A Mini-Review on Straw Bale Construction

: Straw bale building construction is attracting a revived public interest because of its potential for reduced carbon footprint, hygrothermal comfort, and energy savings at an affordable price. The present paper aims to summarize the current knowledge on straw bale construction, using available data from academic, industry, and public agencies sources. The main ﬁndings on straw ﬁbers, bales, walls, and buildings are presented. The literature shows a wide variability of results, which reﬂects the diversity of straw material and of straw construction techniques. It is found that the effective thermal conductivity, density, speciﬁc heat, and elastic modulus of straw bales used in construction are in the range 0.033–0.19 W/(m · K), 80–150 kg/m 3 , 1075–2000 J/(kg · K), and 150–350 kPa respectively. Most straw-based multilayered walls comply with ﬁre resistance regulations, and their U-value and sound reduction index range from 0.11 to 0.28 W/m 2 K and 42 to 53 dB respectively, depending on the wall layout. When compared to standard buildings, straw bale buildings do provide yearly reductions in carbon emissions and energy consumption. The reductions often match those obtained after applying energy-saving technologies in standard buildings. The paper ends by discussing the future research needed to foster the dissemination of straw bale construction.


Introduction
Buildings are responsible for 36% of the world's energy load and 37% of the CO 2 emissions [1].The immense untapped potential for energy and carbon emission savings of the building sector makes it the prime player in energy transition and climate change mitigation.Global concern about environmental and energy issues has led many governments, worldwide, to issue new building regulations.In western countries, such regulations promote the use of low embodied energy and sustainable materials.For example, the 2020 French environmental regulation includes a new carbon requirement that must be met by all buildings constructed after 1 January 2022.Due to their affordability, low carbon footprint and interesting thermal, hygric and mechanical properties, bio-based materials are attracting a renewed interest from many academic researchers and private contractors.In this context, straw, as an agricultural waste product often produced close to the construction site, is increasingly considered and used as a construction material.Figure 1 shows the increasing number of academic and industry papers devoted to straw bale construction from 1990 to 2020.It should be noted that straw buildings appeared first in 1886 in the USA [2].
The present review aims at summarizing the current knowledge on straw bale construction.More detailed reviews can be found in [3][4][5].This paper first presents the results of the thermophysical characterization of straw fibers and bales.It then discusses the hygrothermal and energy behavior of straw-based walls and buildings, along with their life cycle assessment.Last, the main open research questions regarding straw bale buildings are detailed.The present review aims at summarizing the current knowledge on straw bale construction.More detailed reviews can be found in [3][4][5].This paper first presents the results of the thermophysical characterization of straw fibers and bales.It then discusses the hygrothermal and energy behavior of straw-based walls and buildings, along with their life cycle assessment.Last, the main open research questions regarding straw bale buildings are detailed.

Straw Fibers and Bales
Straw stalks are generally composed of three main parts: grain head, nodes, and internodes, as shown in Figure 2. Internodes are hollow tubular structures of a few millimeters in diameter, whose shell includes the epidermis, the parenchyma, and vascular bundles [6].Researchers have mostly used digital microscopy and scanning electron microscopy (SEM) devices to get visual access to the internode and fiber microstructure [7][8][9][10].They found that although the internode may be several centimeters long, the fibers located inside its shell are mostly 2 to 7 mm long, and pores are 2 μm to 100 μm large, depending on the straw type.Figure 3 shows the microstructure of a wheat straw internode, using digital microscopy and SEM analysis.Straw fibers are mainly composed of cellulose (40-80%), responsible for the thermal insulation property; lignin, (10-30%) responsible for the mechanical stability; moisture (6-8%); and ash (5-10%) [11-14].

Straw Fibers and Bales
Straw stalks are generally composed of three main parts: grain head, nodes, and internodes, as shown in Figure 2. Internodes are hollow tubular structures of a few millimeters in diameter, whose shell includes the epidermis, the parenchyma, and vascular bundles [6].Researchers have mostly used digital microscopy and scanning electron microscopy (SEM) devices to get visual access to the internode and fiber microstructure [7][8][9][10].They found that although the internode may be several centimeters long, the fibers located inside its shell are mostly 2 to 7 mm long, and pores are 2 µm to 100 µm large, depending on the straw type.Figure 3 shows the microstructure of a wheat straw internode, using digital microscopy and SEM analysis.Straw fibers are mainly composed of cellulose (40-80%), responsible for the thermal insulation property; lignin, (10-30%) responsible for the mechanical stability; moisture (6-8%); and ash (5-10%) [11][12][13][14].The present review aims at summarizing the current knowledge on straw bale construction.More detailed reviews can be found in [3][4][5].This paper first presents the results of the thermophysical characterization of straw fibers and bales.It then discusses the hygrothermal and energy behavior of straw-based walls and buildings, along with their life cycle assessment.Last, the main open research questions regarding straw bale buildings are detailed.

