The materials selected for the construction of our experimental samples include fired clay bricks (dimensions: 25 cm × 15 cm × 7 cm), raw earth (clay) mortar, and a plaster composed of clay mixed with sand.

For insulation purposes, reed mat and rice straw were chosen. The specific configurations of these materials define the different samples as follows:

Sample 1: wall constructed with fired clay brick and finished with a cement mortar coating (serving as the control);

Sample 2: wall constructed with fired clay brick, incorporating a reed mat, rice straw, another reed mat, and finished with a clay coating;

Sample 3: uninsulated sheet metal used at the roof level;

Sample 4: sheet metal at the roof level, with rice straw and reed mat as insulation layers.

The densities of the different materials were measured in the laboratory of the National Advanced School of Public Works (ENSTP) in N’Djamena, Chad, following the NFP18-554 standard. The main equipment employed included a 500 ml graduated burette, an electronic balance, and a tare. The process for determining the volumes and masses of the materials under investigation involved several sequential steps.

Figure 1(a) , Figure 1(b) display images illustrating the mass measurements of samples taken from a fired clay brick and rice straws.

Volumetric measurements were conducted on the materials under investigation, namely rice straw, reed, and fired clay brick. The procedure began with the use of a graduated cylinder filled with 300 ml of water. Subsequently, pre-weighed samples of each material were sequentially introduced into the cylinder for measurement. The actual volumes of the materials were determined based on the observed changes in water level. The complete set of these experiments is illustrated in Figure 2 . It is important to note that measurements should be taken immediately after immersion to minimize water adsorption by the materials.

The total volume, $V_{total}$, as given by Equation (1), represents the sum of the initial water volume and the volume of the immersed material, $V_{material}$, which is to be measured.

$V_{total}=V_{initial,water}+V_{material}$(1)

The volume of the unknown material, $V_m$, is calculated by subtracting the initial water volume, $V_{iniatial}$, from the total measured volume.

$V_{material}=V_{total}-V_{initial,water}$ (2)

The density of a solid substance is determined by the ratio of its mass to its volume. The mass was measured in the initial step using a balance. The volume of the material can be obtained using Equations (1) and (2). Once the volume is known, the density of the solid is calculated as the ratio of its measured mass to its volume.

$\rho =M_{massofthematerial}/V_{volumeofthematerial}$ (3)

$\rho$: denotes the apparent density of the material (Kg/m³).

$M_{massofthematerial}$: denotes the mass of the solid (Kg).

$V_{volumeofthematerial}$: denotes the volume of the solide (m³).

Experimental tests were performed on a test cell constructed from fired clay bricks and topped with a sheet metal roof (see Figure 3 ). Among the four vertical walls and the horizontal roof of the cell, two vertical walls and one section of the sheet metal roof were selected as representative samples, with and without insulation, as follows:

Sample 1: Fired clay brick wall finished with cement mortar plaster (reference sample);

Sample 2: Fired clay brick wall layered with reed mat, rice straw, another reed mat, and finished with clay plaster;

Sample 3: Uninsulated sheet metal roof;

Sample 4: Roof assembly comprising reed mat, rice straw, and sheet metal.

To provide space for the installation of insulation materials, chevron-shaped templates were fabricated and positioned 14.5 cm from the cell's interior surface along the three walls, enabling the secure placement of reed mats (see Figure 4(a) ). These templates serve to regulate the total thickness of the insulation layer, which includes both the insulating material and the plaster finish. The use of these templates significantly streamlines the process for subsequent construction stages. Figure 4(b) illustrates the insulation configuration for sample 2.

In addition to the use of templates, several supplementary techniques were employed to ensure the planar alignment of the wall surfaces. Specifically, wall-leveling strings were utilized to monitor and maintain the flatness of the template throughout its installation process. The lower-level templates were installed at a minimum height of 15 cm above the finished floor surface ( Figure 4(a) ). Battens or shims were attached to the joists at intervals of 80 cm to guarantee consistent spacing between the templates and the wall surfaces, thereby also contributing to the overall stability of the insulation system ( Figure 4(a) ).

Figure 5(a) and Figure 5(b) illustrate, respectively, the interior wall surfaces of the cell after the application of plaster over the reed mats, and the positioning of rice straw between the reed mats, with one mat affixed to the wall and the other to the templates coated with clay plaster, as exemplified by sample 2.

Figure 6(a) and Figure 6(b) illustrate the roofing configurations. In Figure 6(a) , reed mats are installed on the underside of the roof, functioning as a support layer for the rice straw, as shown in Figure 6(b) . This configuration pertains to sample 4.

Temperature measurements were systematically conducted on both the interior and exterior surfaces of the four samples. In this study, the progression of thermal gradients was monitored by recording temperatures on each surface throughout a full day (see Figure 7(a) ). The primary aim was to elucidate the mechanisms of heat transfer across each wall, regardless of whether it was thermally insulated. Such an approach enables the assessment of the thermal insulation properties of the walls. Additionally, long-term reliability and resistance to environmental factors are essential characteristics, particularly since these materials are protected from adverse weather conditions such as rainfall. It should be noted that exterior walls primarily serve as protective barriers, acting as the first line of defense to limit heat exchange between the interior and exterior environments of a room. Investigation of their thermal inertia thus necessitates monitoring the evolution of ambient air temperature in contact with the internal surface. Accordingly, changes in the internal temperature of the test cell were tracked using an Elitech rc-5 temperature data logger (refer to Figure 7(a) , Figure 7(b) ). This device was installed on both the internal and external surfaces of the walls to record temperature variations for samples 1, 2, 3, and 4 examined in this study.

