A Comprehensive Comparison of Insulation Materials for Timber Building Systems

Abstract

The key objectives of both European Union and Portuguese policies are energy efficiency and carbon neutrality in the building sector. Timber construction offers unique advantages in achieving these goals, such as increased productivity through faster and more efficient building processes, using renewable resources with lower carbon emissions during production and throughout the lifecycle, and contributions to forest conservation. However, in many countries, timber construction remains underutilised due to concerns about its thermal and acoustic performance, fire safety, and limited availability of raw materials. This study addresses these challenges by evaluating the potential of various insulation materials, including polystyrenes, mineral wools, natural fibres, composites, and acoustic mats, for incorporation into prefabricated timber components. Key performance criteria included thermal insulation, sound absorption, fire reaction, environmental impact, and local availability. Among the materials analysed, glass wool, rock wool, and cork emerged as the most favourable options, offering excellent thermal and acoustic performance and presenting strong results in other key parameters. These findings underscore the potential of incorporating these materials into timber construction systems, contributing to developing sustainable and high-performance building solutions.

1. Introduction

In recent years, governments and industry have set ambitious targets to achieve net-zero greenhouse gas emissions, placing the built environment squarely in focus. As buildings account for a large share of operational and embodied carbon, material selection has become a key lever for decarbonisation. Light timber construction has emerged as a key strategy to promote sustainable development as it uniquely combines the advantages of a renewable resource with inherent carbon-storage capacity and optimised material use while also fitting naturally into circular economy models by enabling reuse and recycling at the end of life. By embracing timber-based systems, designers and policymakers can drive meaningful reductions in a sector that must rapidly shift toward low-carbon, sustainable building practices.

Despite these benefits, the lightweight nature of timber buildings presents challenges regarding thermal and acoustic comfort. Overheating in summer due to low thermal mass [1,2] and insufficient sound insulation—especially at low frequencies transmitted through structural elements [3,4,5,6]—remain significant concerns in modern timber buildings.

In building physics, heat exchange through the envelope—comprising glazing, walls, roofs, and floors—is governed by each component’s thermal transmittance coefficient (U-value), reflecting the overall effectiveness of the insulation system [7].

Table 1. Average maximum thermal transmittance coefficients for climates with similar degree-day values. Adapted from [8,9].

Building Element	Average Thermal Transmittance Limit Based on Building Envelope Type (W/m2K)
	Portugal	Germany	France	England and Wales	Spain	Passivhaus System	USA (ASHRAE)
External walls	0.42	0.20	0.36	0.25 (0.30) a	0.57	0.15	0.45
External floors	0.35	0.28	0.27	0.18 (0.25)	0.48	0.15	0.27
Roofs	0.35	0.20	0.20	0.15 (0.20)	0.35	0.15	0.36 b
Windows/doors	2.47	1.30	1.80	1.60 (2.00)	2.70	0.80	1.99
Average building thermal transmittance coefficient (h) c	0.69	0.39	0.51	0.41	0.81	0.25	0.61

^a The lowest allowed value for the best installations is listed in parentheses. ^b Thermal transmittance coefficient when the roof is above a habitable attic. ^c It is assumed that the exterior wall surface equals the sum of the floor and roof surfaces, with approximately 30% windows/doors.

summarises the maximum average U-values permitted in various countries, including the Passivhaus standard. Originating in Germany, Passivhaus is a benchmark for ultra-low-energy buildings with minimal heating and cooling demands.

A comparison of regulations reveals notable discrepancies. Spain has the least stringent requirements among countries with zones with similar climates, while England, Wales, and Germany impose stricter limits, more closely aligning with Passivhaus standards. The United States falls between France and Spain but remains below the European average regarding regulatory stringency. Portugal, with its Mediterranean climate and relatively lenient thermal standards compared to other European countries, faces the dual challenge of retaining indoor heat during winter while mitigating overheating in summer. This challenge is particularly pronounced in lightweight timber constructions with low thermal inertia. These regulatory variations underscore the global disparity in energy efficiency standards and reinforce the importance of adopting high-performance insulation materials to enhance building performance and sustainability.

The sound absorption capacity of materials is influenced by their inherent properties and the frequency of incident sound waves. In the case of porous materials, high-frequency sounds are primarily absorbed through an adiabatic process, where heat loss occurs due to friction as sound waves travel through the irregular pores. In contrast, at low frequencies absorption is driven by energy loss through heat exchange, which is primarily an isothermal process. As a result, porous materials tend to be more effective at absorbing high-frequency sounds [10]. For lower frequencies, however, material thickness plays a critical role as shorter wavelengths need a higher thickness to impede propagation and facilitate energy absorption [11].

Metrics such as the weighted sound reduction index (R_w) for airborne sound insulation, the normalised impact sound pressure level (L’_nTw) for impact noise, and the standardised level difference (D_nt,w) for airborne sound transmission provide a comprehensive assessment of a structure’s acoustic performance, influencing occupant comfort and compliance with regulatory standards.

Rasmussen [12] compiled data on airborne sound insulation (Figure 1) and impact sound insulation (Figure 2) requirements across Europe, highlighting variations in regulatory stringency. It is important to note that the values presented in Figure 1 and Figure 2 are approximate as a direct comparison is not feasible due to the use of different indices across countries. To enable cross-country analysis between European countries, Rasmussen [12] applied a conversion that standardises the measurements to the in situ airborne (D_nT,w) and impact (L’_nT,w) sound insulation indexes. However, this conversion is based on assumptions regarding geometry and absorption properties, making the results approximate for conventional spaces. The red lines represent the average values among the countries considered. Malta, Macedonia, Luxembourg, and Cyprus do not have regulatory requirements in this regard.

Figure 1 highlights Greece and Italy as having some of the least stringent airborne sound insulation requirements, with values below the average of 52 dB. Nordic countries such as Norway, Finland, and Estonia enforce the strictest regulations. Similarly, Figure 2 shows that Spain, Croatia, and Serbia have lower stringency in impact sound insulation, with average values around 55 dB. In contrast, Nordic countries impose the most demanding requirements, reflected in their lower values indicating stricter control over impact noise transmission. Portugal has relatively low requirements for airborne and impact sound insulation, emphasising the need for greater awareness of the importance of thoughtful design strategies. This includes selecting appropriate materials and implementing integrated solutions to ensure adequate comfort, a challenge that is especially pronounced in lightweight structures, particularly at low frequencies.

Survey results [13] indicate that impact sounds, such as footsteps from neighbouring apartments, are among timber buildings’ most disruptive noise sources. In a related study, Müller et al. [14] examined the acoustic performance of timber and concrete floors at low frequencies, comparing various construction solutions. The study highlighted the significant difference in volumetric mass density between these materials, with concrete averaging around 2500 kg/m³ and wood approximately 600 kg/m³, depending on the species. This difference has a pronounced impact on their acoustic behaviour, particularly at lower frequencies where timber structures generally underperform compared to concrete. In response to these challenges, some countries, such as Sweden, have adopted more stringent regulations by extending the frequency range for acoustic performance assessments to 50–3150 Hz, in contrast to the more commonly used 100–3150 Hz range in other nations [15]. To improve the acoustic performance of timber floors, particularly at lower frequencies, researchers recommend enhancing flooring systems, installing suspended ceilings, or combining both approaches. However, these solutions often entail higher costs and may compromise some of the environmental benefits of timber construction. Therefore, integrating sound-absorbing materials and resilient elements into the design is crucial to balancing improved acoustic performance, cost efficiency, and sustainability.

