An updated and expanded set of thermal property data for green roof growing media

Vegetated (green) roofs alter the roof surface energy balance and hence affect both building energy consumption and the transport of heat into the environment. Quantitative evaluation of the energetics of green roof systems requires accurate knowledge of the moisture-dependent thermal properties of the growing media. To support this need for data and to supplement previously published data we conducted a laboratory study to measure thermal conductivity, volumetric heat capacity, and thermal diffusivity of 12 green roof soil samples of varying composition. The results indicate that thermal properties vary significantly as a function of growing media design. Growing media incorporating expanded slate as their aggregate had thermal conductivities that were two to three times those of media that used a porous silica-based aggregate. Media incorporating expanded clay as the aggregate had thermal conductivities roughly in the middle of these extremes. In general the thermal conductivity nearly tripled as the growing media moisture levels were increased from relatively dry to saturated. Also, it was found that compaction typical of green roof systems that have been installed for multiple seasons can increase thermal conductivity of moist soils by 30–40% over their uncompressed values.

► Moisture increases green roof soil thermal conductivity by up to a factor of three.
► Soil compaction can lead to a substantial increase in thermal conductivity.
► Green roof aggregate choice can double thermal conductivity.