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Community and Q&A

Spray foam on 2×4″ roof trusses?

troubleshooter | Posted in Green Building Techniques on

I am going to have spray foam sprayed on the bottom side of the roof decking on my shop in southern Colorado zone 6b.. The trusses are 2×4″ and 16″ OC.  My thought was 4-5″ of closed cell. Should I have enough foam applied to completely cover the 2×4? To eliminate thermal bridging and condensation? If so, should I put 6″ in each 16″ bay and have two inches cover the 2×4? In other words all you would see is continous foam, the 2×4 would be completely enclosed in foam

At this time there will not be a ceiling in the shop. Later I will install sheetrock or sheet metal

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  1. troubleshooter | | #1


  2. Expert Member
    BILL WICHERS | | #2

    A typical 2x4 is actually only 3.5" deep, so "4-5" of spray foam would already be enough to cover the 2x4s.

    With what you're planning, a little extra to get an even surface and cover all the 2x4s with at least an inch or more would probably be a good idea. You're likely to not see a lot of extra cost to do this, and you'll get better thermal performance with the 2x4s covered. 2" of cover would be a 5.5" average thickness layer, and would give you pretty good performance -- somewhere around R33 or so, and you wouldn't have major thermal bridging issues with the studs covered. With something as shallow (compared to deeper rafters) as a 2x4, thermal bridging is more of an issue.


  3. troubleshooter | | #3

    Thanks Bill

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