Rating System

LEED BD+C: New Construction, LEED O+M: Existing Buildings, LEED BD+C: Core and Shell, LEED BD+C: Schools

Rating System Version

v3 - LEED 2009, v3 - LEED 2008, v2 - Schools 2007, v2 - LEED 2.2

Inquiry

The project is a multiple building (campus) submission, it is for 2 1/2 million square feet of office, amenity and associated support functions. The flat roofing type selected by the team for the majority of roof area is a Protected Roof Membrane assembly, which consists of a roofing membrane on the roof slab, covered by rigid insulation, which is then covered by medium weight (17-20 PSF) gravel ballast. The design and construction team was unable to find "white coated gravel" listed in the reference guide for light colored ballast that complies with SRI requirements for the point. The Oak Ridge National Laboratories published a four-year study of ballasted roofing assemblies that indicated that medium weight ballasted roofing systems performed as well or better than white roof systems for reducing the heat of the roofing membrane, and recommended that these systems be considered "cool roof" systems as well. The study also noted that these values were constant over a four year period, while the temperature reduction values for the white membrane roof degraded within the first year to a much lower value. We are aware that the study focused on the thermal performance of the system specifically related to heat transfer through the ballast to the membrane, but since the study showed that approximately the same (or less) heat got to the membrane than on a white (SRI compliant) roof at peak, we conclude that the rest of the heat is being rejected back into the air similar to the performance of the white roof. We spoke to the primary investigator on the study, Andre Desjarlais from ORNL, and he said the following: "What the ballast does is redistribute the temperature profile. Unlike a cool roof that simply reflects the solar load away, ballast stores the load and distributes it differently over the daily cycle." By using ballast, we are meeting the peak temperatures of the credit, although it is likely that we will end up with slightly higher temperatures emitting from the roof later in the day. The design team believes that this system, with its ability to dampen the temperature swings associated with a dark-colored roof, meets the intent of the LEED Credit. Can we consider this roofing system to meet the intent of the SS Credit 7.2 as an alternative method through performance? The study is called "Evaluating the Energy Performance of Ballasted Roof Systems" and was published in April, 2008. It is available at http://www.spri.org under "Technical Reports."