This parameter is among the most significant in construction-oriented studies that seek to assess the thermal inertia of a wall. Accurate determination of the thermal phase shift is essential during the experimental stage. The resulting data will subsequently be compared to those stipulated in the 2012 Thermal Regulation. In general, the thermal phase shift corresponds to the time interval between the peak of an external thermal load and the moment when this peak equivalently affects the building’s interior within a 24-hour period. This metric reflects the capacity of a material to delay heat transfer. Denoted as D, it is calculated using the following formula:

$D=t_{T_{ext\_max}}-t_{T_{int\_max}}\left[h\right]$ (4)

where

$D=\frac{\sqrt{T}}{e\times \sqrt{\Pi }}\times e\times \sqrt{\frac{\rho C}{\lambda }}$ (5)

Here, T represents 24 hours, equivalent to 24 × 3600 seconds; e denotes the thickness of the material, ρ is the density, and C is the specific heat capacity.

$C_p=\frac{Q}{m\Delta T};avecQ=\phi \times \Delta t$ (6)

In this context, Q denotes the amount of heat, m is the mass of the body in kilograms, and ∆T is the temperature difference in kelvin. φ represents the heat flux per unit area, and ∆t is the duration over which the heat quantity is calculated, taken as one hour in this study.

To determine the thermal conductivities of the materials investigated in this study, two empirical methods were selected: those proposed by Elangovan R. and Marie Viel [16] . The absence of suitable measurement equipment necessitated the adoption of these approaches. Specifically, the Marie Viel method is utilized for insulating materials with low bulk densities (<500 kg/m³), and it yields results that are comparable to the λ conductivity values reported in other experimental studies.

Following the determination of the thermal conductivity for each material and the measurement of the exterior and interior temperatures of the walls—used as initial conditions, numerical simulations were performed to ascertain the temperatures at the inner surfaces and at the interfaces of each material layer composing the walls or samples.

Table 1 displays the results of the temperature evolution measurements conducted over a 24-hour period on both the internal and external surfaces of the samples.

The density values for rice straw, reed mats, and fired clay bricks are presented in Table 2 . Among these, fired clay brick exhibits the highest density. Generally, a higher density corresponds to lower insulation performance in materials.

Table 3 presents the thermal conductivity values obtained for the various materials investigated in this study. For rice straw, the measured thermal conductivity is λ = 0.041 W/mK, which closely aligns with the value reported by Valin Colson in 2019 for bio-based fiber insulation panels and related techniques [17] . It is noteworthy that the value obtained in the present study is lower than that reported by Mariette T. ADAGBE in her 2021 dissertation, which focused on using earth reinforced with rice straw stems as a structural material for load-bearing elements in buildings reinforced with rônier [14] . However, the thermal conductivity values determined in this work differ significantly from those reported by DOUZANE et al., 2016 [18] , who demonstrated that the thermal conductivity of rice straw bales depends on their orientation relative to the direction of heat flow (parallel or perpendicular). Nevertheless, the conductivity measured in this study is close to the reference value of 0.04 W/mK established by the 2024 Thermal Regulation. It can therefore be concluded that rice straw exhibits good thermal insulation properties, with a conductivity nearly equal to the average value for insulating materials, which is 0.04 W/mK.

For reed mat, the measured thermal conductivity is 0.071 W/mK (see Table 3 ), which exceeds both the regulatory threshold of 0.04 W/mK set by the 2024 Thermal Regulation and the value of 0.05 W/mK cited by architect J-M Pupile, DPLG [19] . This indicates that, while reed also functions as a thermal insulator—similar to rice straw—it serves more as a supplementary material in this study.

In contrast, fired clay brick exhibits a thermal conductivity of 0.51 W/mK, which is nearly nine times greater than the regulatory limit of 0.04 W/mK according to the 2024 Thermal Regulation. This high conductivity indicates that fired clay brick acts as a thermal bridge, allowing significant heat transfer.

The ability of a material to withstand heat conduction is quantified by its thermal resistance. A higher thermal resistance value indicates superior insulating properties. As shown in Table 3 , among the materials tested, rice straw exhibits the highest thermal resistance for the thicknesses considered. Specifically, with a thickness of 14.5 cm, rice straw achieves a thermal resistance of 3.53 m²K/W, significantly exceeding the minimum recommended value of 2.9 m²K/W for building insulation. Given that the straw layer will be combined with additional layers to form an insulating wall assembly, it can be regarded as possessing excellent thermal capacity. It is important to note that the thickness of the reed mat in this study is limited to 2 cm; increasing its thickness could further enhance its thermal performance. Thus, the reed mat can also be considered to have good resistance to heat conduction. In contrast, fired clay brick demonstrates a thermal resistance of only 0.294 m²K/W, which is at least nine times lower than the minimum recommended threshold of 2.9 m²K/W. Consequently, fired clay brick can be characterized as having a very low thermal resistance.

