Hi, we are working on a project with green roofs in climate 3b. According to www.coolroof.org “green roofs do provide similar energy savings and urban heat island mitigation benefits as cool roofs” so we are pursuing to comply with the U factor assembly specified for insulation entirely above deck (U-0.466) in table 5.5.3.1 “High Albedo Roof Insulation”. The real assembly factor of our green roof is U-0.385, better than the requirement in table 5.5.3.1 but worst than the value specified in table 5.5-3 = U-0.273. Our question is:

1. It´s OK to simulate green roof as a high albedo roof in the energy simulation model?
2. In case we are able to simulate our green roof as a high albedo roof, the baseline building model simulation according to ASHRAE 90.1-2007 should have a roof with a U Factor of 0.466? Or should we use the U factor specified in table 5.5-3?

We think that if we use the roof characteristics specified in Appendix G for the baseline model (Roof entirely above deck U-0.273, and reflectivity of 0.30), performance during winter will be worst, due to the gap between the U-factor requirements from table 5.5.3.1 “High Albedo Roof Insulation” and table 5.5-3. What should we do?