Spray foam in unfinished attic?
We are building a home in Zone 6. The room above our three car garage will remain unfinished for some time. Long term intent is to make a guest quarters.
While the room is unfinished (no insulation, no heating or cooling) the insulation folks recommended a thin coat of closed cell foam spray to temper the space somewhat and reduce the swings in temperature.They also tell us that it might avoid condensation. They suggested 1 1/2″ foam in ceiling, no insulation in walls. The roof on the house is a shingle roof. The rest of the house has a spray foam “hot” roof.
My questions: Is this a useful thing to do, or should we leave it completely uninsulated? When, in the future, we decide to finish the room, can we add R23 roxul over the foam, or must the rest of the ceiling insulation be spray foam?
Frank
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Frank,
First of all, insulating this roof is unnecessary. You can insulate it if you want, but millions of barns, sheds, and garages have uninsulated roofs without experiencing moisture problems.
If you plan to store temperature-sensitive items in your garage -- candles, perhaps, or maple syrup -- you might want a little roof insulation. But it isn't necessary.
Second, installing closed-cell spray foam on the underside of your roof sheathing will probably invalidate the warranty on your roofing shingles. I'm not sure whether that matters to you -- shingle warranties are notoriously worthless -- but you should at least be aware of the issue. Most manufacturers of asphalt shingles require a ventilation channel under the roof sheathing.
Third, exposed spray foam is a fire hazard. Most building codes require that spray foam be protected by a layer of drywall to reduce the fire hazard. That reason alone might give you second thoughts.
Fourth, when you eventually decide to finish this space, you may want to install a vented roof assembly instead of an unvented assembly. If you install the spray foam now, the spray foam will preclude that option.
Frank,
Q. "When, in the future, we decide to finish the room, can we add R-23 Roxul over the foam?"
A. I'm assuming you meant to write "Can we add R-23 Roxul under [not over] the foam?" In other words, I'm assuming that it is your intent to install the mineral wool on the interior side of the spray foam.
The answer is no. For a flash-and-batt installation in a sloped roof assembly in Climate Zone 6, building codes require that the spray foam layer have a minimum R-value of R-25. If you install 1 1/2 inch of spray foam, the foam layer will have an R-value of only about R-9 or R-10 -- much less than the code minimum for a flash-and-batt installation.
For more information, see Flash-and-Batt Insulation.