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Air and vapour barrier questions

Wanted to confirm I have correct understanding:

1. It is ok to have 2 or more air barriers in an envelope assembly
2. It is not ok to have 2 or more vapour barriers, in case moisture got into the wall, it would not be able to dry out.
3. A vendor told me that vapour diffusion only occurs 'in a straight line', i.e. perpendicular to wall or roof. If I have a shiplapped layer, the vapour will not follow the serpentine profile to go in or out of the house.

Asked by Jerry Chwang
Posted Wed, 04/02/2014 - 15:28

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Jerry,
Q. "Is it OK to have 2 or more air barriers in an envelope assembly?"

A. Yes. For more information on this issue, see One Air Barrier or Two?

Q. "Is it not OK to have 2 or more vapour barriers, in case moisture got into the wall, it would not be able to dry out?"

A. It depends where these vapor barriers are located. For example, it's possible to install two layers of foil-faced polyisocyanurate foam, one on top of the other, on the exterior side of your wall sheathing. Each layer of foam is a vapor barrier, so this wall has two vapor barriers. But because they are contiguous and adjacent, there is no harm.

It's almost always a good idea for a wall assembly to be able to dry out in at least one direction.

Q. "Is it true that vapour diffusion only occurs 'in a straight line', i.e. perpendicular to wall or roof? Is it true that if I have a shiplapped layer, the vapour will not follow the serpentine profile to go in or out of the house?"

A. No, it's not true that vapor diffusion only occurs in a straight line. Hygroscopic materials like framing lumber and drywall can take on moisture that moves by diffusion. Once these hygroscopic materials take on moisture, the moisture is distributed throughout the material, not just at the single point that first got damp.

In general, though, the ability of a vapor barrier to reduce moisture transfer by diffusion is a function of its area. A vapor barrier that is 95% intact and 5% holes is 95% as effective as a vapor barrier that is 100% intact. This is probably the point behind what you were told. While an air barrier needs to be almost perfect to work well, a vapor barrier doesn't have to be perfect to be effective.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 04/03/2014 - 09:08

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