A Rapid Review of Hygrothermal Performance Metrics for Innovative Materials in Building Envelope Retrofits

Abstract

With government, industry, and public pressure to decarbonize the building sector through reducing embodied and operational emissions, there have been a wide range of innovative materials used in building envelope retrofits. Although these innovative materials, such as super insulating materials, bio-based insulation, and many others, are assessed on thermal performance and code requirements before use in retrofits, there is no unified standard assessment metric for hygrothermal performance of innovative materials in building envelope retrofits. This paper performs a rapid review of the available literature from January 2013 to March 2025 on hygrothermal performance assessment metrics used in retrofits. Using rapid review methods to search for records in Scopus, Web of Science, and Google Scholar, fifty-nine publications were selected for bibliometric and qualitative analysis. Most selected publications include discussions and analysis of relative humidity in the wall assembly post retrofit, moisture content, and mould index within the envelope. There is a research gap in publications considering hygrothermal damage functions such as freeze–thaw index, relative humidity and temperature (RHT) index, or condensation prediction. There is also a research gap in country and climate studies and analyses of in situ retrofits with innovative materials, and occupant comfort post retrofit.

1. Introduction

The motivation for this review is based on the need for greater acceptance of innovative materials in deep energy retrofits [1] and using reviews to improve access to knowledge [2]. This rapid review is not comprehensive for all envelope components, focusing rather on an exploration of the hygrothermal performance metrics used for vertical wall retrofits using innovative materials.

1.1. Innovative Materials

In this review, the term innovative material is used to classify the retrofit interventions discussed. A material is considered innovative if it is outside of commonly accepted building practice, such as novel materials using super insulating properties, and traditional bio-based materials that are not used in current constructions such as sheep’s wool and straw insulation materials. In this review, exterior panelized retrofits that overclad existing envelopes are also considered. Due to the span of reviewed articles, some earlier publications include materials that would have been considered innovative at the time of publication but may be common and accepted practice in 2025. It should also be noted that some materials considered typical in one region may be innovative for others.

1.2. Building Envelope Retrofits

This review is focused on building envelope retrofits, as they are part of deep energy retrofits implemented to reduce building energy use under climate change agreements. Many government initiatives addressing building renovations include building envelope retrofits, highlighted by European Union (EU) directives to address inefficiencies in 75% of buildings [3]. Considering 85–95% of current buildings in the EU will still exist in 2050 [3], and similar statistics in Canada [4], retrofits must play a role in developing an efficient building stock. This increasing focus on reducing building energy consumption has led to increasing research on deep energy retrofits that include retrofitting the building envelope [5]. Continued research for retrofits should acknowledge the risks to building performance in the form of trapped moisture and changes to heat, air, and moisture transport through building envelopes [6].

1.3. Hygrothermal Assessment

Moisture in buildings plays a role not only in occupant health [7] but also in the long-term performance of the building envelope and structure [2]. Understanding how novel materials respond to heat, air, and moisture transport and storage is an essential part of risk assessment for building designers, engineers, architects, and contractors [8]. Given the importance of understanding hygrothermal performance in building envelopes, there is a lack of cohesive standards for assessing innovative materials in retrofits.

As climate change is increasing the need for deep energy retrofits [4,9], a focus on how hygrothermal performance is altered by retrofits with innovative materials and constructions is necessary. Changing climates can also impact the hygrothermal performance of the existing envelope, with shifting temperature, humidity, and precipitation, along with the changing frequency of extreme weather events [9]. At a material level, there has been significant research on the hygrothermal performance of novel materials in retrofits [10], but there has been less focus on the complete envelope performance [10].

In the context of heritage conservation charters and principles [11], which can restrict the extent of exterior retrofits, innovative retrofits solutions are needed, including interior insulation retrofits with more potential for hygrothermal damage [6]. Considering damage functions in the analysis of potential retrofits is important in understanding the risk of these interventions [12].

While there are reviews of hygrothermal assessments metrics for retrofits, such as the discussion of hygrothermal risk assessment on masonry walls by Havinga and Schellen [13], hygrothermal impacts seen in occupied retrofits of housing by Recart and Sturts Dossick [14], or the discussion of hygric criteria for retrofit materials for historic buildings by Posani et al. [10], there is a gap in reviews providing a clear summary of hygrothermal performance metrics of building retrofits when using innovative materials.

1.4. Aim

The aim of this review is to gain an understanding of the current practices in assessing the hygrothermal performance of innovative materials used in building envelope retrofits, which include building walls. This is accomplished through a review of published English language academic studies with a focus on materials from January 2013 to March 2025. This review considers the following research questions with respect to the hygrothermal assessment of building wall retrofits with innovative materials:

• What hygrothermal performance metrics were used most frequently to assess the innovative materials used in wall retrofits?
• What is the scale of the retrofit, scope of hygrothermal monitoring, and application of retrofit?
• What innovative materials were used most frequently when discussing hygrothermal performance in retrofits?

2. Review Method

This review used database and reference searching to generate the literature for screening. Systematic screening was used to narrow the selection of the literature used for bibliometric and qualitative analysis. This mixed method review process was adapted from the work by Weerasinghe et al. [5], with rapid review methods detailed by Garrity et al. [15]. This section discusses the process of the literature search and selection, the literature included in the study, and the temporal and geographic spread of studies selected.

2.1. Literature Search

The literature search for this topic was restricted to publications available from January 2013 to March 2025, with a focus on conference and journal articles. Per rapid review methods [15], record searching was limited to a select number of databases with publications in English and with limited searching in gray literature. Studies were found through searching on Scopus, Web of Science, and Google Scholar with terms such as “(novel OR innovative) AND (hygrothermal OR moisture) AND (retrofit OR renovation) AND (model OR simulation OR experiment* OR in*situ) AND (build* OR envelope)”. These searches and further citation searches based on selected papers resulted in 488 articles to screen after duplicates were removed. Database searching began on 18 December 2024, using Scopus and Google Scholar, and Web of Science was used from 6 January 2025. Citation searching was ongoing between the start of the review (December 2024) and the end of the review, with the last database search conducted on Google Scholar on 24 February 2025, and Web of Science and Scopus on 2 April 2025.

A systematic web-based review platform, Covidence [16], was used to organize and screen papers with two of the authors conducting abstract screening and full text review on all screened records. The reporting for this rapid review was guided by the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) statement. The results of the screening process are summarized in the PRISMA flow diagram in Figure 1 below. Conflicts between reviewers during screening were resolved through analysis of the publication in joint meetings.

Abstract and title screening took place between two authors who screened out irrelevant articles, resulting in 176 articles being eligible for full review. Abstract screening considered the relevancy of articles but also removed publication types such as patents, grant applications, and most thesis publications that had been included from the general search terms.