Timber construction benefits from a variety of insulation materials, yet the lack of comprehensive performance data often drives reliance on traditional solutions [16]. This study comprehensively reviews various absorbent and resilient materials and evaluates their suitability for integration into timber-based prefabricated construction systems, especially in Portugal. The primary aim is to identify the most suitable thermal and acoustic insulation materials for incorporation into modular timber panels based on an in-depth analysis of their performance characteristics. Natural and composite materials were selected, and their behaviour and suitability were evaluated qualitatively based on their properties, characteristics, and experimental findings documented in the literature.

2. Methodology

2.1. Methodological Approach to the State-of-the-Art Review

This state-of-the-art review employed a systematic approach to identify, evaluate, and synthesise relevant literature. The scope, including geographical, temporal, and thematic boundaries, was defined following an initial literature review, which excluded studies published before 1990.

A rigorous selection process was applied to include a diverse range of sources, such as academic databases, peer-reviewed journals, conference proceedings, books, book chapters, technical reports, government regulations, European Technical Approval documents, and manufacturers’ datasheets, all in English.

The search was conducted across platforms, including Scopus, ScienceDirect, ResearchGate, and Google Scholar, which provide extensive access to scholarly content across various disciplines. These platforms offer user-friendly interfaces, free access to relevant materials, and citation metrics, facilitating the identification of influential publications and authors. Specific search terms were used, including “timber”, “wooden”, “modular”, “buildings”, “construction”, “systems”, “material”, “insulation”, “thermal”, “coefficient”, “transmission”, “conductivity”, “acoustic”, “absorption”, “airborne”, “impact”, “sound”, “absorbent”, “fire”, “reaction”, “environmental”, “impact”, “carbon”, “neutrality”, “cost”, “market”, and “availability”. Boolean operators such as “and”, “or”, and “not” were utilised to refine search results and ensure the relevance of the retrieved documents.

The selected documents underwent a screening process based on relevance and quality, resulting in 80 full-text records. Figure 3 illustrates the temporal distribution of the selected documents.

This methodology facilitated the compilation of data for the comparative analysis outlined below, thereby contributing to identifying the most suitable materials for enhancing the efficiency and sustainability of timber buildings.

2.2. Evaluation Parameters

The performance of insulating materials is evaluated based on criteria that reflect their suitability for use in timber buildings. This section outlines the key parameters selected to address technical, economic, and sustainability considerations.

I.

Thermal Performance

Thermal performance plays a pivotal role in determining the energy efficiency, comfort, and durability of buildings. This aspect encompasses two primary parameters: thermal conductivity (λ) and water vapour diffusion resistance (μ). Thermal conductivity measures a material’s capacity to conduct heat, with lower λ-values indicating superior insulation. Conversely, water vapour diffusion resistance assesses a material’s effectiveness in mitigating moisture penetration, with higher μ-values signifying enhanced performance in humid conditions. These properties define the material’s overall ability to maintain insulation efficiency and structural integrity across varying environmental scenarios.

This study evaluated materials based on their thermal conductivity (λ) and water vapour diffusion resistance (μ) values, comprehensively analysing their thermal insulation capabilities. A dual-focus approach was employed, emphasising the balance between thermal resistance and moisture management. Materials with λ-values below 0.040 W/(mK) were categorised as excellent insulators, aligning with industry benchmarks, while those exceeding 0.050 W/(mK) were deemed less efficient. For water vapour resistance, materials with μ-values exceeding 5 MNs/g were identified as optimal for humid conditions, ensuring durability and consistent thermal performance.

II.

Acoustic Performance

Acoustic performance plays a crucial role in selecting materials for timber buildings, directly influencing occupant comfort and well-being. This parameter evaluates a material’s ability to absorb sound and improve sound insulation, encompassing both airborne and impact noise reduction, a critical consideration in multi-story timber buildings where dynamic forces affect floor systems. The sound absorption coefficient (α) is the primary metric for assessing this performance, with experimental data given significant weight to reflect real-world applications.

This study analysed materials based on their sound absorption coefficient (α) values across various frequencies. Those with α greater than 0.5 were classified as excellent sound absorbers, while materials with values between 0.4 and 0.5 were rated as good. Depending on their specific results, materials with lower α values were categorised as intermediate or poor performers.

III.

Fire Reaction

Fire reaction is an essential parameter for ensuring the safety of timber buildings, addressing concerns related to the combustibility of wood. This study evaluates various materials based on the European fire classification system, which categorises them from Class A1 (non-combustible) to Class F (worst fire reaction), providing a standardised measure of their combustibility. In addition, experimental data were considered to understand their behaviour under fire conditions, considering factors such as ignition resistance, contribution to fire spread, smoke production, and heat release.

Materials with classifications of A1 or A2 are considered highly effective in enhancing fire safety due to their minimal contribution to fire development, while those rated as Class E or F demonstrate limited resistance, posing greater risks. This dual approach—combining standardised classifications with experimental insights—provides a robust evaluation framework for selecting materials that enhance the fire safety of timber construction systems.

IV.

Environmental Impact

The environmental impact of construction materials is critical in selecting sustainable building components. This analysis focuses on embodied carbon, recyclability, and overall life cycle performance to assess the environmental performance of insulation materials.

Embodied carbon, quantified through environmental product declarations (EPDs), is the central metric, reflecting emissions across the material’s life cycle stages. Materials with embodied carbon values below 1 kgCO₂eq/m² are considered to have a low environmental impact, while those exceeding 6 kgCO₂eq/m² are regarded as having a significant carbon footprint. Recyclability and potential for reuse were also evaluated, with an emphasis on materials that are either recyclable or have high repurposing potential, aligning with sustainability and circular economy principles.

V.

Cost

The economic feasibility of insulation materials is a crucial consideration in their selection. This analysis assesses materials based on their market price (EUR/m²), production costs, and performance-to-cost ratio.

Materials that provided a favourable balance between cost and thermal/acoustic performance were assigned higher evaluations. Those with a market price below 10 EUR/m² and low production costs were classified as highly cost-effective, while those exceeding 10 EUR/m² with high production costs were considered less cost-effective. Additionally, long-term economic benefits, such as energy efficiency and durability, were incorporated into the evaluation, recognising their potential to reduce overall construction costs over the lifespan of the building. The cost data used in this analysis were sourced from the CYPE price generator [17].

VI.

Local Availability

The local availability and production of insulation materials are crucial factors that directly affect economic feasibility, environmental impact, and supply chain efficiency. Locally manufactured materials reduce logistic costs, support regional industries, and contribute to sustainability objectives.

This analysis prioritised materials produced within Portugal as their availability minimises transportation costs and aligns with environmental goals. Materials produced outside of Portugal with limited availability were considered as less favourable. As a secondary criterion, the production of materials within the Iberian Peninsula was also considered, recognising the advantages of lower transportation impacts and the positive contribution to local economies, fostering a more efficient and sustainable supply chain.

2.3. Qualitative Evaluation Criteria Overview

The rating system presented in

Table 2. Criteria for material classification by performance rating.

