What is correct installation of rigid foam board on roof, at seams of each panel?
What is the correct means to seal the seams between foam board applied on top of roof sheathing? What is the impact of incorrect sealing? Background: we contracted with a roofer to apply 4″ of Atlas nailbase on top of the roof sheathing, before shingling. I happened to look at the roof on a recent cold morning and noticed gridmarks on the roof, indicating areas where frost had either melted, or had not formed, due to heat loss between the seams of the nailbase. I would think if the seams had been sealed correctly there would be no visible frost lines. How should the contractor have sealed the seams? What is the impact, besides energy loss? Is there significant risk of moisture condensing at the seams? What is the right way to correct? I can access only part of the roof underside, due to a cathedral ceiling – I’ve considered spray foaming the areas I can reach – would that help significantly?? Thank you – jbb
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Replies
Jim,
Q. "How should the contractor have sealed the seams?"
A. The usual methods include canned spray foam, caulk, or high-quality tape.
The classic example of this problem occurred on Joe Lstiburek's barn. He has written many articles about the problem. Here is one: Complex Three Dimensional Airflow Networks.
Jim,
Q. "What is the impact, besides energy loss? Is there significant risk of moisture condensing at the seams?"
A. The risk of moisture condensing at the seams is real. How large the risk is depends on whether there is an air barrier under the nailbase. If all you have is old board sheathing under the nailbase, and if there is a path for warm interior air to reach the board sheathing, the risk is high. Ideally, your contractor would have installed a bulletproof air barrier under the nailbase.
Q. "What is the right way to correct? I can access only part of the roof underside, due to a cathedral ceiling - I've considered spray foaming the areas I can reach - would that help significantly?"
A. Installing spray foam on the underside of the sheathing -- where access is possible -- would certainly help. In the cathedral ceiling area, correcting this problem will be difficult.
Martin - many thanks - that's what I figured, unfortunately. The contractor did use a number of cans I spray foam at irregular joins, but I don't believe used any at the butt joints (despite my repeated requests). Argh.
I've attached a picture of the roof showing the grid pattern caused by heat loss between the seams of the 4" foam.
Martin, I applied 1-2" of closed cell spray foam last night to the upper 1/3rd of the underside of the roof, which I was able to access - how thick do I need to go to prevent moisture condensation issues at the seams of the 4" nailbase? We're located in Maryland. What are your ideas for dealing with the roughly 1/3rd of the roof which has cathedral ceiling? If it were just an insulation issue, I'd be able to use cellulose, but since I need to air seal, would a slow rise closed cell foam work, at least for the rafter bays which do not have ductwork? Thank you