Thermal property measurements for ecoroof soils common in the western U.S.

To model the impacts of ecoroofs on building envelope heat transfer accurately, thermal property data for ecoroof soils are needed. To address this need we have measured thermal conductivity, specific heat capacity, thermal emissivity, short wave reflectivity (albedo) and density for ecoroof soil samples over a range of moisture states. To represent a wide range of commonly used ecoroof soils we created eight test samples using an aggregate (expanded shale or pumice), sand, and organic matter in varying volumetric composition ratios. The results indicate significant variability in properties as a function both of soil composition and soil wetness. Thermal conductivity ranged from 0.25 to 0.34 W/(m K) for dry samples and 0.31-0.62 W/(m K) for wet samples. Specific heat capacity ranged from 830 to 1123 J/(kg K) for dry samples and 1085-1602 J/(kg K) for wet samples. Albedo was consistently higher for dry samples (0.17-0.40) decreasing substantially (0.04-0.20) as moisture was added. Thermal emissivities were relatively constant at 0.96 ± 0.02 regardless of soil type or moisture status. These results are discussed in the context of their impacts on building energy consumption and the importance of including daily and seasonal property variation within models of the ecoroof energy balance.