By Suda Molleti, PhD, and Bas Baskaran, PhD National Research Council Canada
Designing buildings in an energy-efficient manner has become of primary importance in order to meet energy code target levels. It has been clearly demonstrated that air leakage is a key influencing parameter in the energy efficiency of the building envelope. To mitigate air leakage and also to control heat and moisture movement, air barriers have been identified as the key component for the proper functioning of the building envelope. In a low-slope roof, the roofing membrane will certainly perform as an air barrier, provided it is well constructed. Factors such as lack of continuity of the membrane seams, improper detailing around rooftop preparations, improper selection of flashing materials, and improper connection of roof membranes to the exterior wall barrier could compromise the air leakage performance of the roof assembly. Therefore, continuity, structural sufficiency, and durability are critical parameters in the effectiveness of roof air barrier assembly. This holds true irrespective of roof membrane type and its attachment mode.
Air impermeability of seam-fastened, mechanically attached roof membranes is not compromised as a result of inherent membrane fluttering behavior. Though roof membrane flutter does not affect air leakage through roof membranes, it is associated with air intrusion into roof assemblies. In functioning roof membrane assemblies, air intrusion is separate and distinct from air leakage.
Much of the industry discussion about air leakage through the building envelope revolves around the design community’s uncertainty about air leakage performance of roof assemblies. There are those who recommend roofs be treated as though they are horizontal walls; others maintain roofs perform differently with respect to air leakage. The National Research Council of Canada (NRCC), in collaboration with the Canadian Roofing Contractors Association (CRCA), the National Roofing Contractors Association (NRCA), the Roofing Industry Alliance for Progress, and the Single-Ply Roofing Institute (SPRI) addressed the issue of air movement in roof assemblies through a research and development project designated as Air Movement Impacts on Roof Systems (AIR). This project evaluated the air barrier performance of low-sloped roofing assemblies and established a test protocol for air leakage measurement in roofs. Under the auspices of ASTM’s D08 committee on roofing and waterproofing, this test protocol was transformed into a new ASTM test standard: ASTM D8052/D8052M: Test Method for Quantification of Air Leakage in Low Sloped Membrane Roof Assemblies.
The air leakage test method consists of installing a roof assembly with five typical rooftop penetrations between two chambers: a bottom chamber where the roof assembly is installed in a horizontal plane, and a top chamber as shown in Figure 1. The test method uses the dynamic load conditioning of CSA A123.21, which accounts for the wind fatigue expected during the life span of a roof. Through this wind conditioning protocol, the structural sufficiency and continuity of the roof air barrier assembly is thoroughly evaluated. The resultant airflow measured pre and post-dynamic wind conditioning at the speciﬁed static pressure differences across the roof assembly determines the air leakage of the roof assembly.
On a general note, it is critical that each assembly of the building envelope be investigated for air leakage performance with appropriate standards. What cannot be captured in the material and full envelope air leakage testing—i.e., the structural strength and continuity—can be quantified in the assembly testing. To achieve energy efficiency in building and to adapt for climate change, comprehensive data on materials, assemblies, and full envelope air leakage testing are needed. Efforts are underway towards the codification of the ASTM D8052 standard to serve as a platform for supporting code compliance air leakage requirements and for constructing energy-efficient and sustainable roof assemblies.