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Insulation for a cathedral ceiling

I'm working out the details for an addition I'll be building next summer. I'm in northern Minnesota. I'll have a cathedrial cieling at a 10/12 pitch and I'm planning on using 2x12 rafters wo have the room for insulation. The room is 29' wide the direction that the rafters run.
From what I've been reading I don't necissarily need to ventilate the roof as long as I make certain to seal the space very well. I have a couple ideas I'm looking at for that and I'm looking to see if either of my potential ideas have any red flags against them and perhaps pros and cons between them.

Idea one is to frame up the rafters. Sheet with 5/8 osb. Asphalt shingles on top. Between the rafters place tight fitting, maybe sealed with caulk, 2" foam sheets against the osb. Then put 2" fiberglass between the rafters. Finish with with poly and sheetrock. This would be unvented and I would place a sealing block between the rafter tails on top of the wall to prevent air movement.

Another idea is similar except I've read some that you can simply place the foam sheets between the rafters and the roof decking. It would basically be, working down, asphalt shingles, roofing felt (ice and water wear needed), 5/8 osb, 1 1/2 foam, with 12" fiberglass between the rafters finished with poly and taped sheetrock insides. My main concern here is if it's ok for the foam to sit drectly on the rafters with out any additional support. I would simply be using longer nails to nail through the osb, foam, and into the rafters.

Also I haven't yet decided for certain that I'll be using aspahlt shingles. I may use metal. If I switch to metal does that change any of this? Also, if I switch to metal would it be ok to eliminate the osb and replace with furring strips (thinking of the idea of having the foam above the rafters)? Or would elininating the osb be as bad of an idea as it sounds?

Asked by Mark Larson
Posted Sun, 12/02/2012 - 18:53
Edited Mon, 12/03/2012 - 05:23

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2 Answers

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You could potentially do your #1 idea, with the foam tight to the roof deck, but I wouldn't. I definitely wouldn't do your #2 idea, you need the roof deck nailed right to the rafters.

My suggestion is to install 2-3" foil-faced polyiso foam between the rafters, spaced 1" below the roof deck. To start, once the roof deck is installed, rip 5/8" x 1" spacers out of leftover sheathing and run them up and down each rafter bay, tacked to the sides of the rafters, up against the sheathing. Then, rip your foam 1/2" to 3/4" less than the bay width, place it against the spacers, and squirt aerosol foam into the gaps all the way along both edges. Tape the butt joints. At that point you have an air-tight insulating baffle with vent space above.

Then, either install blown-in or batt insulation to fill the remaining depth, then drywall.

I would not omit the vent space.

Answered by David Meiland
Posted Sun, 12/02/2012 - 19:37
Edited Sun, 12/02/2012 - 19:38.

2.
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David,
Neither idea is particularly elegant.

If you want to include rigid foam sheathing in your roof assembly -- and I think it's a good idea to do so -- you need to install the foam in such a way that it addresses thermal bridging. That means that you need to install a continuous layer of rigid foam ON TOP of your roof sheathing, not under your roof sheathing. (Rigid foam is too squishy to be installed between the roof sheathing and your rafters.)

Of course, if you want to install asphalt shingles, you need a second layer of OSB on top of the foam. You end up with a site-built SIP. If you prefer, just install a SIP roof. (If you switch to metal roofing, however, all you need is 2x4 purlins on top of the rigid foam instead of a second layer of OSB.)

There is a second problem with your plan: The foam is too thin. The thickness of the required foam in this type of roof is dictated by the building code. In your climate zone (zone 7), the rigid foam needs to have a minimum R-value of R-30. That means that you need about 5 inches of rigid polyiso.

All of this (and more) is explained in this article: How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 12/03/2012 - 05:20
Edited Mon, 12/03/2012 - 05:23.

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