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Community and Q&A

Closed cell foam in in vented roof

Jud Aley | Posted in Energy Efficiency and Durability on

I have been able to avoid using spray foam up until now but with recent code changes in our area we are now forced to use it.

On a current job we are framing a new roof directly on top of the existing old roof sheathing because the owner wants to expose the hand hew beams of the original frame below. The new rafters are 2×8 and the plan is to spray the bays with closed cell foam on a warm day and then immediately sheath and roof.

I’m a concerned because the rafters will have no time to dry out before being foamed and I’m concerned about lumber shrinkage

I have been in the attic of several 3-5-year-old homes in our area that were sprayed with closed cell foam and I often see a 1/8” +/- gaps between the rafter and the edge of the foam. I’m assuming this is because the rafter has shrunk as it dried out over the years and would think that this gap will be a cold spot in the winter where condensation and eventually rot will form?

My concern is on this job that the gap will be even worse because we will be spraying up against the 2x8s that have not had any time out of the weather and they will also shrink as they dry?

Should I be concerned? Is there some way to prevent this?

Thanks.
Jud. Norwalk CT

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Replies

  1. Jud Aley | | #1

    I meant to title it as "Closed cell foam in unvented roof"

  2. Expert Member
    Armando Cobo | | #2

    I think a good option would to use taped and sealed rigid foam with staggered seams over the existing roof decking and then a new roof system. You could buy recycled polyiso to make it more affordable.

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