To focus this review, building wall retrofits were emphasized, with retrofits exclusively studying other elements of the building envelope (roofs, fenestration, foundations) removed during screening. In abstract screening, exclusively innovative roof retrofits (such as green roofs or novel membranes) were removed. There were many reviews of green roof performance available, with installation- and operation-specific performance metrics such as in scoping and systematic reviews by Andenæs et al. [17] or Shafique et al. [18], for example. Limiting the search to building wall components was intended to remove some of the variability and complexity in projects that would have been seen if the entire building envelope was included. Future reviews could include innovative foundation, roofing, and fenestration retrofits as well.

In terms of relevancy of abstracts, the following questions were asked about each publication:

• Is this publication focused on a building retrofit?
• Is there a hygrothermal performance assessment or discussion of moisture or condensation?
• Is there a novel material, innovative construction method, or innovative assembly for the retrofit?

2.2. Exclusion Criteria

From the 176 eligible publications, full paper reviews were conducted to select the publications for inclusion in this review. Exclusion criteria were varied, but the most common reason for removal was due to no novel material or constructions in the retrofit (36% of excluded papers). The second most common exclusion criterion was no specific retrofit (18% of excluded papers), as many papers mentioned retrofits and renovations as reasons for considering hygrothermal performance and innovative materials but did no analysis on a specific retrofit scenario. In these publications, usually the concept of using the novel material in a retrofit was mentioned but not linked to an existing building or wall assembly type.

Material property testing of innovative materials for hygric properties only was also a frequently excluded category of research. These publications often focused on novel bio-based materials that were at an early stage of technological readiness [10], which could theoretically be used in retrofits at a later time, but did not include assessment as part of a wall assembly. From a practical perspective, thesis publications were excluded from this review, as well as publications with English abstracts but full texts in another language. Finally, although the best attempts were made to access all articles using interlibrary loan services and requests to authors, three articles were excluded for reasons of inaccessibility.

2.3. Selected Publications

There were 59 publications selected for further analysis including journal and conference papers. These were published between 2013 and 2025, with publication year frequency given in Figure 2 below. Note that the number of papers selected from 2025 reflect only those selected in the first quarter of 2025 (*Q1): from January to March 2025.

Each publication was reviewed using an extraction template, with fields for authorship, geographic location studied, aim of study, materials used, scale of retrofit, type of simulation tools, time duration of retrofit study, and finally types of hygrothermal assessment metrics used in the research. This review template was filled out by at least one author, with a second author checking, to form for each publication. The data from these forms were used for bibliometric analysis, with further qualitative analysis of each publication when relevant.

Given the potential for international collaboration in experimental research and the potential for different climate file locations in hygrothermal simulations, a review of research location for each paper was given. This worldwide distribution is shown in Figure 3a. The research location and authorship affiliation skewed heavily toward European, with research locations in Europe given in Figure 3b. European authorship is heavily reflected in the selected studies, with the top six most researched countries (Italy, the United Kingdom, Germany, Norway, Portugal, and Switzerland) representing 68% (40 of 59) of all countries by research location. Only 9 of 59 (15%) papers selected were published by authors focusing on research outside of Europe, with 5 of those publications based in Canada.

This search was limited to English publications, which may have reduced the diversity of publications available on this topic, so it would be valuable in future research to use multilingual searches, if possible. As this review focused on retrofit projects, governmental policies and incentives towards retrofits may help to explain the geographic distribution of research, as there has been significant investment in building retrofit strategies in Europe [3], with multiple research programs focused on retrofits of buildings and energy savings in buildings [10]. The lack of publications from some regions may also be impacted by government policies and financial incentives. For example, in China, there has been less emphasis on the retrofitting of existing buildings to reduce energy use, with more emphasis rather on building new buildings with improved energy efficiency [19].

Of the 59 studies selected, there were many interconnections between groups and themes. This is seen using the literature review visualization tool, Litmaps [20], in Figure 4 below. This tool gives the citations between studies as linkages, with increasing citations of the publication as larger circles, for a given publication where first author and year published are noted.

As expected, earlier works have more citations, but there are some interesting trends with the publications that have more frequent citations than expected for the date of publication. The most highly cited work in this review was the paper by Vereecken et al. [25], which focused on hygrothermal performance of capillary open insulation materials containing Calcium Silicate. There is a cluster of articles around capillary open interior insulations such as Calcium Silicate boards in 2015–2017, which reflects the interest in this innovative material at that time [21,22,23,24,25,26].

The work by Zhou et al. [27] was also highly cited; it investigated aerogels and other novel renders for hygrothermal performance using models developed in COMSOL Multiphysics. This work used the relative humidity and temperature (RHT) index as an assessment tool, which considers the risk of moisture problems at different locations in a building envelope [27]. This study considered a number of parameters in modelling, including varied climates in Switzerland, multiple brick wythes, high and low capillary behaviors in brick, vapour control strategies, and water leakage [27].

Another highly cited publication was the research conducted by Bottino-Leone et al. [28], which looked at a wide range of innovative materials, including Calcium Silicate, perlite bricks, cellulose, wood fibre, and cork insulations in addition to base case materials. This project assessed hygrothermal performance such as energy use and the lifecycle of materials for these novel material retrofits [28]. This project considered hygrothermal risk factors such as mould and critical moisture content per WTA standards when analyzing the relative humidity and moisture content results.

In terms of innovative hygrothermal assessments, two publications by Coelho et al. [29,30] were included in this review and discussed the long-term performance of materials inside a building with changing hygrothermal performance under climate change. This research was also tied to recent work by the same group, which was not included in the scope of this review [76] that discusses modelling methods to reduce simulation time and cost when conducting large time- and geographic-scale hygrothermal simulations. This group of research not only considered the hygrothermal performance of the building envelope, but also the resulting change in interior conditions and, therefore, resulting to risks to historic artifacts in the project space.

Based on the highly cited papers, it is evident that there are some common themes in the reviewed publications, which were organized into a keyword map as shown in Figure 5 below. The keywords from each publication were linked using VOSViewer 1.6.20 [80] after using a thesaurus file to harmonize similar keywords and multiple spelling variations (for example terms like wood fibre, wood fibre, and wood wool were combined into a wood-based material term). Of the keywords displayed, each term required a minimum occurrence of five publications, with 22 terms meeting this threshold.

As expected from the focus of this review (hygrothermal performance of innovative materials in building retrofits), common keywords include building retrofit, insulation, hygrothermal performance, and modelling. Also, the frequency of heritage buildings and heritage retrofits can be expected for the research locations representing western Europe and the funding available for heritage retrofits [9]. The hygrothermal metrics that were most frequently mentioned beyond hygrothermal monitoring, performance, and analysis were moisture performance, which included moisture content and related terms, and mould risk. Both these categories were frequently used metrics in the reviewed publications.

