Insulate a solid wood cathedral ceiling / roof?
Scott Lieurance| Posted inGeneral Questionson
I have a cabin in central Oregon with a cathedral ceiling/roof. it is 2×3 laminated to form a solid roof and, with asphalt singles and no insulation. In the next year or so I plan to roof it and would like to add insulation. How can I do it to minimize the weight and gain insulation.
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Rigid foam above the roof held in place with a nailer-deck through-screwed to the structural roof. The lightest material per R would be foil faced polyiso, which is typically 1lb per cubic foot density and R6- 6.5 per inch.
IRC 2012 code requires a U-factor of 0.026 for cathedralized ceilings, which is R38 when it's continuous foam, so you're looking at ~6" of polyiso (about 0.5lbs per square foot) + 1/2" plywood or OSB (about 1.5lbs per square foot plus the timber screws for a total of about 2- 2.5 lbs per square foot.
Scott,
Dana gave you good advice.
The usual approach is to install multiple layers of rigid foam above the roof sheathing. These layers should be installed with staggered seams.
If you want your roof assembly to include vent channels above the insulation, the usual method is to install 2x4s on the flat above the top layer of foam, running from soffit to ridge, with one 2x4 above each rafter. Then install another layer of OSB and plywood, followed by roofing underlayment and roofing.
If this sounds complicated, you may prefer to install SIPs or nailbase (SIPs with OSB on only one side) above your roof sheathing.
For more information, seeHow to Build an Insulated Cathedral Ceiling.