Spray foam – how thick to air seal a rim joist?
WEG
| Posted in General Questions on
What is the minimum thickness of spray foam on a rim joist for effective air sealing? I’m using a disposable 2 part 200 board foot kit.
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Replies
Walter,
Assuming you are doing a perfect job, I would put the minimum at 1 inch.
A thicker installation may allow you to be somewhat imperfect in your application technique (although it's always possible to screw up this type of work, regardless of how thick the foam is).
New home tract built I am adding electric circuits in basement. Check this out, they sprayed closed cell on the rim joist and that's it. The thickness, 1/8-1/4".
The builders are loved for the attention to improving the lawn after you move in though... House "looks" nice, who is gonna be looking at insulation.
For air-sealing purposes you can first "picture frame" it shooting a thin initial shots directly at the seams you're trying to seal, then go over it with a 1/2-1" shot directly on the major field of the wood for vapor retardency.
Even a half-inch of 2lb foam is a ~ 2perm vapor retarder that won't wick moisture during condensation events, but a mere 1/8-1/4" shot is only about as vapor tight as standard latex primer paint.
Who remembers Flash and Batt? I recall articles illustrating moisture and mold problems in wall cavities where the foam is less than 2 inches. Why would this application be any different when the foam is less than 2 inches? Aside from the perm rating any thickness will block air flow. The risk of moisture and mold is another issue to consider.