This Q&A Spotlight begins with a confession of sorts from Ryan Lewis, who thinks he may have botched an insulation job.
“Let’s suppose you screw up insulating a flat roof from the exterior in Climate Zone 4A,” he begins in a post at the Q&A forum.
The code minimum for attic insulation in this climate zone is R-49. Lewis’ roofing contractor has added 1 1/2 inches of polyisocyanurate foam insulation on top of the roof deck, which Lewis assumes will perform at its nominal R-value of 9.75.
If he adds enough fluffy insulation between the rafters to get the total roof R-value to 49, Lewis runs the risk of creating moisture problems on the underside of the roof deck. Why? The roof sheathing will be cold, cold enough to become a condensing surface for moisture working its way through the cavity insulation. (For more information on this issue, see “How to Install Rigid Foam On Top of Roof Sheathing” and “Combining Exterior Rigid Foam With Fluffy Insulation.”)
If the layer of foam is too thin, can’t he simply add more to keep the roof sheathing above the dew point? Not now. The roof is loaded with solar panels. In order to add more foam, Lewis would have to remove the panels and the racks as well as the roofing.
What Lewis wants to know now is the maximum amount of fluffy insulation he can install between the rafters below the roof deck without incurring moisture related problems.
Balancing interior and exterior insulation levels
The issue is the proportion of interior to exterior insulation. In Climate Zone 4A, the International Residential Code prescribes R-15 to the exterior, and a total R-value of 49, writes Dana Dorsett. About 30% of the total R-value should be to the exterior of the…
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