end-of-life [197]. Current development targets two primary applications: thermal insulation and protective cushioning for packaging. Due to their rigid mechanical properties, Innofibre suggests that these foams can be implemented as structural panels within wall assemblies. Furthermore, the water-based manufacturing process facilitates the seamless and uniform integration of essential additives, such as fire retardants and moisture-resistant agents, which are critical for meeting industry standards. The startup Fibu has engineered a versatile cellulose-based foam with customisable density, thickness, and geometry. This adaptability makes it an ideal solution for protective packaging in high-end sectors—such as cosmetics, electronics, and laboratory glassware—as well as for thermal insulation in the medical and grocery cold-chain markets [198]. While the material exhibits lower tensile and tearing strength compared to traditional plastics, its compressive strength remains competitive. Furthermore, its patented mono-material composition, consisting exclusively of wood fibres, ensures 100% biodegradability and seamless integration into paper recycling streams. A life cycle assessment conducted by RISE underscores its environmental superiority, showing a 60% to 90% reduction in carbon footprint compared to fossil-based equivalents, significantly advancing global decarbonisation goals. Woamy, a spinout from Aalto University, was conceived from the insights gained during the Foamwood research project. This work resulted in a patented bio-foam technology, which mimics the directional strength of wood [199]. The outcome is a high-quality solid foam, exceptional in its strength and remarkable for its lightweight nature. While its primary application is currently protective packaging, its versatile properties suggest a vast range of potential uses. The team has successfully transitioned the manufacturing process from laboratory scale to an expanded pilot stage and is currently commissioning a larger production line to facilitate the first commercial packaging trials.

3.3.3. Standardisation and Certification

Construction products, such as foams, within the European Economic Area (EEA) must comply with the EU Construction Products Regulation (CPR), also known as the Construction Products Regulation. This law states that all products traded or sold in Europe must bear a CE mark, when a harmonised standard exists for this product [200]. A CE marking is important in that it ensures the construction products meet the community harmonisation legislation and consequently freely circulate in the European Union Internal Market. This marking is affixed by the product manufacturer [201].

The CE marking procedures is obligatory when a specific product is covered by a harmonised standard that is already on the Official Journal of the European Union. For construction products that are not covered by a harmonised standard, it is also possible to affix the CE marking through a voluntary route. In this process, it is necessary to verify if a European Assessment Document (EAD) exists in which construction products can be included [201]. This process ensures that the functional and performance requirements of the products are defined in accordance with the Basic Requirements for Construction Works established by Regulation (EU) 2024/3110, and the performance of the products is assessed through a set of experimental tests selected based on their intended use, exposure conditions, and role within construction works. These tests need to address all the essential characteristics relevant to the applicable Basic Requirements for Construction Works, covering different fields such as: Fire safety of construction works; Protection against ad-verse hygiene and health impacts related to construction works; Safety and accessibility of construction works; Resistance to the passage of sound and acoustic properties of construction works; Energy efficiency and thermal performance of construction work. Also, the bio-based construction products need to ensure chemical and physical compatibility with adjacent construction materials and systems, ensuring safe and durable integration into construction works.

Bio-based foams, and lignocellulosic-based foams specifically, for insulation of buildings are not covered by a harmonised standard and the most similar would be the one for phenolic foams—EN 13166 for Thermal insulation products for buildings—Factory-made phenolic foam (PF) products [202], but not specifically for bio-based foams. Also, the EN 13171 Thermal insulation products for buildings—Factory-made wood fibre (WF) products [184] is limited to rolls, batts, felts, boards, or slabs and do not cover foams. Also, the EN 13170 Thermal insulation products for buildings—Factory-made products of expanded cork (ICB)—Specification [203] is limited to panels produced exclusively from cork. Looking at European Assessment Documents, we could consider the application of the European Assessment Document for Factory-made thermal and/or acoustic insulation products made of vegetable or animal fibres (EAD 040005-00-1201) [204] as a reference to define the technical requirements needed for assessment, although it was not designed for foams.

