The project is planning the installation of high albedo and vegetated roof to cover 93.1% of the roof area (the remaining roof area consists of parapets and skylight openings). The high albedo roof consists of a membrane covered with white calcite ballast. The team had previously requested a CIR to ascertain that tests could be made on this ballast to ensure conformity with the reflectance and emissivity requirements to achieve this credit. The tests have been performed by a qualified laboratory as suggested in your CIR response dated 05/12/03. The results indicate that the emissivity criterion is respected but that the reflectance is .59 rather than the stipulated .65. According to previous credit interpretation rulings, namely CIR SS7.2 dated 1/21/2002 and CIR SS7.1 dated 12/05/2001, the lower reflectance can be compensated for by enlarging the area covered according to the formula specified in CIR SS7.1 dated 12/05/04. This would result in the following calculation: Required .65 x 75% + .05 x 25%=50 Design case .59 x 83.3% + .05 x 16.7%=50 Therefore in installing 83.3% of high albedo roof instead of 75%, the requirement would be satisfied. The question is the following; the team could increase the high albedo roof and reduce the vegetated roof to achieve 83.3% of high albedo roof (the roof composition is currently split as follows 78% high albedo and 15.1% vegetated roof for a total of 93.1% roof coverage of a combination of high albedo and vegetated roof). Reducing the quantity of vegetated roof however would reduce the amount of green roof available for occupant use, water retention, CO2 gas absorption and vegetation. Since the intent of the credit is to reduce heat island effects and that LEED 2.1 allows for a combination of 75% high albedo and vegetated roof , we would propose that covering the roof with 93% combination high albedo and vegetated roof would meet the intent. This provides an additional 10% roof coverage (93.1% versus 83.3%) with a combination high albedo and vegetated roof that would be required if all high albedo roof were utilized as shown in the design case equation above. Would this be acceptable?
Per CIR SSc7.1 dated 12/05/01, 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 it covers (75% min.). This calculation results yields a baseline reflectivity average of 0.4875. If the project\'s calculation results in a value greater than .4875 then the overall reflectance criteria would be satisfied. From the information and calculations provided in this inquiry, some misinterpretations of the calculation methodology were identified. Since skylights, parapets and equipment are excluded from the calculation, the only portion of the roof under consideration is the area covered by the membrane and the vegetated roofing. For calculation purposes, this area represents 100% of the roofing. The percentage of roof area covered by the membrane can be adjusted to reflect its relative size in comparison to the vegetated roofing. Based on 100% coverage, the adjusted coverage percentage for the ballasted roofing would be 83.7% (78% divided by 93.1%). Consequently, the vegetated roof area percentage would be adjusted accordingly and represents 16.3% of the applicable area. Using the white ballast reflectance of .59 and multiplying it by 83.7%, results in an average weighted reflectance of .4938. This value exceeds the minimum weighted average of .4875 and thus you have achieved the reflectance portion of the requirements. Applicable internationally.