Climate zone differences
GBA Editor| Posted inWebinar Follow-up Q&Aon
What are we to understand about differences in the united states. I build in Pennsylvania (outside of Philadelphia). We are on the border of climate zones and it makes it difficult to know where to properly place vapor barriers in different conditions and some guidance would be appreciated.
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There's no one answer other than 'It depends." It depends in large part on the wall assembly. Cladding, insulating foam (if any), what type of foam (if any), building paper / housewrap, structural wall sheathing (if any), whether the wall sheathing has sealed joints, wall cavity depth, insulation type, interior wall finish (wood, drywall, plaster, FRP..., and even what is on the wall - paint type, thickness, coverage, wall paper.....
What really needs to be done on every wall is a thermal and vapor profile to determine where condensation points can occur and under what conditions.
Pete will probably have some good links either to GBA information or other sources - I'll ask him to post those here.
I've gotten in the habit of installing NO vapor retarder on the inside or outside of my walls. As Pete pointed out, air movement is more an issue than diffusion when it comes to moisture transport through walls and ceilings. The walls I build are air tight. They can dry to the exterior and to the interior. Like your climate in PA, people around here run AC June, July, August and often in early September. In the winter they heat. Even though we're both in what most people would consider 'cold' climate zones (4 and 5) they end up being sort of mixed climates where 1/3 of the year you have the vapor retarder on the 'wrong' side of the wall.
我想象的建筑规范equire vapor/temperature profiles of wall assemblies be submitted at permitting (along with roof and floor engineering).
Anonymous,
Unfortunately, your question is too vague to answer. Vapor diffusion rarely leads to problems, so concern over vapor retarders is usually misplaced. In general, air barriers are far more important than vapor retarders.
Do you have a question about a specific wall or ceiling assembly?
If there is any doubt about where to place a vapor barrier, don't use one. It shouldn't be long before that requirement is removed from the model codes, as it's an anacrhonism.