How much additional insulation does a green roof provide?
I posed this question to two green roof experts that I am working with on an upcoming project. Their responses were consistent and somewhat surprising.
The short answer: NONE.
Green roofs provide innumerous benefits including shading by foliage, retarding heat transfer by advection (transfer of heat or matter by the flow of a fluid), providing thermal mass effect, prompting evapotranspiration, increasing stormwater retention, and protecting waterproof roofing assemblies.
However, a green roof does not provide additional insulation.
Completely dry growing media (the “dirt” of the green roof, so to speak) has a minimal R-value (typically about 0.5/inch). Plants need water to grow, so the green roof is designed to retain some degree of moisture. There is almost always some amount of water existing in a green roof and that moisture prompts a highly accelerated thermal energy transfer via advection. Thus, one can not really count a green roof assembly towards a roof’s total R-value.
It appears to be one of those cases where inaccuracies become perpetuated so much over time that they eventually become accepted as truths. The essence of the confusion lies in the difference between insulation and thermal mass. Rather than keeping heat from migrating through the roof assembly, the green roof acts as a heat sink that soaks up and retains thermal energy.
The thermal mass effect is the most important energy-related benefit of a green roof. The saturated growing media and vegetation has the ability to absorb and store considerable amounts of heat. This has the effect of dampening the effect of extreme outdoor temperatures.