Agave (AF) and wheat straw (SF) fibers are good raw materials for thermal insulation. In this study, loose agave or wheat straw fibers and hybrid boards of agave/wheat straw with different compositions and densities are tested for their thermal insulation qualities. Three new novel hybrid specimens are considered. In all specimens, cornstarch (CS) was used as a binder for the fibers. Thermal conductivity coefficient is obtained for each specimen in the temperature range 10–60°C with the resulting average value in the range 0.04555–0.06835 W/m K. Without binding, loose fibers of agave and wheat straws have an average thermal conductivity of 0.043592 and 0.044678 W/m.K, respectively, at the same range of temperature. Micrographs of the wheat straw fibers (SF) are conducted using the scanning electron microscope (SEM) which showed an average outer diameter between 1.28 and 1.98 mm and an average diameter of the bound porous tubes (multicellular) within the straw in the range 34.5 to 75.6 μm. Fourier transformation infrared (FT-IR) analyses of the wheat straw fiber are carried out which indicated stretching wavenumbers which present different chemical functional groups. Thermogravimetric analysis of the wheat straw fiber (TGA and DTGA) showed that the straws lose less than 3% of its mass at temperatures as high as 184°. Finally, the differential scanning calorimetry (DSC) analysis test of the wheat straw fiber indicates that endothermic transition starts at 342°C with a peak value at 749°C. Three-point bending moment tests for bound specimens are made and show an increase in both flexural stress and flexural modulus of the hybrid specimens. Using these hybrid specimens, agro, renewable and environmentally friendly materials in buildings will share in energy saving when used as insulation materials for building walls.