Venting under standing seam?
I’m in the design phase of a ranch home (my own), 5:12 hip roof, in zone 3, mixed/humid region of the U.S. Since the attic will be used for HVAC, the plan is for it to be unvented. Initially, the roof construction was to be: plywood deck (taped), tarpaper, two staggered layers of ¾ inch polyiso, 2X sleepers laid on a diagonal for venting and a standing seam metal roof. Unfaced fiberglass batts in the rafters to get close to an R40 roof assembly.
In discussions with my very experienced and convincing metal roof provider, it was suggested the diagonal sleepers were a bit fussy for this design and unnecessary for our area. His suggestion (if I wanted to use this out-sulation design) was the first layer of polyiso, 1X on the horizontal with polyiso infill between. I have seen this assembly here and in other web discussions and assemblies with solid sheathing below and over the foam, no airspace below metal.
Given this lovely mixture of heat and humidity, is the reduction in heat conduction, some radiant reflection from the polyiso metal coating and potential aid in outward drying worth the extra effort of creating a vent space below the metal roofing? Recent discussions of unvented attics reducing the potential for wildfire spreading into the vents puts a mark in the negative column, since this home will be in a rural location.
I appreciate any remarks concerning this assembly. Since we are still designing, any opinions for best dealing with an unvented attic in a mixed/humid climate are welcomed. I have gotten a lot of great information from Green Building Advisor and Building Science and hope to also hear from some experienced builders who write with a southern drawl.
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Replies
Tim,
My recommendation for CZ3 is to go with the 2 layers of Polyiso (R11-R16) and horizontal notched battens (Any roofing supplier should sell those), they allow air and moisture to flow. The unfaced fiberglass batt insulation in the rafters I think is a mistake; eventually gravity will pull down the insulation, creating a cavity, thus negating all insulating properties. Also, the insulation will have gaps by the framing members with no insulation at all. I would consider closed or open cell insulation under the roof decking to meet the full R30 on the roof assembly, or better, an R38.
BTW, 1 layer of Polyiso is enough to deal with condensation and/or thermal bridging on CZ3.
Why not put all the insulation above the roof deck?
SHHHHH, David, keep it down.... It can be a good option as well, but that risks starting a second Civil War down here in TX, or seciding from the Union. Most roofing contractors will not install pitched roofs with foam above the sheathing, its just down right unnatural or plain wrong, y'all.
Armando and David, thank you both for responding.