Straw Fibers and Bales
Straw stalks are generally composed of three main parts: grain head, nodes, and internodes, as shown in Figure 2. Internodes are hollow tubular structures of a few millimeters in diameter, whose shell includes the epidermis, the parenchyma, and vascular bundles [6].Researchers have mostly used digital microscopy and scanning electron microscopy (SEM) devices to get visual access to the internode and fiber microstructure [7][8][9][10].They found that although the internode may be several centimeters long, the fibers located inside its shell are mostly 2 to 7 mm long, and pores are 2 μm to 100 μm large, depending on the straw type.Figure 3 shows the microstructure of a wheat straw internode, using digital microscopy and SEM analysis.Straw fibers are mainly composed of cellulose (40-80%), responsible for the thermal insulation property; lignin, (10-30%) responsible for the mechanical stability; moisture (6-8%); and ash (5-10%) [11-14].Straw bales are made of stacked and compressed straw stems.The bale's shape, dimension, and level of compression depend on the baler used.Rectangular bales used in building walls are usually from 60 to 130 cm high, 70 to 120 cm wide, and 50 to 280 cm long.Watts et al. [15] found that the density of barley straw bales ranges from 54.6 kg/m 3 to 78.3 kg/m 3 , whereas that of oat and wheat straw bales ranges from 81 kg/m 3 to 106.3 kg/m 3 .Many national building regulations impose a minimum bale density of 80 kg/m 3 for straw construction.Straw bales are made of stacked and compressed straw stems.The bale's shape, dimension, and level of compression depend on the baler used.Rectangular bales used in building walls are usually from 60 to 130 cm high, 70 to 120 cm wide, and 50 to 280 cm long.Watts et al. [15] found that the density of barley straw bales ranges from 54.6 kg/m 3 to 78.3 kg/m 3 , whereas that of oat and wheat straw bales ranges from 81 kg/m 3 to 106.3 kg/m 3 .Many national building regulations impose a minimum bale density of 80 kg/m 3 for straw construction.
Straw bales are classified as hygroscopic materials with moisture buffer values around 1.8 and capillary absorption of about 0.0155 kg/(m 2 .s 1/2 ), due to their 90% porosity [16].Several teams investigated the effect of straw type, fiber main orientation, bale density, average temperature, and relative humidity (RH), on the bale's effective thermal conductivity.The methodology was mainly experimental, using the hot guarded plate technique or flux meters to measure the thermal conductivity.It was found that the latter increases with increasing density, RH, and temperature, and decreases with higher cellulose content and when the fibers are predominantly oriented perpendicular to the heat flow.Increasing density reduces the amount of air trapped inside the bale, which explains the adverse effect on conductivity.High RH relates to the higher moisture content in the bale, which badly affects the conductivity because of the higher water conductivity.When the bale temperature rises, the number of molecules colliding increases due to the higher microscopic kinetic energy, thus increasing the thermal conductivity.Straw type relates to cellulose content and a reduced amount of lignin increases the thermally insulating porous microstructure.Last, when fibers are perpendicular to the heat flow, there is no solid conduction path through the bale, whereas in the parallel case, such paths are through the fibers, which are less insulant than air.
Table 1 presents research results on bale conductivity as a function of the mentioned parameters.The variability of the results reflects the diversity of the straw material, which depends on the straw type and production site.Therefore, bales from each site have specific thermophysical properties that should be measured before their use in construction.The thermal conductivity is always in the range [0.033-0.19W/m.K] whereas the bale's density, temperature, and RH are in the range of 100-150 kg/m 3 , −5 to 40 ˚C and 5-60% respectively, regardless of the fiber main orientation and straw type [17][18][19][20].The thermal conductivity of a straw bale is lower than that of concrete (0.3-1.8 W/m.K), bricks (0.3-1.1 W/m.K), and wood (0.14-0.22 W/m.K) [21].Straw bales' specific heat capacity varies from 1075 to 2000 J/(kg.K), with the measured value increasing with increasing bales' density and temperature [22,23].Straw bales' heat capacity range is close to that of concrete, brick, and wood which is about 1000-2500 J/(kg.K) [24].Regarding thermal effusivity and diffusivity, they are respectively in the range 417-775 J/(K.m 2 .s−1/2 ) and 0.1 × 10 -6 -3.6 × 10 -6 m 2 /s, Straw bales are classified as hygroscopic materials with moisture buffer values around 1.8 and capillary absorption of about 0.0155 kg/(m 2 •s 1/2 ), due to their 90% porosity [16].Several teams investigated the effect of straw type, fiber main orientation, bale density, average temperature, and relative humidity (RH), on the bale's effective thermal conductivity.The methodology was mainly experimental, using the hot guarded plate technique or flux meters to measure the thermal conductivity.It was found that the latter increases with increasing density, RH, and temperature, and decreases with higher cellulose content and when the fibers are predominantly oriented perpendicular to the heat flow.Increasing density reduces the amount of air trapped inside the bale, which explains the adverse effect on conductivity.High RH relates to the higher moisture content in the bale, which badly affects the conductivity because of the higher water conductivity.When the bale temperature rises, the number of molecules colliding increases due to the higher microscopic kinetic energy, thus increasing the thermal conductivity.Straw type relates to cellulose content and a reduced amount of lignin increases the thermally insulating porous microstructure.Last, when fibers are perpendicular to the heat flow, there is no solid conduction path through the bale, whereas in the parallel case, such paths are through the fibers, which are less insulant than air.
[42] conducted tests on several types of straw bales by applying a 10 kN load.They got elastic modulus between 150 and 350 kPa for density values between 83 kg/m 3 and 100 kg/m 3 which is very low compared to those of concrete of about 10-30 GPa [43].The deformation curves demonstrated the material's elastic behavior, with the bales almost fully recovering their initial shape after the release of the load.

