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Vapor Barriers & moisture control

Posted by The Insulator on April 13th, 2000 06:25 PM
In reply to questions to insulator to clarify by bc on April 12th, 2000 06:16 PM [Go to top of thread]

The difference between a vapor barrier & an air movement barrier is in the type of moisture movement that is being limited. A vapor barrier is designed to stop the moisture that conducts through a material, where as an air movement barrier stops the moisture from being carried around a material. Although the same material can do both, it does't need to be.

A vapor barrier is a material resistant to water vapor passing through it & whatever material it is in contact with (for a vapor barrier to do it's job it must be in contact with the wall material. If you install kraft fiberglass pushed back & not in contact with the wall material, you may as well not even have it.)

On the other hand, an air movement barrier prevents the more serious concern on air passing around the wall & vapor barrier material & carrying the moisture into the wall cavity toward a cooler surface where it can condense. This is move serious because the amount of moisture that is carried by convection through even a small opening for air to move is greater than the potential of the whole wall's conduction.

The best way to stop moisture from being a problem in a wall. or anywhere else, is to prevent it from ever getting there. In the application of a wall over a block foundation, the more you can prevent moisture from ever getting into the walls, whether from the interior or from to outside, the less likely of ever having a problem. Therfore, on both sides, measures need to be done for prevention. Especially on the inside, one needs to detail and seal any possible areas of leakage.

As for the creating of a pocket by 2 vapor barriers, for this to be an issue there needs to be insulation between the 2 layers, creating a temperture difference that allows for condensation. Without that, both are at the same temperature (If it didn't condense on one, why would it condense on the other?). Also, it the first layer is designed to stop both conduction & convection as good as you possibly can, the issue is moot.

The Insulator

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