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John O'Hara Posted: Apr 22, 2008 08:07 AM	Foam insulation in unvented horizontal roof rafters I have a timber framed building. The timbers are cross framed with 2 x 12's. The 2x12's are sheathed with plywood and I have asphalt shingles on top. I want to have the timber framing exposed. Install insulation between the horizontal rafters, and cover the 2x's with drywall.What is the best method of maximizing my insulation in this construct. I am concerned about ventilation, heat build-up and pre-mature deterioration of the plywood and roofing.
mason Posted: Apr 22, 2008 09:06 AM	Check out the SPFA technical document AY 141, Unvented attics and cathedral ceilings. There are details and descriptions of using SPF in unvented attic assemblies. Closed cell SPF woulp provide approximately R 6 per inch and open cell SPF would provide approximately R 3.5 per inch. Open cell SPF would require a vapor retarder element over the foam after installation in colder climates. Contact your supplier for details. Properly installed either foam can be used in an unvented attic without deck or shingle deterioration. PS if the area is inhabited space, the foam requires a thermal barrier over it. In an attic, it would require an ignition barrier.

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John O'Hara

Posted: Apr 22, 2008 08:07 AM

Foam insulation in unvented horizontal roof rafters

I have a timber framed building. The timbers are cross framed with 2 x 12's. The 2x12's are sheathed with plywood and I have asphalt shingles on top. I want to have the timber framing exposed. Install insulation between the horizontal rafters, and cover the 2x's with drywall.What is the best method of maximizing my insulation in this construct. I am concerned about ventilation, heat build-up and pre-mature deterioration of the plywood and roofing.

mason

Posted: Apr 22, 2008 09:06 AM

Check out the SPFA technical document AY 141, Unvented attics and cathedral ceilings. There are details and descriptions of using SPF in unvented attic assemblies. Closed cell SPF woulp provide approximately R 6 per inch and open cell SPF would provide approximately R 3.5 per inch. Open cell SPF would require a vapor retarder element over the foam after installation in colder climates.

Contact your supplier for details. Properly installed either foam can be used in an unvented attic without deck or shingle deterioration.

PS if the area is inhabited space, the foam requires a thermal barrier over it. In an attic, it would require an ignition barrier.

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