The lack of a specific harmonised standard acts as a major constraint to the development and integration into the market of bio-based insulation foams [205]. Under the current framework, to place a lignocellulosic-based foam on the market, a manufacturer must develop a product with performance comparable to existing fossil-based products on the market and initiate the process for developing aa new EAD. This process was designed to be a bottom-up process, precisely to allow the entry of new innovative products into the market, but it is nonetheless time-consuming and costly. The manufacturer (or a group of manufacturers) must approach a Technical Assessment Body (TAB) in any EU Member State, specifying the uses and applications of the product in the construction system, and request the preparation of a new EAD. The TAB will therefore carry out a detailed characterisation of the product, conduct a detailed study of the product, and develop tests to assess its performance, thereby ensuring the product’s durability, safety, and overall performance throughout its life cycle and in accordance with the intended conditions of use. The EAD proposal drawn up by the TAB will then be submitted to the European Organisation for Technical Assessment (EOTA), which will assess whether the product is relevant to the market and whether there are similar products, and will establish a Working Group comprising experts from industry, academia, and a group of TABs to define: the essential characteristics of the product, the product testing methods, using existing standards (EN, ISO) wherever possible or, where this is not possible, proposing new ones, and the assessment methodologies. This is followed by a consultation phase until the adoption and publication of the new EAD in the Official Journal of the EU. Only once this process is complete can the manufacturer apply for a European Technical Assessment (ETA) based on that EAD and the CE marking for placing the product on the market.

Beyond the difficulty of the lack of certification for placing products on the market, there has been a lack of policy and regulatory incentives for the creation of bio-based products. While some European countries, such as France, have already established supportive policies that favour bio-based materials, others continue to prioritise conventional materials due to existing standards that focus on thermal efficiency without fully considering environmental impacts. A key drawback is the reduction to thermal operational efficiency over the incorporation of environmental impacts [206]. If these impacts were considered, it would foster the selection of materials based on their environmental impact rather than just on their thermal efficiency [205]. Some authors argue that to reach a progressive push towards a decarbonised construction, there should be a bio-economy directive and clear CO₂ reduction targets [207]. We can argue that early standardisation of bio-based materials can increase the number of market players and hence the average greenness of products in the market, inducing further demand for biomaterials.

Furthermore, research and development of more bio-based insulating foams will lead to the development of more robust products, with improved technical characterisation and increased performance studies. This will encourage manufacturers to proceed with the request for European assessment documents, guaranteeing market acceptance through CE marking.

4. Challenges of the Integration of Bio-Based Foams in Building Systems

This section focuses on the barriers to the integration of bio-based building materials. Specific studies regarding the challenges of integrating lignocellulosic foams into construction systems are non-existent. From a more global perspective, considering bio-based materials in general, several studies can be found that identify the primary challenges of integrating these materials into building systems, as will be explored below. Consequently, this section provides an overview of the current landscape. This is highly relevant to the development of lignocellulosic foams because, given their nature, they will encounter the same obstacles. Therefore, their development must consider challenges related to performance (compared to the most used insulation materials), long-term durability, production and installation, regulations and incentive policies, costs, and the perception of stakeholders. These interconnected challenges are addressed in a concise manner.

4.1. Performance of Bio-Based Materials

Many bio-based materials remain niche due to batch production and lack of CE marking but also constraints in the performance, mainly structural capacity, durability and fire constraints. Indeed, the absence of CE marking is an obstacle due to the time-consuming nature of bringing products to market, but also because of the difficulty in identifying minimum performance properties and testing procedures whilst the products are still at the development stage. However, performance limitations are undoubtedly a greater challenge, requiring further research to overcome.

While significant progress has been achieved at the material-development level, the performance of bio-based insulation under construction-relevant conditions remains a critical factor to a widespread adaptation. A primary challenge concerns the stability of thermal performance over time and in different environmental conditions such as moisture ingress, temperature fluctuations, and ageing. The presence of hydrophilic components and partially open cellular structures may lead to increased thermal conductivity under elevated humidity conditions, raising concerns regarding the long-term validity of declared thermal performance [80].

