The Martian surface has several regions where thermal inertia measurements indicate a porous ice‐free insulating surface, yet are mechanically competent enough to sustain substantial slopes. In support of the interpretation of those regions within the Martian polar layered terrain, we report measurements of thermal conductivity for loess from the field and in the USA CRREL Permafrost Tunnel. Permafrost Tunnel loess is a desiccated material that can form vertical walls, but is of low density (800–1000 kg/m³), modest shear strength (4 kPa), and has a low thermal conductivity (0.1 W/m‐K at 1 bar). These properties are similar to the inferred properties of the Martian polar layered terrain. The Birch Hill field sample has a density of 1160 kg/m³ and a conductivity of 0.15 W/m‐K. The Chena Spur Road sample has a density of 1360 kg/m³ and a conductivity of 0.7 W/m‐K. The relatively high conductivity for the Chena Spur Road is due to the cementation of soil grain contacts, its higher density, coarser grain size, and higher quartz grain content.