GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

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

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. troubleshooter | | #1

    bump

  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.

    Bill

  3. troubleshooter | | #3

    Thanks Bill

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |