Vapor Barriers. Please proof this new inspection article.

[Nick Gromicko]

Vapor Barriers.

[Brian A. MacNeish]

Originally Posted by gromicko

Vapor Barriers.

The emphasis on vapour barriers/retarders for walls and ceilings in the absence of information on air barrier systems is far off base and leads participants in this thread in the wrong direction.

In normal house conditions, the bulk (98-99%) of interior generated moisture that ends up in the attic, or some times walls, is from vapour moving with the air, not vapour diffusion through permeable materials like drywall, wood, etc. The vapour barriers/retarders stop/slow down the vapour diffusion but do not attack the massive movement of vapour with air movement unless properly sealed, thus becoming part of an ar barrier system!!!

http://www.nrc-cnrc.gc.ca/eng/ibp/ir...igest-175.html

An excerpt from the above 1976 Canadian document:

"It would be more precise to define a vapour barrier as a vapour impermeable layer that resists the diffusion of water vapour under the action of a difference in vapour pressure. By defining it with reference to its resistance to the diffusion of water vapour it has been implied that diffusion is the prime cause of condensation problems. It is probably more accurate to say that vapour diffusion by itself never initiates a problem.

Air leakage is now considered to be the prime cause of most condensation problems in walls and roof spaces. If, therefore, a building can be made tight against air leakage it may not need a vapour barrier, as defined. On the other hand, if there are openings that permit air to leak from the warm side to the cold side of the insulation, adding a vapour barrier (even of zero permeance) that does not seal off the openings will be useless."

Another earlier (1960) document from the same series ends with these words:

"The conditions to which the vapour barrier is exposed after installation are a factor, but the circumstances during application are usually of most importance. The most important general principle to be followed in both design and installation is to reduce to a minimum the number of openings in the barrier. Where such openings are necessary, special care should be taken to seal the barrier so as to approach complete continuity."

http://www.nrc-cnrc.gc.ca/eng/ibp/ir...-digest-9.html

So you can see that the idea of sealed vapour barriers is not a new one that arose out of the energy crisis. Since the release of these great papers (available at no cost), it became good building practice that no one really paid attention to.

[Kenton H. Shepard, CMI]

Vapor barriers and vapor retarders are two different things, depending on their perm ratings. The article should start by explaining the difference. Judging from the second paragraph, the author seems to think that the two terms are interchangeable.

" ...specific application, weather it is for a new build or a retrofit."

If correct application is dependent on climate zone, what difference does it make if it's a "...new build or a retrofit"?

Inspectors usually can't see the vapor barrier. One exception is when floors are insulated. When floor insulation is retrofit, the vapor barrier (paper or foil) is usually facing the cold side of the floor (crawlspace or unheated basement), which is a defective installation. In humid climates this can cause decay or corrosion problems from condensation.