3. Results and Analysis

Of the 59 publications included, key themes and commonalities were analyzed to determine retrofit materials implemented, scale and application of retrofit, modelling and simulation platforms, and finally hygrothermal assessment metrics used.

3.1. Retrofit Materials

A wide range of materials were used in the retrofit projects reviewed, which were classified through different insulation types, cladding systems, coatings, and intervention used. This follows the approach used in other review studies [10], separating materials into thermal renders/plasters, organic natural fibres, inorganic materials, and super insulator materials. As the review exclusion criteria focused on researcher-described innovative materials, some studies that included innovative materials described below were excluded, such as the study by Pagoni et al. [81]. Although glass foam, autoclaved aerated concrete, aerogel, and hemp lime insulation were used [81], this publication did not highlight the novelty of the materials in their hygrothermal risk assessment.

Innovative materials were considered for retrofits over conventional materials for a variety of reasons in the publications selected. Innovative materials were selected based on design restrictions due to historic or heritage considerations, such as restrictions on façade changes, colour, and preservation requirements discussed by Pedersen et al. [31], or the inability to change the exterior of a building such as the interior insulation retrofit seen in Biseniece et al. [32]. Bio-based and other natural fibre materials were considered to improve the sustainability of building materials, such as using Typha insulation modelled by Cascione et al. [33], or to reduce embodied carbon and global warming potential calculated in a lifecycle assessment such as in the work by Bottino-Leone et al. [28]. For occupied buildings, reducing the impact to interior space was a concern, so considerations for thin super insulated materials were given [34] or the use of exterior aerogels was discussed [38]. Considering the specific goal of the retrofit also influenced the decision to use innovative materials, such as the projects specifically intending to improve moisture transport through the wall with capillary open insulation materials, such as the work in [21,22,23,24,25,26].

Generally, novel materials were considered where conventional systems could not fit a given project’s design constraints, or there were environmental or efficiency policies in place to consider alternate options.

3.1.1. Bio-Based/Organic Natural Fibers

Innovative bio-based materials are classified as materials that have partial or full biological origins [82]. While there are many innovative bio-based materials available, especially in early material characterization testing and development [82,83], this review focused on bio-based materials that were used in building retrofits, with wood-based materials being the most common. Many of the novel bio-based materials currently tested at scale are seen more in new builds, such as hempcrete building structures and new straw house buildings [83].

The largest represented category of bio-based insulation was in wood-based materials, such as wood fibre insulation or wood wool insulation, seen in 24% of publications (14 of 59). This includes works in [28,36,37,38,39,40,41,42,43,44,45,46,47,48]. Other wood material retrofits included cross laminated timber (CLT) panels [41,47]. Cork as an insulating material was included in works by 12% of publications (7 of 59) including the works in [28,42,49,50,51,52,84].

Some of the bio-based materials, even though they were seen in new construction projects [83], were not as often noted in retrofit applications. For example, straw insulation was only noted in a single publication, with included laboratory testing for straw insulation in a retrofit application [53]. Hemp-based materials such as hemp lime plasters [54,55] and hemp fibre insulation [55,56] were only noted in 5% (3 of 59) of publications. Other novel fibres, such as Typha, were seen in retrofits by Cascione et al. [33].

Finally, in terms of bio-based materials, there were 14% of publications (8 of 59) that considered cellulose materials in retrofits [24,28,36,39,45,56,57,58].

Bio-based materials at an early level of technological readiness, such as corn pith insulation or seed pod insulation [82], were not seen in any of the retrofit studies selected in this review.

3.1.2. Super Insulators

Super insulating materials such as aerogel coatings and blankets or vacuum-insulated panels are innovative materials used in many of the projects described in this review.

Aerogel insulating materials were represented in 22% (13 of 59) of publications [11,23,24,27,31,32,34,35,40,59,60,61,62]. Vacuum-insulated panels (VIPs) were used as a retrofit option in seven publications. Of these 12% of publications (7 of 59), three works also considered using aerogel systems [23,31,32], and four publications considered VIP systems as the main retrofit [63,64,65,66].

3.1.3. Inorganic Materials

Calcium Silicate (CaSi) as a capillary active interior insulating material was represented in 24% (14 of 59) of publications, often as a comparison case to standard practice or other innovative materials. There was a cluster of CaSi research from 2015 to 2017, and again from 2019 to 2022. Interestingly, after 2022 there were no articles included in this review that used CaSi systems as an innovative retrofit. The novelty of CaSi insulation was not discussed or mentioned in recent retrofit studies [81], with these systems commercially available in select European markets.

In terms of other inorganic materials, perlite as an insulation material was also considered in multiple publications, including the works in [26,28,29,44,45,67,68,76]. Also included in the inorganic materials are phenolic foam insulation panels, which have innovations in formulations to reduce failure [48,51,52,69]. While there are many retrofit projects using phase change materials (PCMs) to improve energy efficiency in buildings [85], there was only one publication included in this review that discussed hygrothermal performance [70].

For deep retrofits, multiple publications used smart vapour barriers, vapour control layers, and membranes [24,25,58,71]. These innovative membranes and layers were often in combination with other innovative insulation materials, forming a new assembly in the retrofit scenario.

Similar to the bio-based material section of this review, there are innovative inorganic materials with low technological readiness, such as smart materials, including 4D/3D-printed systems [86], that have seen material testing but no specific retrofit application and analysis.

3.1.4. Historic Materials

Historic and heritage materials are classified as a separate category in this review, as technically these materials were in use before modern building codes, so they are not new materials [24]. However, the use of historic materials such as sheep’s wool (also a bio-based material [82]), could be considered innovative when using historic materials with new constructions. Also, we look at how to retrofit historic or traditional buildings with modern materials can be considered innovative when the historic structure is unusual.

For this category, a retrofit of old timber constructions with heritage materials by Whitman et al. [42], a sheep’s wool insulation study by Campbell et al. [69], and the renovation of a traditional Kyo-machiya house with soil walls by Liu and Iba [72] were included.

3.1.5. Façades and Modular Cladding Systems

Finally, the last category of innovative materials considered were exterior retrofit solutions, which include façade systems and modular overcladding. While exterior retrofits can be a challenge in historic or heritage-designated buildings, they are possible in some applications. While there are a variety of façade systems, including smart glazing, photovoltaic facades, or dynamic/adaptable façades [87], the innovative façades in this review are only building-integrated photovoltaic panels and exterior overclad panels. For historic buildings, Pedersen et al. [31] modelled many stages of building retrofits, including building integrated photovoltaics. In terms of overcladding, a retrofit initiative based on the Energiesprong initiative [88], called Panelized Exterior Energy Retrofit (PEER) by Natural Resources Canada [4], included multiple case studies and test projects [57,73,77]. Other novel cladding or panelized retrofits with exterior applications were seen in the works in [37,41,43,46,47,58,74,75].

