Dean Manoogian has a Cape Cod style house in Portland, Maine, and is puzzling over the best way to retrofit the roof with rigid foam insulation.
Working with both an insulation company and a roofing contractor, Manoogian has come up with a plan: apply 2-in. rigid foam on the interior of the dormered roof and then fill the rafter bays with dense-packed cellulose.
“My question,” he writes in this Q&A post at GreenBuildingAdvisor, “is the air/moisture permeability against the roof sheathing/rafters.”
The roof has been built without ridge, soffit or gable vents. To protect the sheathing and rafters from moist air, mold, and possible decay, Manoogian proposes covering the underside of the roof sheathing and rafters with a peel-and-stick membrane before adding the foam and cellulose.
The rigid foam would be capped with 5/8-in. gypsum wallboard to meet fire codes.
Is Manoogian on the right track, or courting a problem? That’s the focus of this week’s Q&A Spotlight.
Not the most elegant solution
The plan has already won the city’s approval, but to GBA senior editor Martin Holladay, it has its drawbacks.
“No,” Holladay writes, “you can’t solve the problem by adhering Ice & Water Shield to the underside of your roof sheathing and your rafters, for several reasons: the materials would be expensive, the work would be awkward, it’s impossible to do a perfect job, and you would be creating a vapor-barrier sandwich with roof sheathing in the middle — a disastrous approach if you ever get a roof leak.”
There are only two reliable ways of keeping water vapor away from the roof assembly, says Holladay. One way is to spray closed-cell polyurethane foam on the…