David Amenhauser is buying a home near Boston, Massachusetts, that’s apparently still under construction, but far enough along to have the roof framed and insulated.
“When I decided to purchase the home, it was already framed and the roof deck/rafter bays were sprayed with open-cell spray foam,” Amenhauser writes in a post at Q&A post at GreenBuildingAdvisor. “I do not know what was used for the roof underlayment, but I know there are asphalt shingles. The rafter bays are made out of 2x12s and are completely filled with foam.”
Because the roof is insulated with open-cell foam, which is vapor-permeable, Amenhauser had been planning to ask the builder to prime the ceiling drywall with a vapor-retarding primer/sealer. Then, he came across an article by building scientist Joseph Lstiburek entitled “Cool Hand Luke Meets Attics.”
Now, Amenhauser isn’t so sure he needs a vapor retarder after all. The article implies that may not be necessary as long as the attic has supply and return air ducts. Amenhauser, however, is still concerned about the area behind the kneewalls where the HVAC equipment and ducts will be housed.
“So my question is,” he writes, “what do the experts here recommend to make sure I don’t have moisture problems in the attic?”
For starters, there’s not enough insulation up there
The photo of the attic that Amenhauser posted along with his question suggests to Dana Dorsett that the roof is under-insulated and probably doesn’t meet code requirements for this Climate Zone 5 location.
“The pictures don’t look anything like a complete fill with foam,” Dorsett writes. “It looks (being generous) closer to an ~8-10-inch average depth which, by the way, does not meet code minimum in Massachusetts, with some spots as thin as 6 inches.”