Although there were retrofits in this category, given the innovation in façade systems [87], there is a gap of conducting hygrothermal performance research on novel façade systems in retrofits.

3.2. Scale and Application of Retrofit

Given the variety of publications and research aims, the scale of the projects varied, and they were classified into different types, as seen in

Table 1. Type of study by publication.

Study Type	Quantity	Publications
Simulation	50	[11,21,22,23,24,25,26,27,28,29,30,31,32,33,34,36,37,38,39,41,42,43,44,45,47,48,49,50,53,54,55,57,58,59,60,61,62,63,64,65,67,68,70,71,72,74,75,76,77,79]
In situ partial retrofit	9	[24,32,37,50,58,65,67,68,78]
In situ whole building retrofit	8	[11,35,40,57,63,69,73,79]
Laboratory Testing	9	[36,37,42,49,53,64,66,74,79]
Material Testing	7	[21,42,43,53,59,60,67]
Test Hut	7	[34,43,46,51,52,56]

. The number of studies was greater than 59, as multiple publications included multiple methods, such as simulations, laboratory testing, and in situ retrofits. Simulations are discussed in a later section.

In terms of experimental work, material testing publications were included in this review if they had additional testing or modelling, as hygric material testing of innovative materials alone was a prior exclusion criterion. There were some excluded studies that implemented hygric testing of novel materials such as the work by Peterková et al. [89], which implemented hygrothermal models but focused on building components, such as window casings. Of the many excluded material testing publications, the materials were often earlier levels of technological readiness, produced only in quantities suitable for laboratory testing and not available for any in situ installations.

Laboratory testing differs from material testing, as this includes research where mock-up wall samples were tested in climate chambers, hot box/cold box systems, or other similar equipment at a research facility. These materials were produced in-house or externally, and some materials were available from select suppliers.

Simulations were grouped together even if researchers looked at a wall section or the complete building. Depending on the modelling tool used, such as a hygrothermal or thermal-specific platform or building energy modelling, the scale of simulated wall intervention changed.

Test hut projects generate experimental data with exposure to local climate conditions, with a potential to test many samples, but do not include occupants who can give feedback on comfort or generate moisture loads. This testing setup allows for longer-term monitoring of wall panels, including multi-year test hut projects in Denmark [51,52,56].

In situ retrofits of existing buildings were of particular interest in this review, as the experimental data from actual retrofits can also be used to validate models through the analysis of real-world hygrothermal performance [90]. In addition, some of the projects included in this review were implemented in occupied buildings, and having the whole retrofit team, including builders and occupants involved in the retrofit, is important for ensuring innovative materials are implemented at scale [2]. Construction projects can introduce moisture at unexpected stages [91], which is why full building retrofits are discussed in depth in this review. Given different research restrictions, some projects analyzed only retrofitted portions of a building envelope, while other projects implemented a full building retrofit. A brief review of some of these in situ projects (partial and whole-building retrofits) is given in the following paragraphs.

Andreotti et al. 2022 [50] performed large-scale in situ monitoring of three different interior insulation options (CaSi, cork, and stone wool) for energy retrofits of a historic university building façade, with the total affected wall size being 6.2 m wide by at least 3.4 m tall. This project involved constructing interior metering boxes with controlled interior space conditions, monitored over a four-month heating season (November to March), with interstitial monitoring with temperature and relative humidity sensors. Based on the monitored data, risk for interstitial condensation was analyzed, looking at conditions when relative humidity was greater than 80% [50]. The in situ data in this project was also used in the calibration of hygrothermal models, where sensitivity of interstitial relative humidity was considered with different initial conditions and material files where possible.

Johansson et al. [65] investigated the performance of vacuum-insulated panels (VIPs) on old buildings; the total energy use reduction from damaged VIPs, U-Value of envelope, experimental temperature and humidity measurements, air tightness, and thermal gradients were analyzed. This project also involved in situ infrared thermography and hygrothermal simulation in WUFI. From the simulation, the moisture excess was considered in comparison to calculated moisture excess in experimental data [65].

Piasecki et al. [78] considered the impact of novel retrofits on the indoor environmental quality of a former military barrack that was repurposed as a university building. This project, while not a complete envelope retrofit, provides a context of how occupants perceive hygrothermal retrofits in the context of comport and environmental quality. As this project focused on experimental data, mould spores were collected from the air of non-retrofitted rooms, which provided baseline data on the species observed in an institutional building [78].

Pihelo and Kalamees [58] considered hygrothermal performance in the context of deep energy retrofits using panelized systems, with novel materials on two walls of an apartment building. In situ data was collected over ten months and analyzed in addition to hygrothermal simulation results. This research used a wider range of performance metrics than most other publications, calculating a mould index with critical humidity calculations, as well as considering the moisture content, drying time, and mould growth risk in multiple locations through the depth of the wall assembly [58].

Campbell et al. [69] focused on two single-family homes and novel retrofitting of walls with sheep wool, phenolic foam, and plasterboard insulation [69]. They analyzed the relative humidity data during the in situ monitoring as their hygrothermal metric [69].

In Canada, research related to overcladding existing building for energy retrofits in the PEER project [4] led to multiple publications on hygrothermal performance for an unoccupied pilot project [57] and a full building application [73]. These projects did not necessarily have novel materials, but did include innovative panel assemblies and construction methods in the context of Canada’s building industry [4]. They used mould index, moisture index and relative humidity, vapour pressure, and moisture content as hygrothermal metrics [57,73,77].

Other panel retrofit projects tested modular retrofits, such as the research by Biswas et al. [63], which overcladded an institutional single-storey building with over 330 panels, each with approximately 0.3 m² surface area and 25.4 mm thickness, with stable monitoring from November 2015 to April 2016. In this exterior retrofit project, simulations with WUFI were also implemented in addition to in situ installation and monitoring [63]. Relative humidity, moisture content, and calculated vapour pressure were analyzed at different depths in the wall assembly from simulation results, and relative humidity, temperature, and heat flux were monitored in situ, in addition to energy consumption data [63].

In situ hygrothermal monitoring of five case study timber framed buildings was conducted by Whitman et al. [79], with one of the case buildings being a commercial 17th century building retrofitted with wood fibre and wood wool insulation panels. This in situ monitoring was also supplemented by energy modelling, hygrothermal simulation, and hot box–cold box laboratory testing of the new insulation panels. In terms of hygrothermal assessments, this research considered relative humidity, occupant comfort, and risk of biological attack and degradation in hygrothermal simulations [79].