Moisture sensitivity and hygrothermal behaviour constitute another critical performance-related challenge. Bio-based materials frequently exhibit higher water absorption and lower water vapour diffusion resistance than synthetic materials. This may have a positive effect, absorbing moisture into the internal porous system at increased air humidities, and conversely, gradual moisture release into the surroundings with decreasing air humidity [208]. This mechanism favourably influences the indoor air humidity, primarily in winter when prolonged periods of low indoor air humidity may be experienced. But this may be problematic if the material is exposed on a long-term basis to an environment with high humidity or if it is in contact with liquid water [209]. It is known that the relative air humidity affects the effective thermal conductivity of polyurethane foam (PUF) [210]. According to these studies, when the relative humidity varies from 40 to 95%, the moisture content of closed-cell foam changes from 0.7 to 5%. The thermal conductivity coefficient changes from 0.024 to 0.026 W/m⸱K at 10 °C. With the same relative humidity parameters, the moisture content of open-cell foam changes from 2% to 41%, and the thermal conductivity increases from 0.039 to 0.61 W/m⸱K. Moreover, the excessive exposure to humidity can also increase the risk of moisture accumulation within the insulation layer, affecting thermal conductivity, dimensional stability, and mechanical properties and potentially reducing the durability of the insulation system and cause biological corrosion, i.e., degradation by bacteria, mildew and fungi acting on the material [209]. This fragility can be overcome by integrating it into a constructive system, ensuring protection against excessive moisture, such as through ETICS.

From a mechanical performance standpoint, bio-based foams often exhibit lower and more variable compressive strength compared to established synthetic insulation materials. Mechanical behaviour is strongly dependent on foam density, cell morphology, and cell-wall strength, which are influenced by raw material variability and processing routes. In load-bearing or semi-load-bearing applications, insufficient compressive strength or long-term creep resistance may limit the applicability or necessitate additional protective layers, increasing system complexity. Again, this performance can condition the application demanding the use of protective elements, such as in sandwich panels. In general, a material with higher density has better compression performance; however, this may not be true for non-homogeneous materials [211]. Rodrigues et al. [80], in their study on sandwich panels with tannin foams, confirmed the link between the more fragile and irregular cellular structure of tannin foams with reduced resistance to compression and bending. Some additives have shown improvements in the compressive performance by generating stronger foam cell wall structure, such as soybean protein isolate in tannin foams [212]. In fact, these authors argued that foam density is not the only factor influencing compressive strength, but also the cell morphology and cell wall strength.

Fire performance remains a decisive performance-related challenge. In general, insulation products are typically vulnerable to fire and contribute to smoke production. Although certain bio-based foams, such as tannin-based systems, have demonstrated inherently favourable fire behaviour due to char formation [98,213], many bio-based materials remain vulnerable to ignition and flame propagation without modification. Achieving adequate reaction-to-fire classification while maintaining thermal and mechanical performance often requires the incorporation of flame retardants or structural modifications, which may affect sustainability credentials and processing complexity.

Finally, one of the main concerns with bio-based materials stems from the variability of bio-based raw materials, which introduces additional uncertainty in terms of consistent performance. Natural feedstocks are subject to variations in chemical composition, molecular weight distribution, and impurity content, which can influence foam morphology and, consequently, thermal, mechanical, and hygrothermal properties. Ensuring the reproducibility of performance on an industrial scale remains one of the challenges for the diffusion of these new materials and their regulatory and market acceptance.

4.2. Regulations and Policies

The absence of harmonised standards, as mentioned in the previous section, constitutes a challenge of major relevance as identified by various stakeholders [205,214]. The possibility of obtaining European Technical Assessment (ETA) certification is recognised, but the high costs and significant time required to achieve it are considered major barriers. A disorganised and fragmented certification process in some countries regarding these products is also seen as a barrier [205]. In other countries, such as Belgium, it is considered that the government lacks sufficient rules or initiatives to promote construction using bio-based materials [215].

Strict fire safety standards are also considered a barrier [205,214,216]. When German stakeholders were questioned about the primary obstacles, DIN EN 13501-1 was identified as a challenge since most bio-based products require treatment with fire retardants to satisfy the standard, and research into natural fire retardants remains scarce. On the other hand, the addition of fire-retardants to enhance the fire performance improves the cost of bio-based products.

In the field of policy, incentive-related measures have received special attention, both for their positive role in encouraging the adoption of these materials in construction and for their currently limited application [214]. For example, in Germany, the potential of the Bavarian Wood Construction Subsidy program is recognised; however, it is considered limited as it only applies to large-scale construction projects [205]. Another example is Finland, where the Housing Finance and Development Centre provides loans for public and private building projects. If these projects incorporate the use of wood, the costs can be partially compensated by the government [214].

4.3. Long-Term Durability

The study of the influence of environmental factors on thermomechanical properties is essential for identifying challenges related to thermal performance, dimensional stability, mechanical behaviour, and biological corrosion of materials over time. Ageing studies are therefore crucial, as they enable the identification of challenges in the long-term performance of insulation materials. For bio-based foams (fully or partially bio-based), there are few ageing studies, which prevents a comprehensive discussion of the long-term durability of these materials and their comparison with fossil-based foams. The main conclusions are presented, but it remains impossible to clearly identify the specific challenges in this area. Consequently, future research in this field is a priority.

