Ask Jon Eakes

Last Updated: Thursday, March 28th, 2013, Created: Friday, October 15th, 1999

MYTH: VAPOUR BARRIERS ARE NECESSARY TO PROTECT THE WALLS FROM CONDENSATION.

False. Vapour barriers do help to protect the walls from moisture accumulation, but the help they provide is almost insignificant compared to that provided by air barriers. Moisture gets into walls and attics by two paths: air exfiltration through cracks and vapour diffusion through the wall materials themselves. Their relative importance is almost 100 to 1.

That is to say, a typical house expels over 1,100 kilograms of water a year into the attic through minute air leaks or cracks. The same house diffuses less than 20 kilograms of water through ceiling plaster covered with only a single coat of oil paint. (That's 50 to 1) A vapour barrier paint or even just a couple of coats of good oil paint -- without a plastic or aluminum vapour barrier as such -- will reduce the diffusion to less than 10 kilograms a year (that's 100 to 1), an insignificant quantity. Cracks are the real culprits and air barriers the real solution.

In rewinterizing, where it is often not easy to install a traditional vapour barrier, we can safely leave it out, add oil or latex vapour barrier paint to your shopping list, and concentrate your efforts on creating a good air barrier with good caulking techniques.

 

Keywords: Damage, Moisture, Vapour Barrier

Article 778

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