Stahl et al. [11] retrofitted a heritage building with aerogel exterior insulation to improve the building envelope, reducing the thermal transmittance in the walls to a third of the original value. As this was a heritage building from the 14th century that had an exterior rendering applied in the 1970s, it was possible to remove the 20th century rendering and replace it with exterior insulation in the form of the SiO₂ aerogel. The aerogel in this project had been tested previously for hygric material characteristics, and the researchers were confident that there would be minimal risk to applying the innovative material to a building in situ. In this project, fire resistance and hygric tests are discussed at length as part of the background and methods for this retrofit. For the in situ monitoring, the interface between the existing façade and the aerogel coating was instrumented with temperature and relative humidity on the western side for about 15 months starting at the end of summer 2012. In addition, this research modelled the base building, an aerogel retrofit with a damp initial façade and a retrofit with a very wet wall in WUFI, considering water content per unit surface and moisture accumulation. This publication mentions mould growth risk but does not explicitly calculate this metric, discussing instead the temperature ranges seen on the in situ wall during infrared thermography of the east and south façades during a winter day [11].

Another project with connected members to the previous in situ research [11] considered the retrofit of a prefabricated concrete building with aerogel coating [35]. This project is one of the largest building retrofits discussed in this review, with a building height of 33 m (eight stories) and total applied surface area of 11,100 m². This project involved training of tradespeople in the installation of aerogels, hygrothermal modelling of drying time and construction details, worst case scenarios, and other extreme conditions. There were three retrofit variations modelled, in addition to the base wall designs for both the west and north orientations, and the calculated water content was analyzed. This project also made use of infrared thermography, taking images of the façade before retrofit and post retrofit to determine areas of thermal bridging [35]. U-value was also measured in situ for this envelope retrofit. This project shows the value of implementing hygrothermal modelling prior to the retrofit to discover potential areas of concern and monitoring of the retrofit post construction. In addition to the in situ aerogel research described in [11,35], this research group has continued to investigate aerogel coatings as an innovative material for retrofits, with more material and laboratory testing experiments [92], with a focus on the stability of the material rather than hygrothermal performance.

While the focus of retrofits was generally on improving the energy performance of the building envelope, there were a few researchers that focused on seismic or structural retrofits as the main retrofit goal, with some energy improvement as a secondary goal, such as the work by Besen and Boarin in New Zealand [45] and Evola et al. in Italy [47]. The multidisciplinary nature of seismic reinforcement research, innovative materials, and hygrothermal performance assessment may explain the low frequency of publications in this area but is a gap in research given the importance of retrofitting older buildings for seismic protection and energy efficiency [93].

Again, the wide variety of retrofit applications, methods, and hygrothermal performance metrics are seen in these in situ projects.

3.3. Simulation Tools

Of the 59 papers reviewed, 50 included hygrothermal modelling (85%), with 23 of the selected 50 publications (46%) focused solely on simulation and modelling. A variety of simulation tools were used, such as hygrothermal specific platforms such as WUFI and Delphin. General physics modelling tools such as COMSOL Multiphysics were also used, as well as a variety of internal institutional modelling solutions. Given that the aim of many of the retrofits researched was on improving building energy use, modelling was also conducted in EnergyPlus and THERM. The publications using specific tools are referenced in

Table 2. Hygrothermal or energy simulation platforms by publication.

Simulation Program Used	Quantity	Publications
WUFI	32	[11,21,22,23,24,29,30,31,33,34,36,37,42,43,44,45,49,59,60,61,62,63,64,65,66,67,68,70,71,76,77,79]
Delphin	9	[26,28,32,41,48,50,55,58,75]
COMSOL	3	[27,71,74]
EnergyPlus and other building energy models	4	[24,54,70,79]
THERM	3	[24,31,45]
HAMFEM	2	[25,38]
Other platforms (internal software, hand calculations)	5	[39,47,53,68,72]

below.

In every modelling project, researchers had to choose geometries, boundary conditions, material properties, and many other design considerations that had an impact on the results of the simulations. While many of the simulations were conducted with one- or two-dimensional models, there were some full building models in tools such as WUFI. Many researchers used multiple modelling techniques, such as combining energy modelling tools and hygrothermal models, with some researchers extending this to city-wide modelling that looked at the effect of urban morphology using GIS platforms to inform exterior boundary conditions as seen in the research by Claude et al. [55].

Some researchers also used the Glaser method to calculate vapour pressure at set exterior and interior conditions [24,42], often analyzing the results of these calculations in reference to results from hygrothermal modelling platforms such as WUFI [24,42].

The diversity of modelling platforms, including commercially available systems and in-house modelling, reflects the state of hygrothermal modelling systems, with no unified approach for all researchers [94].

3.4. Hygrothermal Performance Metrics Included

Through these 59 publications, not a single metric for hygrothermal performance was used in every single study. There were frequent criteria used, as summarized in

Table 3. Hygrothermal assessment metric by publication.

Assessment Metric	Quantity	Publications
Relative humidity	45	[11,21,22,23,24,25,26,27,28,30,31,32,33,34,36,38,40,41,43,44,46,49,50,51,52,53,56,57,58,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,78]
Moisture content	34	[11,21,22,23,24,25,28,32,33,35,36,38,41,42,43,44,45,48,49,53,56,58,59,60,61,63,64,66,67,70,72,74,77,79]
Mould risk analysis	29	[21,22,24,26,29,30,34,36,37,41,43,46,47,48,51,52,54,55,56,57,58,59,62,66,70,75,76,77,78]
U value, thermal and energy	22	[11,22,24,26,31,32,35,39,42,43,46,49,59,63,64,67,68,69,74,75,78,79]
Condensation risk	14	[24,25,26,33,38,39,41,47,48,50,54,58,59,74]
Freeze–thaw risk	7	[24,25,26,32,33,48,64]
Occupant comfort metrics	6	[29,30,45,60,76,78,79]
RHT and RHTT index	2	[27,44]
Oher hygrothermal metrics	4	[23,45,51,52]

below, but no single comprehensive standard assessment method was used across all publications. The metrics listed are based on what was observed in selected publications, and while representative of what is in use in hygrothermal research, is by no means exhaustive.

While there has been an assessment of individual hygric properties in other reviews such as water vapour permeability or water absorption [10], this review focused on hygrothermal performance criteria that are applied to the building envelope or occupied spaces themselves. Generally, the metrics fall into categories such as observed or measured values (relative humidity, moisture content, heat flux, measured mould growth, etc.), calculated metrics (mould index, freeze–thaw risk, RHT index), and occupant or indoor environmental quality metrics. Further analysis is given of each type of metric as follows.

