<p>In the present study, natural materials, namely, bulrush (water plant) and wheat straw have been characterized to evaluate their potential as thermal insulators for building envelopes via examining their physical and thermal properties. For the characterization of the samples, techniques including X-ray diffraction, scanning electron microscope (SEM), pycnometer method, Archimedes method, thermogravimetric analysis (TGA), and a transient line heat source thermal conductivity measuring approach (KD2 Pro) were employed. The results revealed the density of individual fibers of bulrush and wheat straw to be 0.123 and 0.340&#xa0;g/cm³, respectively. Additionally, the corresponding porosity values of the straw fibers were found to be 92.15 and 74.23%. Furthermore, it was observed that up to the temperature of 162&#xa0;°C, bulrush is thermally more stable than wheat straw. Also, bulrush exhibited lower values of overall thermal conductivity (<i>k</i>-value) for varying densities owning to higher porosities. The study concludes that the investigated materials show promising prospect as potential thermal insulators owing to their low thermal conductivity (<i>k</i> = 0.033–0.050 Wm⁻¹&#xa0;K⁻¹), minimal environmental footprints, and comparable performance with conventional insulating materials.</p>