Study Finds Air Sealing Essential for Thermal Performance

November 7, 2013 1423 Views Building Health Initiative, Coastline Community College, Dan Heinfeld, Kurt Spiering, Northeast Georgia Health System, Northeast Georgia Medical Center Braselton, principles, U.S Green Building Council Northern California, View Inc.

WESTFORD, Mass. — A new study by the Building Science Corporation (BSC) analyzes the findings of the Thermal Metric Project, a multi-year collaborative research project. The project, also headed by the BSC, aimed at finding a new metric for the thermal performance of building enclosures that better accounts for known physical heat flow mechanisms and operating conditions.

“The Thermal Metric Project was initiated in response to renewed focus on building system performance increasing use of a broader range of building materials and systems,” the report said. “These factors highlight shortcomings of the dominant thermal performance metric, namely R-value.”

R-value is the measurement of thermal resistance of insulating materials. The greater the R-value, the greater the insulating power. Six building industry partners were involved in the thermal performance metric and the associated test method: Dow Building Solutions, Honeywell, Huntsman Polyurethanes, Icynene, North American Insulation Manufacturers Association (NAIMA) and Greenfiber. A baseline set of seven test walls used various insulation types including fiber, glass, cellulose, spray foam and extruded polystyrene.

The most significant finding, according to NAIMA, was that sealed walls of the same R-value perform equally as well regardless of the type of insulation used. When the walls are constructed with the same installed R-value in the stud space and are air sealed both inside and outside, they exhibit essentially the same thermal performance, the study said. In the past, R-value has been the standard for addressing the thermal performance of insulation materials.

The study also detailed the impacts of air leakage on temperature, which is considered vital since all insulation materials exhibited temperature-dependent thermal performance. Findings suggest that air sealing is essential for all insulation types.

“Air leakage was found to be a complex factor in thermal performance. Air leakage always increases the total heat flow through the building enclosure,” the study said. “However, air interacts with the materials on an assembly as it travels through. This interaction changes the temperature field in the assembly and through assembly.”

The wall assemblies in the study all experienced a loss in thermal performance due to air movement through the assembly. The insulation material used in each wall made no impact.

According to the study, commercially available 2-D and 3-D heat transfer models and the parallel path method described in the ASHRAE Handbook of Fundamentals offered good predictions of thermal bridging.