Criteria	Very Good	Good	Poor	Very Poor
Thermal Performance	λ ≤ 0.040 W/(mK) and μ ≥ 5, good experimental results	λ ≤ 0.045 W/(mK) and μ ≥ 3, satisfactory experimental results	λ ≤ 0.050 W/(mK) and μ ≥ 2, average experimental results	λ > 0.050 W/(mK) and μ ≥ 1, unsatisfactory experimental results
Acoustic Performance	α > 0.5, good experimental results	α > 0.4, satisfactory experimental results	α > 0.2, average experimental results	α > 0.1, unsatisfactory experimental results
Fire Reaction	Class A1/A2 and good experimental results	Class B and satisfactory experimental results	Class C/D and average experimental results	Class E/F and unsatisfactory experimental results
Environmental Impact	Embodied carbon < 1 kgCO2eq/m2, recyclable, and good lifecycle performance	Embodied carbon 1–3 kgCO2eq/m2, recyclable, and satisfactory lifecycle performance	Embodied carbon 3–6 kgCO2eq/m2, non-recyclable, and average lifecycle performance	Embodied carbon > 6 kgCO2eq/m2, non-recyclable, and poor lifecycle performance
Cost	Price < 10 EUR/m2, low production costs, and good cost-effectiveness in thermal/acoustic performance	Price < 10 EUR/m2, medium production costs, and satisfactory cost-effectiveness in thermal/acoustic performance	Price > 10 EUR/m2, medium production costs, and average cost-effectiveness in thermal/acoustic performance	Price > 10 EUR/m2, high production costs, and unsatisfactory cost-effectiveness in thermal/acoustic performance
Local Availability	Produced in Portugal, highly commercially available	Produced in Portugal, commercially available	Not produced in Portugal, limited commercially available	Not produced in Portugal, very limited/no commercially available

was established to facilitate a consistent comparison of materials based on six performance criteria. For each criterion, materials were classified according to a scale: “Very Good” (5), “Good” (4), “Intermediate” (3), “Poor” (2), and “Very Poor” (1). These classifications were derived from established thresholds in the literature, supplemented by qualitative assessments when applicable. Thermal performance was assessed through the thermal conductivity (λ) and water vapour resistance factor (μ), while acoustic performance was evaluated based on the average sound absorption coefficient (α). Fire reaction was determined in accordance with Euroclass standards, the environmental impact was measured through embodied carbon and recyclability, and cost and availability were appraised based on market data specific to Portugal. This methodology ensures a comprehensive and structured framework for comparing materials aligned with the objectives of the study.

2.4. Weighting of Parameters

The weighting of each parameter was defined according to the main objectives of this study. Although all evaluated aspects are relevant to the performance of timber building systems, greater emphasis was placed on thermal and acoustic parameters as these constitute the central focus of the review. This approach aims to support comparisons and conclusions related specifically to thermal and acoustic performance without undermining the significance of the other parameters. The remaining aspects were assigned equal weights, as shown in

Table 3. Distribution of weights for the different parameters considered.

Parameter	Assigned Weight (%)
Thermal Performance	30
Acoustic Performance	30
Fire Reaction	10
Environmental Impacts	10
Cost	10
Local Availability	10

.

3. Overview of Selected Insulation Materials

This section provides an overview of the primary materials evaluated for suitability as insulating timber construction elements. The selection includes expanded polystyrene (EPS), extruded polystyrene (XPS), rock wool, glass wool, cork, wood fibre, rubber, cement-bonded particleboard (CBPB), industrial hemp, cellulose, sheep’s wool, and acoustic mats.

The materials were selected based on their prevalence in the literature and current market availability, ensuring a representative overview of commonly adopted solutions. Additionally, the selection includes materials that, although less frequently used, have shown increasing relevance in sustainable and innovative construction practices. Their distinct physical and environmental characteristics suggest promising compatibility with prefabricated timber construction systems, particularly in applications that require alternative insulation solutions. This broader inclusion enhances the comprehensiveness of the analysis and supports the identification of emerging options within the field.

No specific form of application—such as boards, batts, loose-fill, blown-in, or spray-applied—was prioritised in this study. The focus remained on the intrinsic properties of each material to allow for a more general and comparative evaluation. However, depending on the application context, it is essential to ensure proper installation practices so that the insulating layers effectively reflect the material’s expected performance. This consideration is especially important for loose-fill and blown-in forms, where compaction, distribution, and containment can significantly influence thermal and acoustic effectiveness.

Each material is analysed regarding its physical properties, performance parameters, and potential applications.

3.1. Expanded Polystyrene (EPS)

EPS is a plastic-based material produced by evaporating pentane, a hydrocarbon, added to polystyrene grains. This process results in a rigid, closed-cell foam with white cells [18]. The term “expanded” comes from the expansion process that gives the material its unique properties [7]. EPS is primarily used in external walls, especially in external thermal insulation composite systems (ETICS), where it helps treat thermal bridges, reducing the risk of surface condensation and ensuring a uniform thermal transmittance coefficient across the facade [19]. It is also applied to floors and roofs. Its excellent climate resistance and ability to withstand temperature fluctuations make it the most used insulation material in all types of buildings [20].

I.

Thermal Performance

Its thermal conductivity ranges between 0.035 and 0.040 W/(mK), ensuring high thermal insulation performance [21].

II.

Acoustic Performance

EPS exhibits poor acoustic performance due to its low density (15–35 kg/m³) and closed-cell structure, which hinders sound absorption [18].

III.

Fire Reaction

EPS falls into Class E for fire performance, indicating high flammability. Ref. [22] found that variations in density or thickness do not significantly affect ignition probability or mass loss rate.

IV.

Environmental Impact

Gomes, Silvestre, and de Brito [23] identified significant environmental impacts during EPS production, particularly in renewable primary energy consumption (PE-Re), ozone layer degradation potential (ODP), and photochemical ozone creation potential (POCP), primarily due to pentane emissions and energy usage. However, EPS waste can be incinerated for energy recovery (25–35% of production energy) or recycled to produce new polystyrene products [24].

V.

Cost and Local Availability

EPS is a cost-effective thermal insulation solution widely available in commercial markets [25].

3.2. Extruded Polystyrene (XPS)

Extruded polystyrene, an organic and plastic material, is produced by melting polystyrene beads with the addition of an expanding agent [18,26]. The product is available on smooth or grooved plates and is commonly used in roofing, wall cavities, ETICSs, ventilated facades, and flooring, offering good thermal performance and versatility.

I.

Thermal Performance

The thermal conductivity of XPS ranges from 0.032 to 0.037 W/(mK) [27]. Compared to EPS, XPS offers better thermal insulation, representing a thickness reduction of approximately 30% [20].

II.

Acoustic Performance

The density of XPS ranges from 32 to 40 kg/m³, slightly higher than that of EPS [18].

III.

Fire Reaction

XPS, like EPS, is classified as Class E in fire reaction, indicating high flammability. Studies have shown that XPS softens, shrinks, and releases combustible gases when exposed to fire, which sustains combustion. Cai et al. [28] found that the flame thickness and height vary with the wall’s length-to-width ratio due to oxygen supply and chimney effects.

IV.

Environmental Impact

XPS, despite its thermal insulation efficiency and resistance to water, presents significant environmental drawbacks [29]. One of its main concerns is its carbon footprint. The material is made from hydrofluorocarbons (HFCs), which have a high global warming potential (GWP), and it releases harmful gases, including CO₂, throughout its entire lifecycle—from production and installation to use and disposal [26]. Additionally, the use of colourants in XPS to differentiate between products worsens its environmental impact, further contributing to its overall ecological footprint [30].

V.

Cost and Local Availability

Despite environmental concerns, XPS is widely available in construction material stores and is commonly used. Additionally, while it offers a better performance-to-thickness ratio than EPS, it comes at nearly double the cost [20].

3.3. Rock Wool

Rock wool is an inorganic material derived from volcanic minerals like basalt. Its production involves melting the rocks at temperatures around 1600 °C to form fibres which are bound together using cohesion agents like resins, starch, or oils [18]. This material is versatile as it can be widely applied in roofs, floors, walls, and ceilings [7] and is resistant to biological and chemical agents, and mechanical impacts, such as vibrations [31].