Straw Walls and Buildings
Straw walls are generally made of a core straw bale layer within other outer finishing layers that may include plaster, cement, hardwood, an additional thermal insulation layer, and an unventilated gap.Depending on the wall structure and the materials' thickness, the resulting U-values mostly range from 0.11 [44] to 0.28 W/m 2 K [40], which is lower than that of building walls using conventional materials such as concrete or bricks.Several researchers [42, [45][46][47] experimentally noted the ability of straw walls to reduce and time shift the variations of outdoor temperature and RH.Most found time lags and decrement factors of value 6-12 h and 0.01-0.08%,respectively [38,48].Regarding the hygric properties, most studies agree on the very slow humidity absorption of straw walls [42,47,49].This is mainly due to the high vapor diffusion resistance of the finishing layers and plasters.Degradation tests conducted on a straw wall at 28.5% RH showed no bacteria development [46,50].Recently, Ghadie et al.
[51] conducted a numerical study on different straw walls to find the best structure depending on the climate.They found that in terms of condensation risk and mold growth, straw walls with cement or wood covering best fit in tropical and temperate climates, coated straw walls with additional air layers in dry climates, and insulated straw walls are recommended in continental climates.
So far, most research on straw walls' fire resistance has used experimental methods.It was found that most structures could withstand fire temperature on one side above 1000 • C for two hours, thus complying with the international building code [52,53].This is due to the limited amount of oxygen within bales of density above 80 kg/m 3 and to the fire resistance of the finishing layers such as plaster.Last, regarding acoustic performance, experiments showed that the sound reduction index of straw walls varies between 42 and 53 dB depending on the wall layout [39,[54][55][56].
At the building scale, there are three predominant construction techniques for a straw building, as shown in Figure 4.In the loadbearing method [57][58][59], straw bales support the weight of the roof.The method's main limitations are the maximum area of openings, which should be less than 50% of the wall area and the height generally limited to a single story.The nonstructural (or infill) method [59][60][61] uses wood or reinforced concrete columns and beams to support the load.This technique allows high-rise buildings but requires more complex construction operations.Last, the prefabricated technique [62,63] allows assembling prefabricated wall panels on the construction site, thus reducing construction complexity and time.

