Spray foam on 2×4″ roof trusses?
![](https://secure.gravatar.com/avatar/317d4f2ed1f2dfb1f71a498040594c5d?s=52&d=https%3A%2F%2Fimages.greenbuildingadvisor.com%2Fapp%2Favatars%2F200x200%2Ft.png&r=g)
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 Detail Library
A collection of one thousand construction details organized by climate and house part
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb1.png)
![](https://www.greenbuildingadvisor.com/app/themes/greenbuildingadvisor/dist/img/modules/detail-library/thumb2.png)
Replies
bump
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
Thanks Bill