I.

Thermal Properties

The thermal conductivity of rock wool ranges from 0.033 to 0.040 W/(mK) [18]. Studies by Karamanos, Hadiarakou, and Papadopoulos [31] show that this material performs well across various temperatures; however, high temperatures and water absorption can increase its thermal conductivity.

II.

Acoustic Performance

The acoustic performance of rock wool depends on several factors, including density, thickness, and porosity. Its density ranges from 40 to 200 kg/m³, which is higher than that of XPS and EPS [27]. Hongisto et al. [32] investigated four variations of rock wool (densities ranging from 25 to 100 kg/m³) across different applications. They observed that higher density did not necessarily lead to better acoustic performance (absorption and insulation). Instead, increasing the thickness of the material (50 mm, 100 mm, and 200 mm) was found to improve sound insulation significantly. Additionally, materials with open pores performed better in sound absorption and insulation compared to those with closed pores. The study further concluded that materials with low thermal conductivity showed poorer acoustic performance, particularly regarding impact sound insulation (e.g., floating floors) and airborne sound reduction (e.g., double-wall systems). Pedroso, de Brito, and Silvestre [33] also observed that rock wool provided better acoustic performance in double-wall assemblies than XPS and EPS, although it underperformed relative to materials like hemp. Additionally, rock wool demonstrated good acoustic performance when installed in air cavities, as noted by Schiavoni et al. [18].

III.

Fire Reaction

One of the major advantages of rock wool is its fire reaction, classified as A1–A2, indicating that it is a non-combustible material [27]. Sjostrom and Jansson [34] analysed rock wool boards (170 kg/m³) in a furnace and identified the following two exothermic processes: the combustion of organic matter, which is related to the binders used in production, and the crystallisation of amorphous fibres, which is linked to the material’s thermal behaviour. Additionally, rock wool’s fire performance can be further adjusted using simple tools, making it a highly versatile option for fire-resistant insulation.

IV.

Environmental Impact

Rock wool balances positive and negative environmental impacts, with its performance ranking well among conventional materials. According to Füchsl, Rheude, and Röder [35], it trails natural materials like cellulose and hemp in environmental performance and is slightly less favourable than EPS. One of its environmental drawbacks is its higher acidification potential (AP). Similarly, Schmidt et al. [36] compared rock wool with natural materials like recycled paper wool and flax, noting that its global warming potential (GWP) is primarily associated with the production process, where fossil fuels are used for melting raw materials and providing energy. Despite these disadvantages, rock wool has one of the lowest energy consumption metrics among conventional materials throughout its life cycle, outperforming even some natural alternatives. The raw material production stage is also relatively energy efficient. Moreover, it can be recycled by manufacturers or disposed of in landfills, offering additional flexibility in its environmental management [18].

V.

Cost and Local Availability

Rock wool is widely available in blankets or roll formats in the Portuguese market and stands out as a cost-effective solution [7].

3.4. Glass Wool

Glass wool, an inorganic material of natural and mineral origin, is produced by heating sand and glass at 1300–1450 °C, lower than the temperatures required for rock wool [18]. Raw materials include quartz sand, dolomite, limestone, and often recycled glass. While resins are typically unnecessary, they may be added in mat production to bind fibres, enhancing mechanical strength while maintaining superior fire reaction [37,38]. Glass wool shares applications with rock wool, requiring skilled professionals for proper installation in construction elements [7].

I.

Thermal Performance

The thermal conductivity of glass wool ranges between 0.030 and 0.050 W/(mK), slightly lower than that of rock wool [27].

Wang et al. [39] found that thermal conductivity increased linearly with temperature but exhibited a significant spike at 100% relative humidity, unlike rock wool.

II.

Acoustic Performance

Its density typically varies from 13 to 100 kg/m³, comparable to rock wool [38]. The sound absorption coefficient of glass wool was analysed [40], comparing samples from a 1997 office building in South Korea before and after its rehabilitation in 2012. The results showed values of 0.52 for the old sample and 0.62 for the new one, particularly in the 500–2500 Hz frequency range. These differences were attributed to fibre ageing and changes in thickness over time, indicating that long-term exposure to environmental conditions can influence the acoustic performance of the material. Pedroso, de Brito, and Silvestre [33] evaluated the acoustic performance of glass wool in double-wall systems, finding a sound reduction index of 56 dB. This value surpasses many materials but is slightly inferior to hemp.

III.

Fire Reaction

Glass wool is classified as A1–A2 regarding fire reaction, making it a non-combustible material [18]. Amir, Roslan, and Ahmad [41] explored hybrid fibres combining glass wool and rock wool, assessing their fire performance. Glass wool, with its low density and short fibres, exhibited higher expansion during charring. In contrast, rock wool provided superior fire retardancy and enhanced steel protection when reinforced with long fibres.

IV.

Environmental Impact

Glass wool exhibits mixed environmental performance. Füchsl, Rheude, and Röder [35] found that it had the lowest acidification potential (AP) among conventional materials (e.g., rock wool, cork, EPS) but displayed average results in other environmental indicators. Life cycle assessments revealed lower impacts than wood fibres, cork, and sheep’s wool and slightly better results than XPS and EPS. Natural materials like cellulose, hemp, and rock wool demonstrated marginally lower impacts. Hill, Norton, and Dibdiakova [37] assessed the embodied energy of various materials, finding that glass wool consumes less energy than organic insulations like XPS and EPS, though slightly more than cellulose. Nonetheless, its embodied energy is close to cellulose, making it a competitive option.

V.

Cost and Local Availability

Glass wool is available in Portugal and is typically sold in rolls, blankets, or boards.

3.5. Cork

Cork, a natural product from the cork oak tree (Quercus suber), is abundant in the Mediterranean basin, particularly in central and southern Portugal [7]. Its composition includes suberin (45%, for compressibility and elasticity), lignin (27%, a cell-wall structural component), polysaccharides (12%, contributing to structure), ceroids (6%, enhancing impermeability), tannins (6%, for colour and protection), and ashes (4%) [42]. Cork is commonly used for thermal and acoustic insulation and as cladding for walls, ceilings, and floors [43].

I.

Thermal Properties

Cork has a thermal conductivity between 0.037 and 0.043 W/(mK), comparable to rock wool and glass wool [27]. Its cellular structure of small, closed cells reduces convection, while its cell walls minimise radiation through repeated absorption and reflection, enhancing insulation properties [44]. Barreca, F., and Fichera [45] studied cork agglomerate boards, noting excellent thermal performance influenced by granule size, density, and thickness, often outperforming natural cork. In Portugal, the use of expanded cork agglomerates in ETICSs is increasing, mirroring trends in other European countries [19].

II.

Acoustic Performance

Its density typically ranges from 110 to 170 kg/m³, making it denser than polystyrene and glass wool [18]. Iannace et al. [46] evaluated the acoustic properties of cork boards using an impedance tube. Good sound absorption coefficients were achieved at mid-frequencies. A reduced air gap between the wall and cork panels improved absorption at higher frequencies. Gil [43] also highlighted cork’s use in acoustic correction, reducing the reverberation time in specific environments. Cork is commonly employed as a discontinuity layer between rigid elements and as a material to reduce impact noise on floating floors.

III.

Fire Reaction

Cork is classified as fire reaction Class E, making it a combustible material [27]. However, its cell-wall composition offers greater thermal stability than synthetic polymers like polystyrene and polyurethane, which degrade at lower temperatures [44]. Cork can retain its cellular structure at temperatures around 350 °C despite cell expansion and wall thinning [44]. Its structural backbone remains intact even at temperatures exceeding 2000 °C [47]. This resilience makes cork viable for fire insulation layers [44].