3.4.1. Relative Humidity

Of the 59 publications, 45 or 76% published information about the relative humidity in the wall assemblies tested. Given that relative humidity is essential for other calculations of hygrothermal performance, some of the publications that do not directly report relative humidity still used it in their calculations. Relative humidity was generally graphed over time, with in situ and experimental research displaying results over a continuous testing period such as in [65] and modelling projects either reporting the last year of simulation or over a complete simulation period. Generally, relative humidity, together with temperature, was reported, and not absolute humidity, as expected by systematic reviews by other researchers [90].

3.4.2. Moisture Content

Moisture content is a broad category, which includes direct measurement of moisture in an assembly using gravimetric methods, indirect measurement with instrumentation such as moisture dowels or pin gauges [56], and modelled moisture content such as total water, liquid and vapour, or moisture excess. In terms of a moisture content value, or excess moisture data, 58% of publications (34 of 59) included a moisture content performance indicator. Given that equilibrium moisture content is part of moisture modelling standards in ASHRAE 160 [12], and it is a key metric that can be used in hygrothermal simulation validation [95], it should be included in assessments of innovative material retrofits. Moisture content can be considered at a specific time, but also over a set duration, for example using total water content over time to calculate the drying potential of a material [65], clarifying whether the moisture content assessed is a one-time value like initial moisture content or given as a varying result over time, such as moisture excess through multiple years [65].

Precisely defining how the moisture content of a material is measured, given the variety of outputs that could be included in this term, would decrease the likelihood of erroneous comparisons between projects.

3.4.3. Mould Risk Analysis

Mould risk was a concern for half the publications reviewed (29 of 59), with many different assessment methods used. A systematic review of mould growth criteria and assessment methods is given in the work by Gradeci et al. [96], with an analysis of the similarities and differences between methods. The most common assessment in this review article was the VTT method developed at the Technical Research Centre of Finland calculating time above a critical relative humidity for a given temperature and material [97], with 16 publications using this method [22,26,34,36,41,46,47,48,51,52,55,57,58,66,75,77]. Researchers also used mould risk assessment tools that were included in WUFI and WUFIBio models [22,24,29,30,33,37,51,52,59,62,70,76].

Different building materials have varying critical moisture levels, with more sensitive materials such as plywood and chipboard having critical exposure levels from 75 to 80%RH, and other materials such as cement-based boards ranging between 90 and 95%RH [98]. Many publications used multiple mould risk models and assessment tools, such as the work by De Masi et al. [46], which used mould assessments developed by Hens [99], Johansson et al. [98], and Hukka and Viitanen [97].

The mould growth development model created by Hukka and Viitanen is based on calculating a critical relative humidity boundary curve using experimental data for wooded materials [97]. Mould risk is then assessed under an index from 0 to 6, with 0 as no growth and 6 as 100% visual coverage of mould [97]. Other scales of mould growth have been used by researchers, with a range of 0 to 4 given in the experimental formulation of a mould risk index by Johansson et al. [98]. There were a few publications that used the 0–4 scale, such as the work by De Masi et al. [46] and Jensen et al. [51], but the data for mould growth in different material is useful for determining critical moisture levels [98].

Another singular mould test was conducted by Agliata et al. [54] in their research project on hemp-lime plaster using ISO13788/2012 mould tests [100]. This was a pass–fail criterion that considered the most critical month for mould risk [54]. Other researchers used similar pass/fail criteria such as time above critical relative humidity [21] and WTA standards [43]. The in situ projects allowed for a collection of material samples and actual measurement of mould growth, as seen in these projects [51,52,56,78]. These actual measurements of mould growth can be used to validate predictive mould growth indices.

It is evident that researchers consider mould risk assessments of innovative material retrofits important, especially in bio-based and wood retrofits [55]. However, as discussed in the review by Gradeci et al. [96], when there are so many assessment options available, it is difficult to determine the best method given the limitations of each individual method.

3.4.4. Condensation Risk

The risk of interstitial condensation was a hygrothermal assessment metric for many studies that included interior retrofits, where moisture would be trapped and condensation could form at layers in the envelope not previously observed. Some researchers also considered surface condensation risks in the interior, which could impact microclimates near interior furnishings and mould risk for interiors [101]. Condensation risk was seen in 24% of publications (14 of 59). The condensation risk can be calculated using the condensation potential comparing the partial pressure of water vapour in air close to the surface less the water vapour saturation pressure on the surface [102], with CP > 0 indicating condensation [59]. Other condensation risk formulas were seen that considered if the surface temperature of a layer was less than or equal to the dewpoint, and the material water content, as discussed in the work by Cascione et al. [33].

3.4.5. Freeze–Thaw Risks

When additional interior insulation is applied to masonry walls, this can change the temperature at the exterior masonry and through the wall sample, resulting in increased risk of frost damage due to the cycling of the material between freeze–thaw states [12]. The freeze–thaw risk in a material is defined as the number of freeze–thaw cycles over a set time period while the material moisture content exceeds the critical saturation value [103]. This metric was seen in 12% of selected publications (7 of 59) with brick buildings [25,32,48,64]; brick, concrete, and limestone models [33]; and 600 mm thick sandstone walls in situ and in hygrothermal models [24].

Prior to retrofits, there can be freeze–thaw risks in masonry wall assemblies, with additional risk after retrofits [24]. In addition to calculating freeze–thaw risk using critical saturation, relative humidity, and temperature values, qualitative assessments for risk can be conducted on in situ installations, checking for moisture management indicators in masonry such as efflorescence, spalling, or other kinds of deterioration of materials [12].

Given that 64% (38 of 59) of publications included brick or masonry elements, freeze–thaw metrics are potentially underrepresented as a hygrothermal performance criterion. It does make sense that the freeze–thaw metric is not applied to all masonry projects, such as retrofits involving overcladding brick with panelized systems [73], where brick is no longer exposed to exterior conditions, or in climates with minimal winter freezing conditions. Some publications discuss freeze–thaw risk as part of a larger discussion on water content in envelope layers [59], considering the systems avoiding freeze–thaw risk if their water content ratio was below 0.30.

3.4.6. U-Value and Thermal Performance Metrics

Given that many of the retrofits studied aimed to reduce the energy use of a building through improving the thermal performance of the building envelopes, it is expected that some of the publications consider thermal performance metrics such as the thermal transmittance of the wall (U value), which is given in 37% of publications (22 of 59) [32]. The thermal transmittance can be used to calculate heat loss through the envelope and can be adjusted for moisture-dependent performance, as seen in the work by Urso et al. [75]. Some of the in situ retrofits also performed infrared thermography surveys [11,65,78], showing the improved performance of the envelope and locations of thermal bridging. Discussions and analysis of thermal bridging were also seen in projects that modeled the retrofits in THERM [24,31,45] or using numerical methods. A consideration of thermal bridging in retrofits does have an impact on predicting locations with surface or interstitial condensation, in addition to reducing potential heat loss [24].

