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Vapor Barrier

Posted by The Insulator on August 13th, 2000 01:46 PM
In reply to Physics (not sure, really) by Dave on August 11th, 2000 10:30 AM [Go to top of thread]

First, we need to define how moisture travels. In a basement / crawlspace, the first possibility is groundwater leaking in do to cracks & poor excavation. This obviously needs to be the first area of concern. After that, the 2 mechanisms are by vapors moving thru materials(for lack of better terms, conduction) and by means of it moving around the materials (Convection). People in general, in fact most people in the construction trade, have a tendency to blur these together in regards to vapor barriers. Vapor barriers, by definition, only deal with moisture conducting thru materials (ai drywall), not with moisture being convected by means of air leakage. By focusing on the vapor barrier, the more serious area of convection gets ignored.
If the ground water issue is resolved, the next area of attention is to stop convection, which is the reason for most condensation. The solution may be in using a vapor barrier that is already an air barrier, as in poly, or by using wall material the is a air barrier, as in finished drywall (If air doesn't blow thru it, it's an air barrier). In each case, there needs to be attention to details. Sealing (caulking or foaming) any penetration that would allow air movement, such as at electrical boxes & the top & bottom plates of a framed all.
as far as the issue of vapor barriers in a basement, if there is no ground water leakage, provide a vb of the foundation to grade line to stop ground vapors. also install a vb to the entire inside of the wall & be sure to elliminate any & all areas for air leakage. This leaves the above ground foundation as well as the top plate for vapors to escape. Some might feel that it isn't enough, but the more you prevent moisture from entering the wall, the less need there is to allow it to leave.

The Insulator

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