What kind of R-Value does a roof garden have?
In a completely dry state, the R-Value of a typical 4″ deep Carlisle roof garden is approximately 6. However, the higher the moisture content of the assembly, the lower the R-Value, as thermal conductivity increases. The massive cooling savings realized from a roof garden are a result of the plants’ evapotranspiration. Plants function as small water pumps operating at high pressure and low volume. When materials experience a phase change from liquid to vapor, they absorb a large of amount of heat energy from the surrounding environment. In the case of water, every gallon transpired by the plants absorbs roughly 8,000 BTU’s of heat energy. As a result, during hot summer days, the roof membrane temperature is typically 5-10°F. cooler than the ambient air temperature.
