fibreglass bats are 3.0 per inch (0.05 W/m K). Straw bale buildings are thermally efficient and energy conserving, with R-values significantly better than conventional construction, depending on the type of straw and the wall thickness [21]. The insulation value for the straw bale walls was estimated in [22]. The R-value for the straw bale walls was R-44 (0.04 W/m K). The thermal prop- erties of sustainable earth materials using a novel thermal probe technique involving an iterative data analysis method for simulta- neously determining the thermal conductivity and diffusivity were discussed in [23,24]. The viability of using coconut fibre as a thermal insulation for buildings was explored by conducting thermal conductivity tests on 200 × 400 × 60 mm slab specimens. The ther- mal conductivity was 0.058 W/m K which occurred at an optimum density of 85 kg/m3 at 38 ◦ C [25]. Hot wire methods are mostmonly used to measure the thermal conductivity of “refractories” such as insulating bricks. Because it is and basically a transient radial flow technique, isotropic specimens are required. The technique has been used in a more limited way to measure properties of liquids and plastic materials of relatively low thermal conductivity. The hot wire method was established to meet the test methodologies specified in the American Society for Testing and Materials (ASTM) Standard Test Methods C 1113 [26]. Within the framework of here presented investigations, the earth bricks were stabilized and thermally improved by the addition of natural fibres. The fibre addition positively improves not only thermal but also mechanical properties. The investigations regarding the mechanical properties will be presented in a future paper. This article presents the results of an investigation into the thermal conductivity of earth bricks made from earth, cement, and gypsum reinforced with wheat and barley straw natural fibres with different mixing ratios