An energy audit on BuildingNewb’s upstate New York home has prompted a recommendation that he insulate the rafter bays with dense-packed cellulose, transforming what is now a ventilated attic into conditioned space.
The HVAC equipment that’s already in the attic would then be located in a conditioned space, Newb writes in a post at GBA’s Q&A forum. The contractor has suggested that existing vents at the ridge and soffit be maintained, but apparently doesn’t see the need for any ventilation space directly beneath the roof deck, which is completely covered on top with Grace Ice & Water Shield, a peel-and-stick waterproofing membrane.
“The contractor told me that when cellulose is dense packed to the proper density, air and water vapor shouldn’t be able to easily get through it from the interior,” Newb says. “In the event any moisture does get into the cellulose, it will disperse throughout the material and find its way to the openings at the soffit or ridge rather than trying to diffuse through the wood which obviously has a vapor impermeable membrane on the other side.”
Newb says that air-sealing the attic floor looks like it would be too complicated, in part because of the three cathedral ceilings that terminate at the attic.
His contractor’s approach should yield R-35 of insulation between the 2×10 rafters, and keep the underside of the roof deck warm enough (the contractor claims) to prevent any water vapor that reaches it from condensing. Spray foam insulation is off the table due to potential health concerns.
If Newb chose not to insulate the rafter bays and live with the leaky attic floor, he says he’d upgrade the R-4 flex ducts for his heating and cooling runs to R-8.
Is the plan offered by Newb’s…
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