In the category of thermal performance, researchers also gave metrics related to energy demand [31,39,40,60,70,72,79], energy cost [30], operational energy efficiency [62], and energy use reduction [25,31,39,65]. While not strictly hygrothermal metrics, an improvement of the energy performance was often the initial goal of the building envelope retrofit.

3.4.7. RHT and RHTT

There has been an effort to develop hygrothermal performance indices that can be used for different projects, such as the RHT index and RHTT index. The relative humidity and temperature (RHT) index was used by the National Research Council of Canada [104,105] in their research on exterior cladding systems and novel building materials in the early 2000s. This index quantifies the long-term moisture and temperature response for an area of interest. These indices were only seen in two studies [27,44], with the RHT index discussed in [27] and calculated as per Equation (1):

RHT = Σ (RH − RH_x) × (T − T_x), for RH > RH_x and T > T_x,

(1)

where RH_x and T_x are the threshold critical relative humidity and temperature values for a certain material over time [27]. An extension to the RHT index, the Relative Humidity and Temperature Time (RHTT) index [103], is a product of calculated time-of-wetness (TOW) and the RHT index. The TOW is the number of hours where the relative humidity is greater than 80% and temperature is greater than a critical temperature [49]. The full development and formulation can be seen in [103]. The higher the RHTT index value, the higher the risk to the envelope [44,103]. The RHTT index was used in a work by Kaczorek who studied capillary active interior insulation retrofits [44].

While these indices are interesting for comparing different designs within a project [27], they have not been widely implemented in standard practice and so cannot be interpreted across different studies.

3.4.8. Occupant Comfort and Object Metrics

In this review, the consideration of occupant comfort and artifact condition through the retrofit was unexpected compared to most hygrothermal metrics that looked at performance through the building envelope. Although occupant comfort has been discussed in respect to building retrofits [14], it has not been frequently analyzed in in situ projects. A consideration of objects inside the building studied, such as art, books, furniture, and artifacts, has also not been frequently discussed, even though retrofitting the envelope can impact artifact health [106].

In terms of occupant comfort, this included consideration of indoor environmental quality models calculated from temperature, acoustic comfort, daylight, and perceived occupant satisfaction surveys [78]. A consideration of occupant comfort also assessed indoor air pollution, with volatile organic compound (VOC) measurement, carbon dioxide measurements, and fungi spore concentration [78].

There is an assessment of occupant comfort based on simulation results using predicted mean vote (PMV) index and predicted percentage dissatisfied (PPD) index in the research by Koh et al. [60], although this does not include an in situ validation of model results. Besen and Boarin [45] conducted occupant comfort surveys in their research project on retrofitting unreinforced masonry buildings in New Zealand using occupant perceived level of satisfaction with temperature in summer and winter but did not explicitly discuss relative humidity or the perception of moisture/dampness in the survey results.

Occupant questionnaires were also implemented through in situ retrofits in the research by Whitman et al. [79], which included short surveys and semi-structured interviews with occupants who gave more qualitative responses to occupant perceptions of historic property comfort.

Artifact metrics are considered by Coelho et al. in multiple projects about hygrothermal performance of retrofit scenarios based on a 13th-Century church in Lisbon, Portugal [29,30,76]. This research used an equivalent lifetime multiplier (ELM) to assess the mechanical decay of various artifacts, including wood sculptures, wood furniture, wood substates of panel paintings, and picture layer of panel paintings [30]. These publications include efficient simulation methods for the modelling of different retrofit designs [76] against multiple climate files from different cities and climate change scenarios, and with artifact assessments based on ELM and thermal comfort assessments based on ASHRAE 55 standards [29,107].

Given that occupants are part of the retrofit process and directly impacted by the changes in hygrothermal performance [14,85], and efforts to standardize artifact risk assessment are ongoing [108], considering these occupant and object metrics would be valuable in more novel envelope retrofit projects.

3.4.9. Other Types of Metrics in the Envelope

Again, given the breadth of research aims in these publications, there were hygrothermal metrics specific to certain studies that were not as frequently used, captured in this general category. This includes corrosion, salt reactions, degradation of building envelope materials, and calculated vapour pressure.

As part of their retrofit analysis, Besen and Boarin [45] used the EN 16883:2017 standard to check corrosion, salt reaction, and biological risks. This is a European standard for improving the energy performance of historic buildings [109], but the publication by Besen and Boarin does not clarify the methods used for corrosion, salt, and biological risk analysis [45], whether these are qualitative or what the damage thresholds are in these categories.

The test hut projects in Denmark discussed in both [51,52] measured the pH-value of mortars over time, investigating the connection between pH and mould growth. In this project, wood decay measurements were assessed on-site, with no obvious decay [52].

Guizzardi et al. [23] investigated mass loss functions for wooden beams in the wall based on fungi activation as a function of relative humidity and temperature. This required knowledge of the damage process of the wood and interpretation after modelling the hygrothermal performance [23].

4. Discussion

From the metrics and publications chosen in this rapid review, there are some central themes and research gaps found in the study of hygrothermal assessment metrics for innovative materials in building envelope retrofits. These themes include metrics used and retrofit types, including the scale and application, use of simulation tools, and gaps in research included in this rapid review.

4.1. Discussion of Hygrothermal Performance Metrics

While many researchers used common metrics for assessing the hygrothermal performance of a retrofit, there was no single metric used by every publication. Generally, metrics were selected that considered the elements at risk, such as mould indexes for bio-based materials or freeze–thaw risk for masonry retrofits. Selecting the correct metric to communicate risk allows the wider building industry to more clearly interpret the results. Without a clear metric, such as publications that simply gave relative humidity results over time, readers of each publication are left to interpret the risk in using the novel material in a retrofit, which can cause misinterpretation of results. This lack of consensus has been highlighted in other review papers of hygrothermal assessment methods [13] and remains a challenge for assessing innovative materials.

Although there were index metrics developed that are less specific to materials (applicable to bio-based or masonry), such as Time of Wetness, RHT, and RHTT, these are still only used by few researchers, so they cannot be interpreted across many studies.

While many researchers provided data about relative humidity and moisture content, further analysis of these metrics to include condensation prediction values have not been implemented as frequently. While the mould index is useful for bio-based materials, there is no common repeated damage function or index used for inorganic materials that can be used to compare performance. Adding multiple metrics to retrofit research using innovative materials would increase confidence in performance for the building industry.

It should be noted that while innovative materials were used in the publications selected, there was a lack of truly innovative hygrothermal metrics used. No metrics unique to the material studied were discussed, with all metrics implemented based on current practices in hygrothermal performance assessments. This may be due to a gap in knowledge of unique failure modes of new materials or challenges in determining the specific hygrothermal performance requirements of innovative materials [110]. The development of material-specific failure criteria may also be more likely in material testing research conducted prior to retrofits, which was excluded from this review. While it would be time-consuming to test for all hygrothermal failure conditions unique to a specific material, it would still be valuable to prevent unexpected failures, as discussed by de Place Hansen et al. [110].

