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Insulating a cathedral ceiling

I am building a timber frame barn in zone 6 (Vermont), part of which is insulated. We hope to use recycled or seconds of polyisocyanurate in 4x8 sheets. The roof from the inside has 6x6 rafters, 7/8" T&G pine boards, moisture barrier, 2 layers of 3" polyiso, taped and foamed in cracks, 2x4 strapping screwed through the insulation into the rafters, another moisture or rain barrier, plywood underlayment nailed to the 2x4s, creating a 1.5" vent space, finally metal standing-seam roof. My questions are:
1. Is this a good way to go?
2. Do I need a vent space created by the 2x4s. Recently I read that vent spaces are not needed, but I also read that T&G ceilings are very permiable.
3. Is the compressive strength of the polyiso enough to withstand the installation (people walking on it) and the compression of the screwed down strapping.
4. We could also add a final layer of 1" or 2" "blue board" for additional srength. Is that necessary?

Asked by Peter Limon
Posted Thu, 03/13/2014 - 13:51
Edited Thu, 03/13/2014 - 13:53

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Peter,
You'll end up with about R-36. That's less than the minimum code requirements (R-49 in climate zone 6, according to the 2009 IRC).

I suggest that you include a layer of roofing underlayment under the metal roofing. Asphalt felt is fine.

You have to have a tight air barrier. The best place to locate that air barrier is near the bottom of your roof assembly. You need to know which layer is your air barrier. I don't know what you mean by a "moisture barrier" above your T&G ceiling; but if you make that either Ice & Water Shield, or a taped European air barrier membrane, that would work.

The vent space if fine; there are several reasons why it is a good idea. But if you are short of money, it makes more sense to add a third layer of 3-inch polyiso than it does to build a vent channel.

The polyiso has enough compressive strength of this application.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 03/13/2014 - 14:19

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