Condensation in roof rafters in a metal framed house?
Hi, I am trying to build a metal framed house in Bonsall CA, climate zone 10 in California, climate zone 3 nationally. 13 miles inland from the Ocean about 40 miles north of San Diego. One half the roof will be metal trusses, the other half will be conventionally stacked using 2″ x 10″ metal rafters. On top of rafters is radiant barrier plywood, 40# felt, followed by Spanish Clay Tiles with O’Hagin vents. My Title 24 engineer told me that I would need R-48 insulation, about 8″ of polyisocyanurate, in all Rafter sections of the roof, to comply with the County’s interpretation of the Code, but only R-38 in the spaces with attics. U Factor requirement is .031. A polyiso Insulation company, RMax, told me NOT to use polysio in a rafter cavity because it would cause condensation. They HIGHLY recommended not doing this! Should I create a ceiling below the rafters such that the lower rafter assembly and its corresponding insulation (which turns out to be 12″ of fiberglass batt, R-38) would be more than 1″ below the bottom of the roof rafter? I think I am effectively creating a new “attic area” of 1 – 2″ of gap between the upper roof rafter and the lower rafter. Is there another simpler way? I am a first time home builder. Any help you can provide is greatly appreciated!
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Replies
Peter,
In a steel-framed building, all of the insulation has to be installed on the exterior side of the wall sheathing and roof sheathing. This is (in my opinion, and the opinion of most building scientists) non-negotiable. I've very surprised that you got as far along in your construction project as you did without realizing this basic fact about steel-framed buildings.
The reason is simple: steel framing provides thermal bridging that negates the value of any insulation installed between the studs or between the rafters. Steel is an excellent conductor.
The suggestion to use 8 inches of polyisocyanurate is a good one. The polyiso should be installed on the exterior side of your roof sheathing, in a continuous layer. It can't be cut into narrow strips and inserted between your rafters. (Note that you might consider installing 8-inch-thick nailbase.)
For more information on the necessary work, see How to Install Rigid Foam On Top of Roof Sheathing.
If you put the polyiso above the roof deck you would only need ~5" of roofing polyiso (or ~6.5" of EPS) to hit U0.031 with that stack up.
If you put 8" cut'n'cobbled between metal rafters it probably WON'T make it (even if the inspectors say it will.)