In addition, with most of the publications focused on projects implemented in North America and Europe, hygrothermal performance metrics that may have higher importance in other climates outside of these zones may have been missed in this review. In particular, a discussion of the hygrothermal risk of bio-based materials in tropical climates would be worthy of consideration.

Beyond quantitative metrics, there is a gap in physical observed hygrothermal damage, such as rising dampness, efflorescence, or algae that may not be easily calculated in simulations or numerical analysis. This leads to the importance of more in situ projects, where physical deterioration and damage can be monitored.

Moving forward from this review, practitioners are encouraged to, at the minimum, consider hygrothermal metrics specific to material type and climate (such as mould risk or freeze–thaw cycles) and generalized metrics and damage functions such as condensation potential, time of wetness, and RHT index. Reporting on hygrothermal damage functions and indicators, beyond publishing relative humidity or moisture content data, will allow industry professionals to better assess materials in practice.

4.2. Retrofit Types

This review highlights that there remains a gap in in situ retrofits with novel materials: being able to monitor sites with occupant feedback [14], have real construction conditions that may increase moisture risk in the wall assemblies [91], and integrate best practices in retrofitting buildings for the future [1,2]. Given the focus of this review was on innovative materials, there was an understandable lack of long-term monitoring of in situ retrofits. Revisiting the in situ projects would provide essential data about durability and hygrothermal performance. Of the reviewed in situ and test hut projects, the longest duration was approximately 5 years [52].

For in situ retrofits, the hygrothermal metrics are important, but there is a gap in research into hygrothermal performance seen by occupants and materials inside buildings. Changes to occupant comfort, interior relative humidity, and air quality have been seen after retrofits [14], which should be studied when possible. Ensuring occupants are satisfied with the retrofit increases the likelihood of better support of future retrofit projects [2,8].

4.3. Use of Hygrothermal Simulation

Given the relatively short monitoring periods for many of the in situ projects included in this review, hygrothermal models could be used to generate long-term performance data. Of the 50 publications with modelling, 16 projects (30%) included simulations for longer than five years, with some researchers integrating changes to the climate model in long-term simulations [29,30]. These longer-term simulations allowed researchers to verify the sensitivity of design changes without the expense, time, and coordination required for in situ retrofits.

The challenge with these hygrothermal simulations is that there still is no agreed standard for model validation [90], so realistically, interpreting resulting hygrothermal performance metrics can be challenging, especially if no clear validation is performed. Attempts at the standardization of hygrothermal model calibration procedures are ongoing, with guidelines adapted from current practice in hygrothermal modelling and energy modelling suggested by Huerto-Cardenas et al. [90], which can be a starting point, especially in building retrofits. A discussion of the parameters of interest to calibrate against, such as temperature, relative humidity, heat flux, and moisture content, is also given in the round-robin hygrothermal model calibration project conducted by Dang et al. [95].

4.4. Gaps in Research

While this rapid review attempted to cover a variety of retrofit projects with innovative materials and the hygrothermal assessment metrics used, there are still gaps in the research. As discussed, there is no singular criterion used by all researchers, with many assessment metrics used in the context of specific materials or retrofit designs. Reporting multiple metrics for novel retrofits would increase confidence in these interventions.

Although there was a wide variety of innovative material studies in these publications, there were still innovative materials not used. This is especially evident in materials with low technological readiness, such as recycled bio-based materials, or novel façade systems with challenging manufacturing and installation conditions. More research should focus on developing test hut projects and partial wall case studies of these innovative materials, even if there is not enough material for a full building retrofit. Broadly, there are minimal to no legislative requirements for material type selection for retrofits that would require the use of innovative materials. There might be a policy incentive to consider novel materials due to changing standards for reporting requirements for global warming potential of buildings in Europe [111], depending on levels of enforcement and financial support.

Of the 59 publications selected for review, less than 30% included an in situ retrofit, whether partial or whole building. In each of these in situ retrofits, valuable knowledge was gained during construction about moisture risk during construction: occupants provided moisture loads and local weather and climate effects. More in situ retrofits with innovative materials are needed to further understand the risk factors of these materials and the construction process.

It should be noted that economic and material availability considerations were not discussed at length or analyzed for any of the in situ projects, but both of these topics are discussed in reviews of retrofits outside of a hygrothermal context [4,85]. Discussing the availability and cost of innovative materials would aid the building industry in comparing the implementation of these materials with typical building designs and practice. Some publications considered the cost savings for each retrofit scenario, in addition to hygrothermal impact, such as the project by Coelho et al. [30], but did not integrate these metrics together into a cohesive indicator for an overall performance assessment of different retrofit options. There is a gap in the research to integrate economic/logistical impact, hygrothermal performance, and innovative material use in envelope retrofits.

Finally, a limitation of this review is the geographic distribution of retrofits with a heavy focus on Europe and North America. Part of this focus is explained by the review method limiting the search language to English, but there still needs to be further research across different climate zones and vernacular building styles that are not implemented in Europe and North America. This is a common gap identified in other hygrothermal research, novel material, and building retrofit reviews [5,7,82,85,112], with a lack of publications focused on the global south and underdeveloped countries. There is starting to be a focus on research related to building energy use and comfort in the global south [113] but little discussion of hygrothermal performance even in humid subtropical climates.

5. Conclusions

This paper aimed to provide a rapid review of recent research on hygrothermal performance assessments of innovative material retrofits. Although no single metric was used, as each retrofit considered had different characteristics, there was frequent consideration of relative humidity, moisture content, condensation prediction, mould indices, and other hygrothermal damage functions where appropriate.

The diversity of retrofits reviewed included occupied residential units, institutional buildings, historic churches and galleries, as well as laboratory testing, test hut experiments, and hygrothermal modelling and simulation.

While this review highlights current research in this field, it was limited by language of publication and exclusion criteria discussed. There are research gaps in retrofits with innovative materials with low levels of technological readiness and in situ experimentation. There is also a research gap in long-term field installations and long-term modelling. In terms of hygrothermal metrics, there is a gap in publications using damage functions beyond mould indices, especially freeze–thaw functions, RHT index, and similar.

Future systematic reviews should consider studies in non-English publications and consider hygrothermal performance metrics used in gray literature and supplemental literature. Meta-analyses by material type of retrofit, base building type, and hygrothermal metric would add to gaps in current reviews of this topic. A consideration of all elements of the building envelope, such as fenestration, roofs, and foundations, in addition to wall retrofits, would further expand the knowledgebase in this field.