Clearing the Air on Sealing Attic Bypasses

Originally published by: Journal of Light ConstructionJanuary 30, 2014

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It's refreshing to see a news story ("Beware the Insulation Contractor Who Does Not Include Air Sealing" in the Kenyon Leader) providing guidance to homeowners that might actually make a difference. In this case, it's recommending that homeowners steer clear of contractors who offer to insulate attics without sealing attic bypasses first.

The advice is solid, although the explanation why is a little shaky. The article reads "Although insulation slows heat transfer, it is easily compromised by air flow." Almost. There are three types of heat transfer - conduction, convection and radiation. Insulation slows conduction. And it's true that the insulation's effectiveness is compromised by air flow, but a loss in R-value typically only occurs in a cold climate when cold air flows through the insulation. In a vented attics, this often happens near the soffit vents when there is no insulation baffle.

The real reason attic bypasses need to be sealed is that these leaks are carrying away conditioned indoor air by convection: Stack pressure, an engine for convective air flow, is pushing all that indoor air through the holes in the lid.

Perhaps this is a little too much information for the average consumer. But not for us, or for those errant contractors, who also need to know specifically what "attic bypasses" to seal. For that, the best resource is the Energy Star Homes' "Thermal Bypass Checklist Guide" (PDF).  Detailed Guidance can also be found by searching "attic" at the Building America Solution Center.

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