Zhou et al. investigated the ageing process of palm-oil polyurethane foams reinforced with CNCs. While they noted that the nano-reinforcements significantly boosted water uptake, the subsequent effects on the foams’ mechanical integrity were not addressed [217]. Dong, Wei et al. manufactured a lignocellulosic foam using multiscale cellulose fibres and sodium lignosulfonate, and in investigating its stability after 120 days of storage, they found that the morphological and structural properties were maintained without significant decrease or collapse. Stability was also observed in terms of compression modulus and thermal conductivity [133]. Zhu et al. produced lignocellulose/clay composite foams for building insulation and found no statistically significant difference in the samples in terms of weight, structural dimensions, or appearance after 9 months under laboratory conditions (22 ± 1 °C and 50% relative humidity) [218]. The hygrothermal ageing of phenolic foams incorporating lignin and wood flour showed a reduction in mechanical properties, between 66–69% in compressive modulus and 57% in compressive strength. Additionally, hygrothermal ageing did not influence the cell size of the foams [219]. Ageing studies on polyurethane foams prepared from castor oil demonstrated that thermal conductivity performance is not compromised when compared to the ageing of petroleum-based polyurethane foams [220].

4.4. Production and Installation

Challenges regarding production and installation are linked to a lack of skilled labour and the absence of training programs throughout the entire value chain [214,215]. Furthermore, the limited size of the market and industry makes it difficult to benefit from economies of scale and technological learning [214]. Such learning is essential to operationalise and optimise the production of these products, ultimately making them more attractive from a cost-efficient perspective. An in-depth analysis of the topic is out of the scope of this paper.

In the specific case of sandwich panels, research has focused on bio-based skins, bio-based adhesives, and bio-based cores [221,222,223,224,225]. Widely studied bio-based core materials include balsa wood (in laminated configurations), plywood, cork stoppers (reused or recycled), and bio-based polymeric foams (such as those made from PLA, starch, or tannin). However, for lignocellulosic cores, research has shown limitations in terms of strength and anisotropic behaviour, core-to-metal sheet bonding, and compatibility with currently used industrial technology [221]. The core-to-metal sheet bonding and compatibility with currently used industrial technology are features of major interest in the production process.

Improving the interfacial bonding strength between the faces (generally biocomposites) and the core (lightweight material) is one of the main challenges affecting the overall mechanical properties of sandwich panels. Lignocellulosic cores do not naturally adhere to metal sheets, contrary to what is observed with PIR-based cores. Adhesive bonding is the simplest method, but adhesives (even bio-based ones) can have the disadvantage of reduced bonding strength over time and across different working environments [226]. Additionally, a concern for bio-based adhesives is the resulting bonding strength and adhesive’s susceptibility to external factors such as humidity, ageing, and the need for surface treatment on the bonding substrate [227]. The intrinsic variability of lignocellulosic raw materials, mentioned in Section 4.1, constitutes an industrial technological limitation. This variability translates into inconsistent dosing, viscosity fluctuations, cellular heterogeneity, density variations across the panel width, and a higher risk of localised defects, including poorly consolidated zones, cellular collapse, or delamination. Conventional industrial PUR/PIR systems work with much more standardised raw materials, thus avoiding these challenges. Still in the context of a sandwich panel production line, lignocellulosic fibres or powders can lead to dust generation, cause increased equipment wear and abrasion, require more frequent cleaning, elevate the possibility of microbiological contamination of the raw material, and, depending on the particle size and concentration in the suspension, pose a potential ATEX (from the French Atmosphères Explosibles) risk due to combustible dust. While this does not render the scale-up unfeasible, it implies the re-engineering of existing production lines regarding raw material feeding, aspiration systems, local inerting, and storage. The challenges mentioned in this section result from the experience of Engineer Jose Junqueira, one of the authors of this article, who works for a company dedicated to the production of sandwich panels. In scientific literature, these types of limitations are not reported.

