Plastic on the inside of the wall, humid air outside Will that poly be protected from the outdoor humidity? Or will it, like the window I saw, be dripping with condensation? Then build a wood-frame wall outside the poly, complete with cladding and air permeable insulation in the cavities. Next, imagine that a layer of drywall separates the poly from the indoor air. Now, imagine that pane of glass is actually a sheet of polyethylene. They were running the air conditioner, so the indoor temperature was probably 75 or below. The dew point of the outdoor air was 78° F. I was there one day in June a few years ago and saw condensation on the outside of a window…at 1 o’clock in the afternoon of a sunny day. If you’re at all familiar with the climate in Charleston and understand moisture, you know that can’t be a good thing. I’m writing this article because one of our HERS raters came across a house in Charleston, SC that had poly under the drywall, on the interior side of the wall assembly. I’m simply going to explain what happens in a wall cavity with and without a plastic vapor barrier installed. In this article, I’m not going to get into the details of vapor barriers or all the possible scenarios of different wall assemblies and moisture loads. A big part of the problem, I think, is that they’ve been told what to do-”Put it on the warm-in-winter side,” or “Never use one”-but they haven’t had the physics of what happens explained to them.
A lot of people have heard advice about vapor barriers and vapor retarders.
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