IV.

Environmental Impact

Despite its renewable nature, cork has been found to have higher environmental impacts compared to many insulation materials, including non-renewables [35]. Sierra-Pérez et al. [48] identified key contributors to these impacts, including granule production, which involves crushing and accounts for over 60% of the total impacts in this phase. Low-quality granules exacerbate waste as they become unusable. Transportation and energy consumption (electricity and diesel) are major factors influencing cork’s environmental footprint. However, transportation impacts are negligible in Portugal, where cork production is localised. Improving production efficiency can enhance cork’s advantages, such as recyclability, non-emission of VOCs during combustion, and releasing only CO₂, similar to wood-based products [45,49].

V.

Cost and Local Availability

Cork is commercially available in Portugal, and cork agglomerate boards offer competitive pricing, particularly when synthetic binders are avoided [45]. However, this material is more expensive than conventional ones like XPS, EPS, and mineral wool [25].

3.6. Wood Fibre

Wood fibre is a natural insulation material derived from trees, sawmill residues, or forest maintenance operations. Its composition typically includes lignin and aluminium sulphate, which acts as a pesticide and protects against insect attacks [18]. According to Schulte et al. [50], wood fibre insulation panels are composed of 83% wood fibre, 6.3% recycled paper, 6% water, 2.4% aluminium sulphate (as a fire retardant), 1.3% polyethylene and polypropylene fibres, and 1.2% sodium silicate (as an adhesive). These panels are commonly used in roofs, facades, ceilings, and interior partition walls [7].

I.

Thermal Performance

The thermal conductivity of wood fibre depends on its density. At lower densities (30–60 kg/m³) it ranges between 0.037 and 0.038 W/(mK), while higher densities (110–250 kg/m³) show increased values of 0.047–0.080 W/(mK) [51]. Experiments by Troppová et al. [52] revealed that temperature and humidity changes significantly affect the performance of wood fibre panels in facade walls, leading to higher thermal conductivity.

II.

Acoustic Performance

Cherradi et al. [53] analysed the acoustic properties of composite materials made from alfa fibres and three types of wood fibres: beech, spruce, and oak. Using the sound absorption coefficient (α) as a metric, the study of Cherradi et al. [53] found that spruce fibres demonstrated the most stable performance within the 500–2000 Hz range. Tiuc et al. [54] highlighted that sound absorption is influenced by material thickness, fibre content, size, and the air gap between the material and a rigid wall. For low frequencies, increasing the air gap significantly improves absorption, especially in thinner samples.

III.

Fire Reaction

Wood fibre has limited fire reaction and is classified as fire reaction Class E [27]. To mitigate this, flame-retardant treatments can be used. Sun et al. [55] found that a double-layer coating of expandable graphite and ammonium phosphate (2:3 ratio) enhanced fire performance and mechanical properties by forming a protective carbonised layer.

IV.

Environmental Impact

As a natural material, wood fibres exhibit strong sustainability indicators [50]. Across 18 indicators, wood fibres performed satisfactorily in 16. Wood fibres, like hemp and cellulose, offer great potential for property enhancement through modifications in additives and binders, enabling more sustainable solutions [35].

V.

Cost and Local Availability

Wood fibre can be a relatively expensive material, with only cellulose surpassing it in initial investment costs [25].

3.7. Rubber

Rubber, a key polymer globally, is a raw material for over 40,000 products [56]. It can be produced naturally, extracted from South American-origin rubber trees, or synthetically, using petroleum byproducts commonly found in tyre manufacturing [57].

I.

Thermal Performance

Recycled rubber has a thermal conductivity ranging from 0.100 to 0.140 W/(mK) [18], which is higher than the other materials analysed so far. Its density falls between 500 and 930 kg/m³ [27].

II.

Acoustic Performance

The acoustic performance of rubber aggregates was analysed by Asdrubali, D’Alessandro, and Schiavoni [58], with key factors including aggregate size, coating, compaction degree, and thickness. Smaller aggregates demonstrated superior sound absorption. The study found that reduced compaction enhanced sound absorption without compromising mechanical properties. Thickness also influenced performance, with increased thickness improving absorption, particularly at low frequencies [59].

III.

Fire Reaction

Rubber has a fire reaction class of D–E, indicating it is flammable [27].

IV.

Environmental Impact

Rubber waste increases annually, primarily due to automotive use, contributing to environmental pollution. Rubber is not easily decomposed, and its degradation time is unknown [60]. However, it is recyclable. In life cycle assessments, recycled rubber shows disadvantages compared to natural materials like cellulose and cork [35].

V.

Cost and Local Availability

As previously mentioned, rubber waste increases annually, primarily due to its widespread use in the automotive industry, leading to greater market availability [61].

3.8. Cement-Bonded Particleboard (CBPB)

Cement-bonded particleboards are composite materials made from a mixture of cement and wood. Their composition by dry weight percentage can be as follows: Portland cement (61.8%), pine wood shavings (22.7%), water (10.7%), non-toxic additives (1.4%), and pigment (3.4%) [62]. These panels can be applied in walls, roof coverings, tiles, fences, and acoustic barriers [63].

I.

Thermal Performance

The thermal conductivity of CBPB is 0.22 W/(mK), which is higher than most previously mentioned materials [62]. Due to the properties of wood, CBPB showed better thermal performance than concrete panels containing sand aggregates in similar proportions to wood [64]. However, the thermal conductivity of CBPB was slightly higher than that of panels made from solid wood waste.

II.

Acoustic Performance

The average density of CBPB is 1350 kg/m³ [62]. Wang et al. [64] assessed noise reduction for low- and mid-range frequencies (32–3150 Hz). CBPB outperformed concrete panels across most of the frequency range, with wood waste showing higher efficiency at high frequencies.

III.

Fire Reaction

CBPB have a fire reaction class ranging from B to A2, making them low-flammability materials and, in some cases, non-combustible [62].

IV.

Environmental Impact

A life cycle assessment (LCA) of various cement–bonded particle boards was conducted by Akin, Irfan, and Hacioğlu [65] across three scenarios: using natural wood, using construction and demolition wood waste, and using construction and demolition wood waste treated with an alkaline solution to improve the physical and mechanical properties. The results revealed that the cement layer primarily influenced the panels’ global warming potential (GWP). Scenario 2, which used construction and demolition wood waste, showed the best LCA results, followed by Scenario 3, where the wood waste was treated with a 2% NaOH alkaline solution, and Scenario 1 with natural wood.

V.

Cost and Local Availability

In Portugal, CBPB are commercially available.

3.9. Industrial Hemp

Industrial hemp (Cannabis Sativa) is a plant with low levels of psychoactive substances, well-suited for cultivation in dry climates. Different parts of the plant, including the outer fibres, inner woody core, and seeds, are used for various purposes. The outer fibres are processed through aggregation, compression, and fire treatment [7]. In construction, hemp can be mixed with lime, clay, or starch to create high-quality insulation products [66]. It is commonly used in panel form or wool for cladding interior and exterior walls [67].

I.

Thermal Performance

The thermal conductivity of hemp fibres typically ranges from 0.038 to 0.060 W/(mK) [18], making them less efficient than other materials discussed earlier. Gaujena et al. [68] highlighted that water absorption significantly impacts hemp’s thermal performance, doubling its conductivity at 20% water content. The study found that hemp mass increased by 198% and its volume by 40% when wet, indicating that water absorption is a key drawback.

II.