Considering what has been presented in this section, the gaps in harmonised standards, and the results of long-term durability studies, it can be stated that the industrial upscaling of lignocellulosic foams into continuous sandwich-panel manufacturing remains constrained by three interrelated bottlenecks: process variability, regulatory qualification, and system-level durability. Unlike conventional PUR/PIR systems, lignocellulosic feedstocks exhibit wider variability in moisture content, particle size distribution, extractives, and thermal stability, which narrows the processing window and complicates density control, cell homogeneity, cure kinetics, and adhesion to facings. From a market-entry perspective, the absence of a fully consolidated, harmonised standard pathway for structural lignocellulosic foam cores may require case-specific ETA/EAD-based assessment, increasing te certification time and cost. In addition, long-term performance/durability under moisture, biological exposure, thermal cycling, and fire must be demonstrated more rigorously, particularly for applications in continuous sandwich-panel lines originally designed for conventional petrochemical foams.

4.5. Costs

From an economic perspective, there is still limited data on the production costs, operational costs, and long-term profitability of bio-based materials. However, several studies point to the high cost of these materials as a barrier to their adoption in building [205,215,228]. A comparison of bio-based insulation products with expanded polystyrene and stone wool using Life Cycle Costing showed that bio-based insulation materials are more expensive than expanded polystyrene—ranging from 70% more for wood fibre-based materials to 9% more for miscanthus-based materials [154]. Lower market prices for expanded polystyrene insulation compared to bio-based products were also identified in [153]. Rock wool and glass wool (loose fill) also have lower market prices than bio-based material [153]. These high costs are associated with high labour costs, the expenses incurred by producers to obtain the necessary certifications [215], the limited availability of raw materials (as many producers must import their supply) [205], and the processing of raw materials, which is sometimes complex and expensive [205].

4.6. Stakeholder Perception

Regarding this point, the challenges are related to the low acceptance of these materials within the construction sector. This is partly due to a lack of knowledge concerning application methods and the absence of solid expertise, leading to resistance to switching from conventional to bio-based materials. Furthermore, a culture rooted in misinformation often perceives these materials as vulnerable and having limited durability [205,215,228,229]. Other studies point to a lack of awareness and knowledge—specifically, an unfamiliarity with bio-based insulation materials among both end-users and sales personnel [229]. There is also poor communication and a lack of common understanding and interests among designers, contractors, and architects, alongside a lack of public drive to emphasise the saving and preservation of resources in construction [230].

5. Future Research

The development of lignocellulosic materials for building applications is a requisite aligned with global sustainability targets, particularly those associated with the European Green Deal and decarbonisation strategies. Recent advances in nanotechnology have further expanded the functional properties of wood-based materials, enabling the development of high-performance systems such as densified wood, transparent wood, and bioinspired wood. These materials demonstrate enhanced mechanical strength, improved fire resistance, optical transparency, and controlled structural properties, highlighting their strong potential to replace or complement conventional materials in sustainable building solutions [231,232].

In parallel, significant research efforts have focused on the valorisation of agro-industrial residues and natural fibres as raw materials for lignocellulosic foams and composites. The agro-industry generates large amounts of residues that can be repurposed into value-added construction materials, contributing to waste reduction and lowering the environmental impact associated with synthetic material production [140,209,233,234,235]. Natural fibres such as hemp, flax, straw, coconut, sisal, and sunflower offer several advantages, including low density, renewability, biodegradability, low energy processing requirements, and reduced CO₂ emissions [209,234,235,236]. Their intrinsic cellular structure contributes to favourable thermoacoustic insulation properties, depending on composition and processing conditions [237,238]. Future research should therefore prioritise the exploration of locally available and underutilised fibres to reduce transportation costs, promote regional circular economies, and diversify the raw material base for sustainable construction materials [209,239].

Another promising research direction lies in the application of bioinspired and biomimetic approaches to enhance the performance of lignocellulosic materials. By mimicking natural processes such as biomineralisation, it is possible to engineer organic–inorganic hybrid systems with improved mechanical strength, thermal stability, and fire resistance without relying on toxic additives [240,241]. These strategies exploit the intrinsic porous structure and surface chemistry of lignocellulosic materials to induce in situ mineral formation, resulting in composites that benefit from reduced heat transfer, enhanced resistance to biological degradation, and improved fire behaviour through stable char formation [240,241]. Such approaches not only preserve the sustainability of bio-based materials but also transform them into multifunctional systems with specified properties for advanced building applications. Overall, the convergence of material innovation, waste valorisation, and bioinspired design provides a clear pathway for the development of next-generation lignocellulosic foams and composites that meet the demands of energy-efficient, low-carbon construction.

