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Flash and Batt foam thickness

I'm in NJ climate zone 5 and have a project with an installed falsh and batt with 1" closed cell foam and Roxul batt in wood framing. I usually spec 2"+ but would like to avoid ripping everything apart and spraying more foam. Is there a chart/rule-of-thumb anywhere showing wtih of foam based on climate zone averages or any specific data to justify thickness besides doing a calc? This seems like a case where it would probably be ok but with a small chance for a big problem??

Asked by Michael Margulies
Posted Jan 25, 2013 10:33 AM ET
Edited Jan 25, 2013 10:35 AM ET

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Michael,
Relax -- your walls should be fine.

The guidelines you are looking for can be found in either of these two articles:

Calculating the Minimum Thickness of Rigid Foam Sheathing (Look for the paragraph with the headline, "What about flash-and-batt jobs?")

Why Flash and Batt Makes Sense ("According to building scientist Joseph Lstiburek, flash-and-batt installations require at least 1 inch of spray foam in climate zone 5, 1 1⁄2 inch in zone 6, and 2 inches in zone 7.")

Answered by Martin Holladay
Posted Jan 25, 2013 10:40 AM ET

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