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Air Conditioning

Air conditioning is a major factor in summertime electricity consumption and costs for industrial, commercial and residential users, and can cause significant peak demand and peak distribution problems for utilities. Data we have collected at the Penn State Center for Green Roof Research suggests that green roofs can significantly reduce air conditioning costs to the consumer and most likely reduce the associated peak demand on the utility. This occurs because evapotranspiration from greenroof plants and media, cools the roof surface. Figure 1 illustrates the effects of the plants on roof surface temperatures. The surface of the roof (immediately above the waterproofing membrane) of non-green roofs reached nearly 70C during the week of June 24 to June 30, 2002. In contrast the waterproofing surface under the greenroof peaked at about 30C, several degrees less than the ambient air temperature. Figure 2 shows another way to look at the same response.

On a different date, with an air temperature of 90F, the exposed black plastic surface reached 145F while the surface of the sedum plants (leaf temperature) only reached 82F. We have used this data to examine urban heat island implications of the use of green roofs.  The energy balance model for greened compared with white roofs suggests that greened roofs have ‘equivalent albedos’ in the range of 0.7-0.85 (Gaffin et al, 2005). Of course reduced thermal loading results in a cooler interior temperature for the associated building (Figure 3). The average interior temperature for our non-green roof buildings during the period in June shown in figure 1 exceeded 40C while the interior temperature of the greenroofed buildings was much lower, only slightly above ambient for most of the period. In an air conditioned building, this of course means less electricity is required for air conditioning. For example during the month of August our small buildings with greenroofs in Rock Springs, PA consumed about 10% less electricity than those with flat black roofs (Figure 4). Total air conditioning savings of course vary with the season and climate, buildings in areas with higher cooling requirements are likely to have even greater savings.

Stuart Gaffin, Cynthia Rosenzweig, Lily Parshall, David Beattie, Robert Berghage, Greg O’Keeffe, Dan Braman. 2005. Energy Balance Modelling Applied to a Comarison of White and Green Roof Cooling Efficiency. In Proc. Green Roofs for Healthy Cities, Washington, DC