In addition to material development and performance optimisation, future research should address critical gaps related to the long-term sustainability and end-of-life management of lignocellulosic materials. Although these materials are inherently bio-based and often considered environmentally friendly, comprehensive evaluations of their full life cycle remain limited. In this context, it is essential to conduct detailed life cycle assessments (LCAs) to quantify their environmental impact from raw material sourcing to disposal or recycling, ensuring that their implementation effectively contributes to carbon reduction and circular economy goals [234]. Furthermore, systematic degradation studies under realistic environmental conditions, such as prolonged exposure to humidity, temperature fluctuations, UV radiation, and biological agents, are necessary to assess durability, performance stability, and potential environmental release of degradation by-products over time [209,237]. Long-term exposure studies will also be crucial to validate their applicability in real building environments, particularly in terms of mechanical integrity, insulation performance, and fire resistance throughout their service life. Addressing these aspects will be fundamental to ensuring not only the initial sustainability of lignocellulosic materials but also their safe, reliable, and responsible integration into next-generation construction systems.

Lastly, research regarding LCC is practically non-existent. Among the challenges identified, the higher cost of bio-based insulation than synthetic alternatives stands out as a significant limitation. However, it is essential to develop LCC analyses specifically tailored to these materials to enable proper benchmarking against market-dominant insulation types. While the initial cost is important to the end consumer, understanding the savings achieved over the service life of bio-based insulation is crucial for evaluating their true economic value and, consequently, identifying market opportunities.

6. Conclusions

The integration of lignocellulosic materials into bio-based construction systems represents a technically credible pathway toward reducing the embodied carbon and fossil dependency of thermal insulation products. Lignocellulosic foams can achieve densities between 10 and 120 kg·m⁻³ and thermal conductivities as low as 0.019–0.046 W·m⁻¹·K⁻¹, approaching those of established synthetic foams. Through control of relative density, pore size distribution, crosslink density, and polymer–biopolymer interfacial compatibility, it is possible to tailor compressive strength, dimensional stability, and fire performance to meet specific building requirements. In particular, chemically modified lignin has demonstrated strong potential for partial or full substitution of petrochemical polyols in polyurethane and phenolic systems, while crosslinked cellulose and nanocellulose architectures enable lightweight structures with hierarchical porosity and competitive insulation performance.

Nevertheless, translating laboratory-scale success into market-ready construction products requires overcoming several interrelated challenges: (1) biomass heterogeneity, arising from feedstock origin and extraction processes, introduces variability in hydroxyl functionality, molecular weight distribution, and impurity content, directly affecting reactivity, foaming kinetics, and reproducibility at industrial scale; (2) moisture sensitivity remains a fundamental limitation for polysaccharide-rich systems, as water uptake increases thermal conductivity and compromises dimensional stability and mechanical integrity, necessitating crosslinking or hydrophobisation strategies that must be balanced against environmental impact; (3) fire performance, although improved in lignin-rich systems due to enhanced char formation, many bio-based foams still struggle to achieve classifications beyond Class E without additives. Achieving consistent Euroclass performance without compromising mechanical and thermal properties remains a central research priority. In addition, processing scalability is a significant bottleneck. Techniques such as freeze-drying and supercritical CO₂ drying enable highly controlled porous architectures but are energy-intensive and economically prohibitive at construction scale. Mechanical foaming, extrusion, and reactive foaming routes are more scalable but require precise rheological and kinetic control to prevent phase separation, cell coalescence, or structural collapse. Bridging the gap between laboratory-scale solutions and industrially viable rigid boards is therefore a major technological hurdle. Comprehensive validation of long-term durability of lignocellulosic foams under realistic service conditions, including cyclic humidity, biological attack, sustained compressive loading, and ageing, remains essential for regulatory certification and market penetration. However, such performance data are still insufficiently documented and lack systematic standardisation.

Finally, considering the bio-based products currently on the market, we can anticipate challenges for lignocellulosic foams related to the lack of skilled labour to handle the final product, the absence of incentives, and a resistance to change from various stakeholders due to a lack of knowledge and poor market disclosure of these products.

Overall, lignocellulosic foams possess the clear potential to evolve into structurally reliable, thermally competitive, and environmentally advantageous insulation materials. However, they must not only match technical performance and compatible processing and integration, but also economic competitiveness to become commercially viable insulation materials capable of contributing substantively to a circular and low-carbon construction system.