Energy Efficient Roofing Metrics
Part 1: Cool Roofs: Reflectivity Matters
Part 2: Cool Roofs: Emissivity Standards
Part 3: Thermal Conductance And Roofing Systems
Part 4: Adding Vegetated Roofs To An Existing Building
Thermal Conductance And Roofing Systems
By Karen Warseck, Contributing Editor - June 2009
The third factor that affects the energy efficiency of the roofing system is the thermal conductance of the roofing materials. Most roofing membranes have high thermal conductance — whatever gets into the membrane gets out just as fast. To counteract this, thermal insulation has been a part of roofing systems for many years. The thermal resistance of the roof assembly — from the air that flows over the top of the roof, to the roof membrane and insulation, to the deck and the air that flows below the roof as well as any ceiling systems — is calculated into a total value for the thermal resistance of the building: the ability of the assembly to allow heat to transfer from the interior to the exterior or from the exterior to the interior depending on the season and climate.
Adding more insulation to resist heat transfer is one way to compensate for a non-reflective roof system in a warm climate. Similarly, the more insulation used in facilities in cold climates, the better the assembly’s ability to minimize heat loss from the warm interior to the cold exterior. This resistance (R-value) has been mandated by building code and in ASHRAE 90.1. As the requirements for energy efficiency get more stringent, so does the R-value of the roofing system that is mandated. The 2010 version of ASHRAE 90.1 is expected to mandate a 30 percent reduction in the energy use of a building from the 2004 version — meaning the R-values of the roof will very likely need to be increased even more.
Energy efficiency of a building can be impacted for good or for bad simply by the choice of roofing materials. Increasing the R-value of the thermal insulation of the roofing system can provide an easy way to increase the energy efficiency of the building.
