Our residential project is comprised of two multi-story towers that are connected on the ground floor with a single-story atrium that is sandwiched between the two towers. The sum of the two tower roofs equals 17,586 s.f. and makes up 70% of the total roof area. In order to comply with this credit and reduce heat islands, the two tower roofs are specified as a white TPO membrane that meets the emissivity and Energy Star requirements. We could easily achieve the credit requirement of 75% of the roof area by specifying white membrane for 1,254 s.f. of the 7,484 s.f. lower roof. However, in this case, we feel that having a high albedo lower roof would actually conflict with the intent of the credit. 97% of the curtain wall is glass, thus the towers would be subject to heat gain because the solar reflectance would project back up into the buildings. In addition, all of the residential units that face the atrium will have direct views of the roof which would expose the residents to glare. We propose to install a gray TPO membrane on the lower roof. This would not produce glare or contribute to as much heat gain as the white membrane, but still have a higher emissivity level than a standard black membrane. Given the circumstances, would you award us credit if the upper roofs are white and the lower roof is gray?
Per the CIR Ruling for SSc7.1 dated 12/05/01, you may perform a weighted average calculation for the reflectivity. Alternatively, you may apply the weighted average for emissivity instead of reflectivity (but allowing both would be too lenient). For example, the reflectance minimum weighted average can be calculated by applying the minimum reflectance of the roof (.65) and multiplying it by the percentage of roof coverage required (75% min.). This calculation results yields a baseline reflectivity average of 0.4875. For this example, each surface must comply with the emissivity requirement independently. If the average reflectance for your total roof area are above the minimum weighted averages, the overall criteria would be satisfied. NOTE: depending on the building orientation, reflection off the curtain walls could result in significant heat gain to the single story atrium. You might consider integrating a green roof over the single story atrium which would provide a pleasant view for the residents, eliminate the glare issue, and meet the intent of the credit.