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The shift towards using materials derived from plants is considered an innovative approach to creating eco-friendly buildings and decreasing the energy expenses associated with the building sector. This study aims to provide a comprehensive analysis of these materials' hygrothermal properties to support their successful integration in both new construction and building renovation projects. It reviews the current state of research on the thermal and hydric properties of plant materials on various scales: raw aggregate/fiber, plant-based composite, wall, and building scale. The first part focuses on the physical, chemical, and hygrothermal characterizations of raw plant aggregates and fibers. The second part covers the main hygrothermal properties of plant-based composites (insulation boards and concretes) and compares their performance with conventional materials. The study then shifts its focus to the hygrothermal performance of walls integrated with plant-based materials in response to external climate conditions. Finally, a review of full-size buildings is conducted to investigate the impact of these materials on hygrothermal behavior at the building scale and to highlight their influence on the reduction of energy consumption. A lack of studies on plant-based materials was observed. There remain numerous areas that require further investigation at each scale. Consequently, the study includes discussion sections that provide guidance for future research to fill the gaps and advance the development of this type of material. • Up-to-date literature on plant-based building materials at various scales is reviewed. • The physical, chemical, and hygrothermal properties of raw plant aggregates and fibers are discussed. • The hygrothermal properties of plant-based composites, insulation boards, and concretes are discussed. • The application of plant materials at the wall scale is reviewed. • The hygrothermal comfort and energy performance of plant-based composites at the building scale are highlighted. • Future studies are proposed to fully explore the potential of plants as sustainable building materials. [ABSTRACT FROM AUTHOR]

• Microscopic structure characterization of straw fibers. • Thermal and mechanical properties of straw bales. • Fire and sound resistance of a straw bale walls. • Life cycle assessment of a straw construction. • Energy and economic performance of straw bale buildings. Bio-based materials such as straw are becoming a promising alternative to improve the building energy performance and to reduce its carbon footprint. When compared to common building construction materials, bio-based materials control the temperature and the relative humidity variation to ameliorate the indoor comfort with a low embodied energy and CO 2 emission. This paper presents a comprehensive review of the thermal and mechanical properties of straw-based materials and buildings. The objective is to synthesis the work that has been carried out by the research community and to compare the results. The paper first introduces straw bale as a construction material from a historical viewpoint and in the context of the current building sector. The second part focuses on the available chemical and microstructural data of the straw fiber. The third part refers to the thermophysical and mechanical properties of the bales. The fourth part reviews the numerical and experimental studies done at the wall scale. The fifth part describes straw bale construction methods considering the regulation, structure requirements, and life cycle assessment data. Last, a critical analysis of the currently available data on straw as a building material is carried out and pending research issues are discussed. It was found that, despite abundant literature on structural and thermal properties of straw bale constructions, there is still a lack of some information. At a fiber scale, more research should be done to compare straw fibers to other natural and synthetic fibers. At a bale scale, further pH-related research is needed because it affects the material's interior conditions and durability. In addition, a thermal conductivity model for straw should be developed. On a bigger scale, the hygrothermal characteristics of various types of walls must be measured and computed experimentally and theoretically under various exterior and internal situations. More research is needed to improve the sound resistance of the straw wall by adding new layers capable of absorbing acoustic waves. Studies on the energy behavior, cost analysis, and how interior air moisture is self-regulated in straw buildings are needed at the building size. Therefore, a lack of consistent data among the different studies was noted depending on the straw characteristics. [ABSTRACT FROM AUTHOR]

• The interconnections distribution in fibers determines the heat transfer model. • The development of a thermal conductivity equation considers the radiative heat transfer. • The bale's thermal conductivity varies according to the fiber orientation. • Thermal conductivity depends on the chemical composition of the straw. Straw fibers are natural fibers that have a strong insulating property and a minimal environmental effect, making them suitable construction materials. In the construction sector, to predict the building energy consumption, the thermal conductivity of the materials composing the envelopes must be precisely known, which is not the case for the straw material. The properties of a straw bale are variable and mainly depend on its density, fibers orientation, chemical composition, temperature and relative humidity. Consequently, this study consists of determining a mathematical equation to predict the effective thermal conductivity of straw bales that can be applied to all straw types, in different circumstances. For this purpose, the size, distribution, and morphology of straw fibers are first determined from microscopic images. Second, a mathematical model for estimating the thermal conductivity is suggested using the heat transfer models of porous and fibrous materials. The numerical model is validated by an experimental study that measures the thermal conductivity of straw bales by altering their density between 80 and 120 kg/m3, their relative humidity between 15 % and 95 %, and their temperature between 15 ˚C and 55 ˚C. The experimental results show that the thermal conductivity increases from a minimum of 0.047 W/(m∙K) to a maximum of 0.09 W/(m∙K) when increasing the three altered factors. For a straw fiber having 40% cellulose content, the thermal conductivity increased from a minimum of 0.05 W/(m∙K) to a maximum of 0.0832 W/(m∙K). This behavior is found the same for the numerical findings. The comparison of numerical and experimental values shows a good agreement, with a root mean square error of 0.005 W/(m∙K) and a scatter index of 5.5 %. [ABSTRACT FROM AUTHOR]

• The temperature and relative humidity variations obtained by WUFI are in good agreement with the literature experimental findings. • The excessive moisture content in straw leads to bio-deterioration, mold, and decay damage in the whole structure. • The hygrothermal issues in a straw wall depend on the interior and exterior covering layers. • Hygrothermal issues can be solved by adding the straw thickness, adding a vapor retarder layer, adding an unventilated air layer, and controlling the indoor conditions. Straw is becoming a promising alternative insulation material to improve the building energy performance because of its low cost, large availability as cereal waste, its good hygrothermal properties, and its low embodied energy. The goal of this paper is to assess the hygrothermal performance of multi-layered straw walls with different boundary conditions. The study consists of evaluating five different straw-based wall assemblies, under a continental (Arkhangelsk, Russia), a tropical climate (Brasilia, Brazil), a temperate Mediterranean climate (Nice, France), and a cold desert climate Xinjiang (China). Numerical models of the heat and moisture transfer through the walls using WUFI software are developed, calibrated, and validated through experimental results from the literature. The chosen evaluation criteria are the total water content, the drying rate, the condensation risk, the mold growth, the moisture quantity, the time lag, and the decrement factor. Results show that straw walls with cement and/or wood covering can be used in tropical and temperate climates, coated straw walls with additional air layers in dry climates, while insulated straw walls are best fitted in continental climates. In the latter, the dryness rate varies between 7 and 40% with a low condensation risk of value in the range of 0–12%. It can be concluded that the straw wall's performance strongly depends on the interior and exterior added thermal insulation layers since they affect the ability of the material to dry out. [ABSTRACT FROM AUTHOR]