ecoHouse 4 - Vapour Barrier Ideas - Try mounting OSB on the inside

We are currently in an evolutionary period of home construction. Climate issues, resource depletion and market demand are driving innovation in both product and building technique. The old standby wall recipe of 2×6s, glass fibre insulation and polyethylene saw us through the last few decades, but doesn’t measure up anymore with building codes, nor the trend towards high-performance housing.

By Mike Reynolds

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One aspect of building envelopes that is garnering a bit more attention lately is how we control the migration of moisture. A polyethylene vapour barrier is one way to do it, but it’s the old way to do it, and by no means the best for our climate. In fact, for homes with air-conditioning it is closer to being the worst than the best. In the last issue we spoke about using vapour retarder primers [paint]; another very effective vapour barrier is Oriented Strand Board sheathing [OSB].

Sheathing provides a necessary structural strength to house frames, but it is written nowhere except in our minds that it has to be on the outside. When installed on the inside it still provides the structural strength, but can additionally act as both an air barrier and vapour barrier.
There is no question that this technique also presents a new challenge to the builder, namely the fact that you have exterior rather than interior cavities to fill with insulation. But this can easily be overcome with foresight and planning.

Some components of a successful air barrier:
- Gaskets, tape or sealants at all joints and junctions
- Sill plate taped to concrete
- Gasket between the subfloor and bottom plate
- Removing window shims and foaming those openings [shims will shrink and leave gaps]
- Wiring chase that does not penetrate the air barrier
- Using drywall as an air barrier [Airtight Drywall Approach or ADA]
- Choosing the proper tapes or sealants for each application

Of all the things you can do to ensure a successful air barrier, one of the more important ones could simply be to not install polyethylene. I say this because we get lazy with poly and tend to see its simple existence as the cure-all for preventing air leakage and moisture diffusion, which it is not. And without it you actually have to put some thought into an air barrier, and that is the first step towards success.


Mike Reynolds is a former home builder, a LEED for Homes Green Rater and the editor of Ecohome.net.

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