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Vapor Retarder Post New Topic | Post Reply
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Robert Holmes
Posted: Oct 24, 2010 09:30 PM
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Vapor Retarder
I have a project spraying 3 inches of foam on the cmu cavity wall. They are asking for a vapor retarder for the three courses of cmu below grade. Is this correct?
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mason
Posted: Oct 25, 2010 09:10 AM
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Below grade applications can vary in design and in the challenges presented. Without specific detail of the design elements I could not make an opinion on whether the foam by itself would be sufficient. Facts to consider: 1. Is there a drainage tile system under the foundation wall? 2. Is there a vapor retarder on the ground below the concrete? 3. Is there a high water table allowing water into the CMU from the outside 4. If so, is the CMU water or dampproofed from the outside? Assuming you have addressed these factors above, you can use the 3 inches of closed cell sprayfoam without an additional vapor retarder element. You could use open cell foam without an additional vapor retarder element in mixed or hot climates but would require one in colder climates. A closed cell foam will have around 1 to 2 perms per inch, so at 3 inches would be well below 1 perms total making it a "semi-impermeable" (Joe Lstiburek's term) vapor retarder and acceptable in all areas of the US without an additional vapor retarder. Check out the articles on basement and crawlspaces at Builldingscience.com. They have good information and design details that should take you in the right direction. For example, check out this article on basements. http://www.buildingscience.com/documents/digests/bsd-103-understanding-basements?topic=resources/cond-crawlspaces |
