Comment on "Building Green From the Ground Up"


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    This is a quibble with the article “”, since I couldn’t find a link to post directly to the article. I see that it was posted to the blogspot site, but under a different title, last fall.

    Regarding the vapor barrier in a building, Peter makes the dismissive statement “it has been found that air leakage – and not vapor diffusion – is the real problem. In fact, air leakage accounts for over 200 times the amount of moisture transmitted by diffusion.” This misses the point of a vapor barrier. That is to keep moisture out of the wall. The moisture inside the building is a (mostly) separate matter.

    If there is a largish temperature difference between the outside and inside of a house, and if the humidity on the warm side of the wall would translate to a saturated condition on the cool side, that moisture will tend to condense. When that happens, mold and mildew and rot will begin in the wall and eventually ruin it.

    Because of this effect, the vapor barrier should be positioned toward the warm side of the wall, to keep moisture from entering there. If it were placed on the cool side of the wall, condensation would simply form on the vapor barrier. This has a different meaning depending on whether you are building for a hot climate or a cool one.

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