UPDATED on August 13, 2015
Most builders assume — and GBA has long reported — that oriented strand board (OSB) is a good air barrier. If a builder uses a high quality tape like Siga Wigluv, Zip System tape, or 3M All Weather flashing tape to seal sheathing seams, OSB wall and roof sheathing can act as a building’s primary air barrier. Energy consultant Marc Rosenbaum used the technique at two dormitory buildings at the College of the Atlantic in Maine and several projects on Martha’s Vineyard, and subsequently gave a presentation at a Vermont conference at which he advised builders who wanted low blower-door numbers to tape the seams of their OSB sheathing.
In recent months, however, many building experts have been surprised to read reports that some brands of OSB may not be airtight. The reported leakage isn’t coming from poorly taped OSB seams; instead, the air is allegedly leaking through the OSB panels themselves, even when the seams are flawlessly taped. While this type of air leakage sounds unlikely, evidence is accumulating that the reports must be taken seriously.
Among recent developments:
What’s the definition of an air barrier? In North America, some building codes have adopted the following definition: an “air barrier material” is defined as a material with a leakage rate of no more than 0.02 liters/sec-m² @75 Pa.
Building scientist Joseph Lstiburek helped develop this definition. “The .02 number was based on a suggestion from Gus Handegord,” Lstiburek told me. “He suggested that an air-barrier material should be defined relative to drywall. We tested drywall and the air leakage rate for drywall was a bit under .02. So that was the basis of the standard.”
The 2009 International Residential Code based its definition of “air-impermeable insulation” on the above definition of an “air barrier material.” The code…