In a sharp exposé of the current use of OBS in high performance houses, GBA Editor Martin Holladay brings to light a number of important details on what is and what isn’t an air barrier by Canadian Standards. The OBC states in 9.25.3.2 (1) [that an air barrier has an] air leakage not greater than 0.02 liters/sec-m² measured at an air pressure differential of @75 Pa, which according to the article is based on the permeance of drywall.
In an earlier study published by Building Science Corp on permeance and sorbtion tests done to OBS samples by P. Christopher Timusk of George Brown College, that suggested the sample’s moisture content increased non-linearly as relative humidity increased. In a recent news letter form Building Science Corp, BSI-038, some cautionary notes were issued about the use of OSB in “Things get very risky if you use a high-density spray foam on the inside of OSB sheathing. There is no way that any appreciable moisture in the OSB can dry inwards. The only drying possible is outwards. We need the gap.”
If you use OSB as sheathing and plan on using it as your air barrier for a high performance project – caveat emptor! Click on the link above to learn more!