Acoustic Performance

Hemp’s density ranges from 20 to 90 kg/m³ [18]. Gomes, Silvestre, and de Brito [23] examined the acoustic performance of hemp mats and compared them to XPS (5 cm thickness). In double walls, hemp outperformed XPS, achieving a sound reduction index of 58 dB with a smaller thickness of just 4 cm. Additionally, Mirski, Dziurka, and Trociński [69] studied panels made of birch veneers and hemp fibres (15 mm thick) with densities ranging from 300 to 1300 kg/m³. Panels with hemp fibres in the 500–900 kg/m³ range, combined with birch veneer on the outer layers, showed excellent acoustic performance, particularly at high frequencies, achieving sound reduction indices that were 30–45% higher than those of mineral wool or fibre panels [69].

III.

Fire Reaction

Fire reaction is a limitation of hemp, which is classified as E [27]. However, its fire behaviour can be improved with increased fibre thickness and volumetric fraction [70].

IV.

Environmental Impact

Hemp stands out for its strong environmental performance, ranking higher than rock and glass wool in sustainability. Schmidt et al. [35] found that hemp outperforms conventional materials like rock wool in environmental indicators such as global warming potential (GWP) and acidification potential (AP). Compared to other natural materials, such as cellulose and wood fibres, hemp shows low environmental impacts, aligning with natural materials, and is more sustainable than synthetic alternatives like EPS and XPS.

V.

Cost and Local Availability

In Portugal, the availability of hemp is still limited, but it can already be used in construction, particularly as a raw material for concrete. A study by Hult, and Karlsmo [71] on the cost of hemp insulation in a Swedish house found that it is more expensive than cellulose or fibreglass insulation.

3.10. Cellulose

Cellulose fibre is made from ground paper treated with inorganic compounds for fire reaction and fungal inhibition. It is typically sourced from recycled newspapers and consists of a mix of hemicelluloses and lignin. This material, often compared to cotton, is available in prefabricated panels or bulk form and is commonly used as insulation in roofing, walls, and hard-to-reach areas [7,72].

I.

Thermal Performance

The thermal conductivity of cellulose fibres varies between 0.037 and 0.042 W/(mK) [18]. Analysis by Pal, Goyal, and Sehgal [73] of various materials’ thermal conductivity showed that cellulose panels offer competitive values when compared to commonly used materials like cork and glass wool. The thermal diffusivity of cellulose panels is approximately 5 times lower than that of cork and 18 times lower than glass wool. It was concluded that cellulose fibres perform much better under dynamic heat transfer conditions, such as those found in buildings, compared to conventional materials used today.

II.

Acoustic Performance

The density of cellulose can range from 30 to 80 kg/m³ [18]. When used in panels, the elasticity of cellulose fibres allows them to be used as a resilient material on floating floors. Nechita and Năstac [74] compared the acoustic performance of cellulose foam composites with conventional materials (XPS/EPS). The properties studied included transmission loss and absorption coefficients, considering fibre composition and density. Laboratory results showed the good performance of cellulose fibres. The best performance was found in the composite material with a higher proportion of shorter and finer fibres, made from 100% recyclable cellulose fibres. Furthermore, when cellulose mats were incorporated into double walls, Pedroso, de Brito, and Silvestre [33] found that this material showed the highest sound reduction index (59 dB) among all materials analysed, only surpassed by polyurethane foam (60 dB).

III.

Fire Reaction

Cellulose fibres have fire reaction classes ranging from B to E, with B and C indicating combustible materials but with a limited contribution to fire [27]. Due to its high flammability, cellulose requires treatment for acceptable combustion resistance and latent fire protection [72]. Li et al. [75] incorporated magnesium oxide (MgO) microcapsules into cellulose fibres, enhancing their acid resistance and improving fire behaviour compared to unmodified cellulose.

IV.

Environmental Impact

The environmental aspect is also a significant advantage of cellulose. This insulation material performed best, especially compared to EPS, rock, and glass wool [35]. Only natural materials like wood fibres and hemp showed results comparable to cellulose. Lopez Hurtado et al. [72] also found cellulose to be environmentally viable when compared to conventional materials.

V.

Cost and Local Availability

In the north of Portugal, a company uses spray-applied cellulose made from recycled paper for thermal and acoustic insulation. It is applied in walls and floors, enhancing acoustic insulation through its tangled structure [76]. The cost of this material is higher compared with conventional insulation materials.

3.11. Sheep’s Wool

Sheep’s wool is a natural material generally consisting of 60% animal protein fibres, 15% moisture, 10% fat, 10% sheep sweat, and 5% impurities. It can be used as an insulation material in rolls, semi-rigid panels, or in loose form, and can be applied in roofing, ceilings, and walls [7].

I.

Thermal Performance

The thermal conductivity of sheep’s wool ranges from 0.038 to 0.054 W/(mK) [18]. Zach et al. [77] studied the thermal properties of sheep’s wool under various temperature conditions (10 °C, 20 °C, 30 °C, and 40 °C) and moisture levels (natural, dry, and wet). They found that increasing density by 50% led to a progressive rise in thermal conductivity, ranging from 15% at 10 °C to 21% at 40 °C. The smallest variation in thermal conductivity occurred at a 40 kg/m³ density.

II.

Acoustic Performance

Sheep’s wool is a light material with a density ranging from 10 to 20 kg/m³ [27]. Zach et al. [77] studied the acoustic performance of sheep’s wool by testing samples of different thicknesses (20 mm, 30 mm, 40 mm, and 60 mm). The sound absorption coefficient was measured across three-octave bands in the 100–3150 Hz frequency range. The results showed that as the insulation thickness increased, the frequency with the highest absorption decreased and the weighted sound absorption coefficient improved. The ideal thickness was calculated to be 170 mm.

III.

Fire Reaction

Sheep’s wool is classified as a Class E material [18]. Despite this classification, studies by Dénes, Florea, and Manea [66] show that sheep’s wool and EPS share the same fire resistance rating, but EPS burns when exposed to flames. In contrast, sheep’s wool self-carbonises, preventing flame spread, thanks to its high nitrogen content which inhibits combustion.

IV.

Environmental Impact

As a natural material, sheep’s wool offers several environmental advantages. However, when comparing various insulation materials, Füchsl, Rheude, and Röder [35] found that wool lags hemp, cellulose, and cork. One area where sheep’s wool underperforms is the ozone depletion potential (ODP), which is linked to the environmental impact of the animal throughout its life. Despite this, the broad applicability of wool in insulation may help offset its negative environmental effects [16].

V.

Cost and Local Availability

A disadvantage of wool-based insulation in Portugal is the lack of local production. This would necessitate imports from countries like the United Kingdom, resulting in higher costs and increased environmental impacts due to transportation.

3.12. Acoustic Mats

Acoustic membranes are used in construction to reduce impact and airborne noise, particularly in floors and walls. They are typically made from materials such as cross-linked polyethylene foam (PE foam) or bituminous compounds, which provide good mechanical properties and resistance to moisture and fatigue. These membranes are available in various thicknesses, generally ranging from 2 mm to 10 mm, and are often used in applications like floating floors and partition walls to help control sound transmission. The effectiveness of these membranes depends on factors like material composition, thickness, and density, with thicker versions typically offering better sound insulation. Some products may also include recycled materials.

I.

Thermal Performance

The declared thermal conductivity of the cross-linked PE foam membrane varies between 0.037 and 0.038 W/(mK) for the 5- and 10 mm thicknesses, respectively [78].

II.

