Foam insulating a roof that has 1×8 sheathing with large gaps between the boards?
I have a sixty year-old house, and want to spray foam in the attic, on the underside of the roof. The roof sheathing consists of 1 x 8 boards, and the spacing between the boards varies from 1/4″ to 1/2″. There is an ice and water shield underlayment and asphalt shingles on the roof. I have two questions: When the foam is applied will it expand through the gaps in the sheathing and cause the underlayment and shingles to lift? Do I somehow need to seal the gaps before the foam is applied? Will it make a difference if I choose to use open or closed cell foam? Also, while working in the attic, with the pull-down attic stairs open, I noticed condensation–not on the sheathing, but on the underside of the underlayment, in the gaps between the boards. Will this cause a problem if I use open cell foam without a vapor barrier? Will a vapor barrier (paint?) or closed cell foam ameliorate the problem? Thanks for your help.
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Replies
Ron,
First, you should talk to your spray foam contractor about this issue. Your contractor may have a standard method for installing a material on the interior side of the sheathing boards before spraying begins.
That said, many spray foam contractors are casual or ill-informed, so you may need to make some suggestions.
I don't recommend that you install spray foam directly against your sheathing boards. It will make future repairs difficult.
There are two options that I often recommend:
1. Install strips of cardboard between the rafters before the spray foam installation.
2. Install rigid ventilation baffles between your rafters to create a vented insulated roof assembly. For more information on this option, see Site-Built Ventilation Baffles for Roofs.
Q. "Will it make a difference if I choose to use open or closed-cell foam?"
A. That's a pretty vague question. You might want to read this article: Open-Cell Spray Foam and Damp Roof Sheathing. If you are worried about damp roof sheathing, a ventilation channel between the sheathing boards and the top of the insulation will help reduce your worries.
Q. "I noticed condensation--not on the sheathing, but on the underside of the underlayment, in the gaps between the boards. Will this cause a problem if I use open cell foam without a vapor barrier?"
A. Once you have installed the spray foam layer, this problem won't happen. But the condensation you see now is an indication that you have air leaks from your warm, heated interior into your attic. The pull-down attic stairs are a big part of the problem. For more information on these issues, see:
Air Sealing an Attic
How to Insulate and Air-Seal Pull-Down Attic Stairs
Insulating Attic Stairs
Thanks for the advice. I put a call into the contractor to discuss options.