Conclusions
The existing literature represents an important step towards the dissemination of straw bale buildings.However, there are still several gaps in the existing knowledge.Measurements on bales' hygric properties other than the vapor diffusion resistance factor are seldom found.The literature also lacks detailed models of the heat and mass transfer through straw fibers and bales, depending on the straw type, bale density, temperature, RH, and fiber orientation.Such models will help predict the hygrothermal and energy behavior of straw buildings.Similarly, the three main straw building construction methods have not been fully investigated in terms of their inherent benefits and limitations from the hygrothermal and mechanical perspectives.For example, it is still unclear which method delivers the highest thermal comfort, energy savings, and seismic resistance.Such studies should also include the building location and its scenarios of occupation and operation.Last, the role of straw as a regulator of indoor air moisture is still insufficiently documented, even though straw building contractors sometimes advertise this characteristic.Future studies should take into account the variability of the results, due to the changing nature of the thermo-physical properties of this bio-based material over time.The main goal of the use of straw is to reduce the building's energy consumption and carbon emissions.Chaussinand et al. [64] assessed a 300 m 2 straw load-bearing office building.They showed a final energy consumption of around 3800 kWh and primary annual energy of around 8.9 kWh/m 2 of floor area.Alcorn et al. [65] numerically assessed the life cycle of a straw bale and timber dwelling with a floor area of 200 m 2 .They noted an annual energy consumption of 37.9 GJ whereas a standard house with conventional insulation, wood timber, and concrete floor consumes about 41.7 GJ per year.Compared with the standard house, they found a reduction of 1230 kg CO 2 equivalent per year that, interestingly, was almost the same as the total emission reduction from applying energy-minimizing technologies, excluding onsite renewable energy generation.Currently, one of the tallest straw buildings in the world is a seven-story apartment building including 15 residential units of 90 m 2 floor area each, located in St. Die des Vosges, France.The Jules Ferry building is labeled "passive house", with a heating load lower than 15 kWh/m 2 /year [66].

Conclusions
The existing literature represents an important step towards the dissemination of straw bale buildings.However, there are still several gaps in the existing knowledge.Measurements on bales' hygric properties other than the vapor diffusion resistance factor are seldom found.The literature also lacks detailed models of the heat and mass transfer through straw fibers and bales, depending on the straw type, bale density, temperature, RH, and fiber orientation.Such models will help predict the hygrothermal and energy behavior of straw buildings.Similarly, the three main straw building construction methods have not been fully investigated in terms of their inherent benefits and limitations from the hygrothermal and mechanical perspectives.For example, it is still unclear which method delivers the highest thermal comfort, energy savings, and seismic resistance.Such studies should also include the building location and its scenarios of occupation and operation.Last, the role of straw as a regulator of indoor air moisture is still insufficiently documented, even though straw building contractors sometimes advertise this characteristic.Future studies should take into account the variability of the results, due to the changing nature of the thermo-physical properties of this bio-based material over time.Based on the energy results, the economic feasibility of straw buildings should be more intensively researched, especially in terms of payback period and life cycle cost.Those research steps are crucial to foster private and public interest in straw construction.

Figure 1 .
Figure 1.Experimental and numerical articles on straw bale construction between the years 1990 and 2020 [3].

Figure 2 .
Figure 2. Different parts of a wheat stalk.

Figure 1 .
Figure 1.Experimental and numerical articles on straw bale construction between the years 1990 and 2020 [3].

Figure 1 .
Figure 1.Experimental and numerical articles on straw bale construction between the years 1990 and 2020 [3].

Figure 2 .
Figure 2. Different parts of a wheat stalk.Figure 2. Different parts of a wheat stalk.

Figure 2 .
Figure 2. Different parts of a wheat stalk.Figure 2. Different parts of a wheat stalk.

Figure 3 .
Figure 3. (a) Transversal cross-section of an internode wheat straw using a digital microscope.(b) SEM analysis of a wheat straw shell [10].

Figure 3 .
Figure 3. (a) Transversal cross-section of an internode wheat straw using a digital microscope.(b) SEM analysis of a wheat straw shell [10].
Energies 2022, 15, x FOR PEER REVIEW 6 of 8 minimizing technologies, excluding onsite renewable energy generation.Currently, one of the tallest straw buildings in the world is a seven-story apartment building including 15 residential units of 90 m 2 floor area each, located in St. Die des Vosges, France.The Jules Ferry building is labeled "passive house", with a heating load lower than 15 kWh/m 2