Acoustic Performance

The material density ranges between 23 and 29 kg/m³. While primarily designed to reduce impact noise, the cross-linked PE foam membrane also contributes to airborne sound insulation in floating floors, with an L’_nT,w index of less than 58 dB for a 10 mm thickness and less than 60 dB for a 5 mm thickness. On the other hand, the bituminous membrane reinforced with mineral fillers enhances airborne sound insulation in gypsum board partitions, improving performance by 2 dB, 5 dB, and 6 dB for thicknesses of 2 mm, 4 mm, and 6 mm, respectively.

III.

Fire Reaction

The fire reaction class of the cross-linked PE foam membrane is F, while the fire reaction class of the bituminous membrane reinforced with mineral fillers is C [78].

IV.

Environmental Impact

Acoustic membranes have varying environmental impacts depending on their materials. Polymers like cross-linked polyethylene, especially when incorporating recycled content, tend to have a lower environmental footprint. However, their production still involves non-renewable resources and emissions. Bituminous membranes, made with mineral fillers and polyethylene coatings, can have a higher environmental impact due to material extraction and processing. Despite this, their durability and noise reduction benefits contribute to sustainable construction. Recycling options at the end of their lifecycle can further reduce their long-term environmental impact.

V.

Cost and Local Availability

Cross-linked PE foam membranes are typically cost-effective, while bituminous membranes may be slightly more expensive due to their material composition. These products are commercially available in Portugal.

3.13. Summary of the Characteristics of the Analysed Materials

Table 4. Parameters of the different materials studied. Adapted from [18,21,27,29,38,51,62,78,79].

Material	Parameters
	λ (W/mK)	μ (-)	ρ (kg/m3)	cp (kJ/kgK)	α (-)	FR (-)	EC (kgCO2eq/kg)
EPS	0.035–0.040	20–100	15–35	1.25	0.22–0.65	E	6.30–7.30
XPS	0.032–0.037	80–170	32–40	1.45–1.70	0.20–0.65	E	7.55
Rock Wool	0.033–0.040	1–1.3	40–200	0.80–1.00	0.29–0.90	A1-A2	1.05
Glass Wool	0.030–0.050	1–1.3	13–100	0.90–1.00	0.45–0.80	A1-A2	1.24
Cork	0.037–0.043	5–30	110–170	1.50–1.70	0.39–0.85	E	0.82
Wood Fibre	0.037–0.038 0.047–0.080	1–5	30–60 110–250	1.90–2.10	0.10–0.32	E	0.12
Rubber	0.100–0.140	14	500–930	a	0.20–0.80	D-E	3.76
CBPB	0.220	a	1350	0.80–1.20	a	B-A2	a
Hemp	0.038–0.060	1–10	20–90	1.60–1.70	0.52–0.60	E	0.14
Cellulose	0.037–0.042	1	30–80	1.30–1.60	0.53–0.90	B-C-E	0.31–1.83
Sheep’s Wool	0.038–0.054	4–5	10–20	1.30–1.70	0.056–1.12	E	0.12
Acoustic Mat	0.037–0.038	a	23–29	a	a	F b/C c	a

^a Data not found. ^b PE foam-based. ^c Bitumen-based.

presents a comprehensive summary of various material properties, including thermal conductivity (λ), water diffusion resistance (μ), density (ρ), specific heat capacity (c_p), sound absorption coefficient (α), fire reaction class (FR), and embodied carbon (EC). Although density is not a direct evaluation criterion, it is intrinsically linked to the material’s properties and performance characteristics and its inclusion provides additional context for understanding the overall behaviour of the materials. Similarly, specific heat capacity was included to offer a more complete picture of the materials’ thermal performance. While it was not used as a standalone evaluation criterion, this parameter plays a key role in the assessment of thermal inertia, influencing how materials respond to temperature fluctuations over time—a relevant consideration in building envelope design and thermal comfort. Properties more closely related to heat capacity, such as thermal diffusivity, were not prioritised in this context as the focus lies on the materials’ insulating capability rather than their role in thermal inertia.

The values are presented as ranges, representing the minimum and maximum values reported in the literature for each parameter. These variations reflect differences in material composition, manufacturing processes, and other factors that influence performance. Presenting these values as ranges rather than fixed values provides a more accurate representation of the variability observed across different sources and applications, offering a more comprehensive and realistic assessment of each material’s potential performance.

Additional information is provided to assess the insulation materials. Appendix A details the performance of various insulation materials when applied in timber buildings. Appendix B outlines the costs of these materials, based on the CYPE price generator [17], with distinctions based on their use in walls, slabs, and their function (thermal insulation, acoustic insulation, or both). The prices, in EUR/m², reflect the latest update from October 2023 and only materials listed in the price generator are included. Appendix C presents a table with the raw materials for each product and the locations of the manufacturing plants. Finally, Appendix D provides the results from the environmental impact declarations research, offering a comparison of the materials’ environmental impacts, particularly in the A1–A3 category (cradle-to-gate).

4. Results

This section presents the final evaluation of the studied insulation materials, considering their thermal and acoustic performance, fire reaction, environmental impacts, costs, and availability in Portugal. Using data from Section 2.4 and the Appendix A, Appendix B, Appendix C and Appendix D, a qualitative assessment was performed, assigning scores to each material based on the weighted importance of each parameter.

Final Scores and Classification

Table 5. Classification adjustments and contributing factors for insulation materials.

Material	Initial Classification	Final Classification	Factors for Change	References
EPS	Poor (Environmental Impact)	Intermediate	Recyclability	[24]
XPS	Very Poor (Environmental Impact)	Poor	Recyclability	[80]
Rock Wool	Poor (Thermal Performance)	Good	Experimental results justify “Good” for thermal and acoustic performance despite low water diffusion resistance	[31]
Glass Wool	Good (Thermal Performance)	Very Good	Excellent thermal performance outweighs low water diffusion resistance	[40]
Cork	Poor (Fire Reaction)	Very Good (Thermal Performance)	Good thermal performance and water diffusion resistance, though fire reaction remains “Poor”	[45]
Rubber	Very Poor (Thermal Performance)	Poor	High water diffusion resistance improved thermal rating; acoustic rating increased to “Good”	[60]
CBPB	Very Poor	Good	Good thermal and acoustic performance justified the improved rating	[64]
Hemp	Poor (Thermal Performance)	Very Poor	Low resistance to water contact	[68]
Cellulose	Intermediate (Thermal Performance)	Good	Strong thermal performance justifies “Good” rating despite low water diffusion resistance	[73]
Sheep’s Wool	Poor (Thermal Performance)	Good (Acoustic Performance)	Varying results, but overall good acoustic performance	[77]
Acoustic Mat	Good (Acoustic Performance)	Very Good	Excellent isolation from impact and airborne noise in floating slabs	[78]

delineates the refinement of initial material classifications following a systematic review of supplementary bibliographic and experimental evidence. Where new data revealed enhanced performance metrics or additional functional attributes—such as improved recyclability, increased moisture resistance, or superior thermal/acoustic test results—reclassification was implemented to more accurately represent each material’s behaviour. The table presents original and revised ratings across four performance domains (thermal, acoustic, environmental, and fire reaction) and cites the specific studies that justified each adjustment. Materials whose preliminary ratings remained valid—most notably wood fibre—are omitted, thereby focusing attention on the substantive reclassifications driven by emerging evidence.

Table 6. Qualitative evaluation of insulation materials based on defined parameters.

