MythBusters

MYTH 6:  Green roofs are good insulators

By and large, green roofs are poor insulators.  Most of the time, green roofs are heat capacitors; that is, they can absorb and release energy.  The effectiveness of green roofs in managing energy is related primarily to two phenomena: 1) heat capacitance, and 2) latent heat of evaporation and freezing. 

Energy benefits will be reaped in climates where latent heat processes dominate – particularly cold climates and hot climates (as opposed to temperate climates).   Work conducted by the National Research Council (Institute for Research and Construction, Ottawa) concluded that green roofs that are 9 inches, or thicker, can begin to have an important benefit in northern latitudes.  With these moderately deep green roofs, roof temperatures in winter can be maintained at around 32 deg. F as long as some free water remains in the profile.  In southern latitudes, evaporation of water from leaf surfaces (called transpiration) can have a large influence on reducing surface temperatures. 

Because green roofs are not good insulators, they cannot be described accurately using ‘R’ values.  However, in order to incorporate a green roof in building envelope analyses assigning an effective R value may be essential.  Effective R values can be back-calculated from simulations and will apply over the specific range of conditions used in the simulation.

The most comprehensive work in this area has been done by the Institute for Environmental Studies (Environment Canada) and by the Technical University of Berlin.  We encourage interested parties to contact Dr. Brad Bass at the Institute for Environmental Studies to learn more.

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