Sample 1 (control) and Sample 2 (insulated)

The simulations were conducted to determine several parameters, including thermal insulation, hygrometric conditions, and summer thermal comfort. The primary indicators for thermal comfort are the attenuation of thermal amplitude and the thermal phase shift.

Thermal resistance, temperature profiles, and thermal conductivities.

Table 4 summarizes the thermal properties, including thermal conductivity (λ), thermal resistance (R), and the minimum and maximum temperatures recorded across the various layers of Sample 1 (uninsulated control) and Sample 2 (insulated). Notably, significant temperature variations are observed. In Sample 1, temperatures range from 45˚C to 42.2˚C, with a thermal resistance of only 0.346 m²K/W—substantially below the minimum recommended threshold of 2.9 m²K/W. This finding confirms that the wall acts as a significant thermal bridge. In contrast, analysis of Sample 2 reveals that the thermal conductivity of the insulating materials ranges from 0.04 to 0.065 W/m·K, with the average remaining below the recommended limit of 0.06 W/m·K. Moreover, in Sample 2, the temperature difference between the exterior and interior spans from 45˚C to 31.5˚C, exceeding 10˚C. The equivalent thermal resistance of Sample 2 is 4.22 m²K/W, which closely aligns with the 4 m²K/W benchmark for Low Energy Consumption Building (BBC) walls as specified by the RT 2024 (Thermal Regulation 2024). These results indicate that Sample 2 provides a high level of thermal comfort.

*Thermal resistance according to DIN 6946 for U-value calculation. For moisture protection and temperature profile, Rsi = 0.25 and Rse = 0.04 were used in accordance with DIN 4108-3. (a) Interior surface temperature (min/med/max): 42.2˚C/42.2˚C/42.2˚C, Exterior surface temperature (min/med/max): 44.5˚C/44.5˚C/44.5˚C; (b) Interior surface temperature (min/med/max): 31.5˚C/31.5˚C/31.5˚C, Exterior surface temperature (min/med/max): 44.9˚C/44.9˚C/44.9˚C.

Transmittance Coefficient, Hygrometry, and Summer Comfort

The thermal insulation property, indicated by the U-value (expressed in W/(m²·K)), also referred to as the surface transmission coefficient, is presented for samples 1 and 2. For sample 1 (the control), the U-value is 2.89 W/(m²·K), which significantly exceeds the recommended threshold of 0.24 W/(m²·K). A lower U-value signifies superior thermal performance of the wall. Therefore, the high U-value of 2.89 W/(m²·K) for sample 1 demonstrates its poor insulating capability, as it allows heat to transfer from the exterior to the interior more rapidly. In contrast, the insulated sample 2 achieves a U-value of 0.24 W/(m²·K), meeting the recommended standard and indicating effective thermal insulation. Regarding hygrometric performance, both samples 1 and 2 exhibit satisfactory results. However, thermal comfort is notably enhanced in sample 2 compared to sample 1.

Sample 3 (Uninsulated Roof) and Sample 4 (Insulated Roof)

Thermal Resistance, Conductivity, and Temperature Measurements

*Thermal resistance according to DIN 6946 for U-value calculation. For moisture protection and temperature profile, Rsi = 0.25 and Rse = 0.04 were used in accordance with DIN 4108-3. (a) Interior surface temperature (min/med/max): 51.8˚C/51.8˚C/51.8˚C, Exterior surface temperature (min/med/max): 53.9˚C/53.9˚C/53.9˚C; (b) Interior surface temperature (min/med/max): 37.3˚C/37.5˚C/37.9˚C, Exterior surface temperature (min/med/max): 55.8˚C/55.8˚C/55.8˚C.

Table 5 provides the thermal conductivity values for samples 3 and 4, which are 0.750 W/m·K and 0.04 W/m·K, respectively. For reference, the recommended value according to RT 2012 (Thermal Regulation 2012) is 0.06 W/m·K. In terms of temperature variation, the measured temperature above the sheet metal under the rafter decreases from 54˚C (due to overheating) to 51˚C. The equivalent thermal resistances for samples 3 and 4 are 0.865 m²K/W and 4.192 m²K/W, respectively, with the regulatory limit for roofs set at 3.3 m²K/W according to RT 2012. These results indicate that the sample demonstrates effective thermal insulation.

Transmittance Coefficient, Hygrometry, and Summer Comfort

The parameters relating to thermal insulation, hygrometry and summer thermal comfort characterising the uninsulated and insulated roofs corresponding to samples 3 and 4 respectively show that the thermal insulation value U of the uninsulated roof is 1.6 W/(m²K), higher than the recommended limit value of 0.24 W/(m²K). In sample 4, on the other hand, it is 0.24 W/(m²K). In view of this parameter, the sheet metal roof, insulated with rice straw and reed matting, acts as an insulator. The latter offers better thermal insulation and summer comfort than sample 3.