Material	Thermal Performance	Acoustic Performance	Fire Reaction	Environmental Impact	Cost	Local Availability
EPS	Very Good	Very Poor	Very Poor	Intermediate	Very Good	Very Good
XPS	Very Good	Very Poor	Very Poor	Poor	Very Good	Very Good
Rock Wool	Good	Very Good	Very Good	Good	Very Good	Very Good
Glass Wool	Very Good	Good	Very Good	Good	Very Good	Very Good
Cork	Very Good	Good	Poor	Very Good	Very Good	Very Good
Wood Fibre	Poor	Very Poor	Very Poor	Very Good	Very Good	Very Good
Rubber	Good	Very Good	Very Poor	Poor	Intermediate	Very Poor
CBPB	Poor	Good	Good	Very Poor	Very Poor	Good
Hemp	Very Poor	Very Good	Very Poor	Very Good	Poor	Very Poor
Cellulose	Good	Very Good	Very Poor	Good	Very Good	Very Poor
Sheep’s Wool	Poor	Good	Poor	Very Poor	Very Poor	Very Poor
Acoustic Mat— PE Foam-Based	Intermediate	Very Good	Very Poor	Intermediate	Very Good	Good
Acoustic Mat— Bitumen-Based	Intermediate	Very Good	Poor	Intermediate	Intermediate	Good

presents the qualitative evaluation for the defined parameters. The final score for each material is calculated by summing the product of each parameter’s weight and the material’s score for that parameter. Materials with equal scores were classified in the same position. Materials with identical totals share the same rank. Figure 4 presents the final scores and respective classifications of the insulation materials based on the final evaluation.

The radar chart in Figure 5 visually represents the performance of the top three insulation materials—glass wool, rock wool, and cork—across the following key evaluation parameters: thermal performance, acoustic performance, fire reaction, environmental impact, cost, and availability.

To guide the selection of the most suitable insulation material for specific applications, the materials were reclassified based on six performance domains that are directly influenced by the intended use of the materials. Figure 6 presents these domain-specific rankings, providing a valuable tool for practitioners aiming to select the optimal material for each unique application scenario.

5. Discussion

5.1. Comparative Evaluation

The comparative evaluation of the materials studied integrates the findings of the previous sections, considering thermal and acoustic performance, fire reaction, availability, cost, and environmental impact. The final analysis of the insulation materials reveals several important conclusions for selecting efficient and sustainable solutions in timber buildings. Based on the radar chart presented for the top three materials, the following highlights are observed:

A.

Glass Wool

• Strengths: Glass wool stands out for its excellent thermal performance, characterised by low thermal conductivity and consistent long-term effectiveness. It also offers good fire reaction, a crucial safety feature, especially in timber construction. Its acoustical performance is satisfactory.
• Drawbacks: Energy-intensive production increases embodied carbon, though this is offset by its long service life and performance stability.

B.

Rock Wool

• Strengths: Rock wool’s fire reaction, due to its incombustible nature, significantly enhances safety, especially in timber structures where fire protection is vital. Its availability in Portugal is another advantage as it is widely sold and locally produced. Its strong thermal and acoustic performance also solidifies its top ranking among the materials analysed.
• Drawbacks: Similarly to glass wool, its production requires significant energy, contributing to higher embodied carbon.

C.

Cork

• Strengths: Cork offers exceptional thermal performance, mainly due to its moisture resistance, making it an efficient insulation material. It is also locally produced in Portugal, increasing its availability. Moreover, its environmental impact is minimal, being a natural and recyclable material.
• Drawbacks: Cork’s fire reaction requires treatment to improve performance and it is more expensive than conventional materials like XPS, EPS, and mineral wool.

D.

Other Materials

• Polystyrenes (EPS and XPS): Some significant drawbacks are their fire reaction, acoustic performance, and high embodied carbon with limited recyclability. On the positive side, EPS stands out for its local availability as it is widely used for thermal insulation. On the other hand, XPS is notable for its thermal performance, especially in humid conditions where it performs quite satisfactorily.
• Wood Fibre Panels: Wood fibre panels stood out primarily for their environmental benefits, alongside sheep’s wool and hemp. While performing well in some categories, the CBPB was hindered by its high cost and did not excel in any particular area.
• Rubber: While rubber excels in water resistance and acoustical performance, its poor fire reaction, high embodied carbon, and limited availability in Portugal are important drawbacks.
• Sheep’s Wool and Hemp: Both materials have environmental advantages but face challenges in terms of production in Portugal, limiting their widespread adoption.
• Cellulose: While cellulose performs reasonably well in several parameters, it did not stand out in any specific area and more research is required.
• Acoustic Mats: These mats have excellent acoustic performance but are limited in other areas, such as cost and fire reaction.

To further guide the selection of the most suitable insulation materials for specific applications, the materials were additionally grouped based on their dominant performance attributes. This complementary classification enhances the applicability of the results by linking each material to contexts where their strengths are most relevant.

• Thermal Performance: For applications where thermal performance is the primary concern, such as envelope insulation to minimise energy loss, EPS, XPS, glass wool, and cork proved to be the most efficient options due to their low thermal conductivity and durability.
• Acoustic Performance: For buildings where acoustic performance plays a more significant role—such as schools, offices, or residential buildings in noisy environments—materials like rock wool, rubber, hemp, cellulose, and acoustic mats were more suitable, offering high sound absorption properties.
• Fire Reaction: Regarding fire safety, which is a critical requirement, particularly in timber construction, rock wool and glass wool were the top performers due to their incombustibility and favourable classification under European fire reaction standards.
• Environmental Impact: For environmentally conscious projects, especially those prioritising low embodied carbon and renewable materials, cork, wood fibre, and hemp emerged as the most sustainable choices.
• Cost: When cost is a major constraint, such as in large-scale developments or affordable housing, conventional materials like EPS, XPS, rock wool, glass wool, cork, wood fibre, cellulose, and PE foam-based acoustic mats offer solid performance at relatively accessible prices within the analysed market.
• Local Availability: In terms of local availability—an increasingly important criterion for reducing transport-related emissions and ensuring supply chain resilience—EPS, XPS, rock wool, glass wool, cork, and wood fibre are among the most widely accessible materials in the Portuguese market.

5.2. Challenges Encountered

The development of this research faced several limitations, which are summarised as follows:

• The scarcity of comprehensive information on construction solutions that integrate both thermal and acoustic performance, specifically tailored for timber buildings and prefabricated timber systems.
• There is a notable lack of detailed acoustic performance data in manufacturer catalogues. Materials are frequently described with vague terms, such as “good acoustic performance”, and without quantitative metrics, such as sound reduction indices or improvements in airborne and impact sound insulation.
• The limited number of in situ studies on timber buildings restricts the understanding of its real-world performance, particularly regarding acoustic behaviour.

6. Conclusions

Key Findings

This research focused on evaluating the performance of various absorbent and resilient materials for their potential application in prefabricated timber construction systems. By analysing key parameters such as thermal conductivity, acoustic absorption, fire reaction, environmental impact, and availability, the study aimed to identify the most suitable materials for enhancing the efficiency and sustainability of timber buildings. Mineral wool (glass and rock wool) and cork emerged as the most advantageous materials. Key findings of each type of analysed material can be found below:

• Glass Wool: Superior in thermal and fire reaction, widely available, and economically viable.
• Rock Wool: Excellent fire reaction, noise insulation, and local production support its high ranking.
• Cork: Environmental benefits and local production make it an ideal sustainable option, though fire reaction is a limitation.
• Cement-Bonded Particleboard: Reliable in structural applications but limited by higher costs and moderate availability.
• Polystyrene (XPS and EPS): Strong thermal performance but limited by poor acoustic performance and fire reaction.
• Natural Fibres (Hemp, Sheep’s Wool, and Cellulose): Environmentally friendly but constrained by low availability and production in Portugal.