Seasonal heat flux properties of an extensive green roof in a Midwestern U.S. climate

Green roofs, or vegetated roofs, can reduce heat flux magnitude through a building envelope as a result of insulation provided by the growing medium, shading from the plant canopy, and transpirational cooling provided by the plants. This study quantifies the thermal properties of an inverted 325 m2 retro-fitted extensive green roof versus a traditional gravel ballasted inverted roof in a Midwestern U.S. climate characterized by hot, humid summers and cold, snowy winters. In autumn, green roof temperatures were consistently 5 °C lower than corresponding gravel roof temperatures. Even during chilly and moist conditions, the heat flux leaving the building was lower for the green roof than the gravel roof. Temperatures at the top of the insulation layer were more variable for both green roof and gravel roof on winter days with no snow cover than on days with snow cover. Variation in temperatures between roof types in spring was similar to those in autumn. Peak temperature differences between gravel and green roof were larger in summer than other seasons (sometimes by as much as 20 °C). Over the course of a year (September 2005–August 2006), maximum and minimum average monthly temperatures and heat fluxes were consistently more extreme for the gravel roof than the green roof.

► Extensive green roof in Michigan reduced building heat flux by 13% in winter and 167% during summer.
► Summer cumulative monthly heat flux showed a net heat gain into the building for the gravel roof while the green roof showed a cooling effect on the building.
► Under quasi-steady state conditions the ratio of the temperature difference to the heat flux, K = (Tinside ceilling − Toutside roofsurace)/Q, exhibited a numerical value that was nearly identical across all four seasons (K ∼ −45 m2 K/W).