Solar Reflectance, or albedo, refers to the amount of solar radiation reflected off a surface, compared to the amount of radiation absorbed through the surface.
Radiation Absorbed – Radiation Reflected = Solar Reflectance
A typical black membrane roof absorbs solar radiation during the day as the temperature rises, and then re-radiates the absorbed heat at night as the temperature drops. This absorption cycle results in large temperature fluctuations of the roof membrane and large amounts of heat flow through the building. In the previous Evaporation blog, we referenced a study conducted by the National Research Council of Canada (NRCC) and illustrated the difference between black membrane temperature fluctuation and green roof temperature fluctuation explaining how green roofs help lower ambient air temperature.
Heat flux is measured by the amount of positive heat flow entering a roof, when it is hot and negative heat flow leaving the roof, when it is cold. The NRCC study also measured heat flux of a black top membrane, compared to a green roof. Their findings showed a median daily temperature fluctuation of 81° F in the black membrane reference roof and only an 11°F fluctuation in the green roof. (http://www.nrc
The degree of a building’s heat flux directly relates to the buildings cooling demand. The figure below illustrates the energy demand of a building due solely to Heat Flow through the roof.
Clearly, the green roof significantly reduces energy demands on a building in hot, summer months. This cooling effect is due to evapotranspiration shading, and thermal mass (Thermal mass is a solid or liquid that will absorb and store warmth or coolness until it is needed). During the warmer months, the daily average energy demand on the black membrane roof was 6.0-7.5 kWh/day (20,500-25,600 BTU/day). On the other hand, average energy demand of the green roof building was only 1.5 kWh/day (5,100 BTU/day). That is over a 75% reduction in energy use due to heat flow.