Rigid foam without furring strips under dry wall
fgmei35nCV
| Posted in General Questions on
I’d like to add 1″ rigid foam (XPS) over my rafters on a cape cod (above knee wall) cathedral ceiling w/o the hassle of furring strips as some of FH Bldng articles show. I’ll finish that w/1/2 inch drywall. I have R 21 fiberglass paper faced batts in that cathedral ceiling and plastic baffles for air flow from eaves to ridge, exhausting out side gables as the back side of that top floor has been dormer-ed. Are the furring strips necessary?
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Replies
Jeffrey,
I assume you mean under your rafters, not over your rafters...
Yes, it's possible to install 1-inch of foam on a sloped ceiling, followed by drywall, without furring strips. You just need long screws and a good system for marking the rafters. Needless to say, it helps if the bottoms of the rafters are co-planar; that's not always the case.
You'll get a few extra screw pops, and you'll have to put in a few extra screws to handle these problems. But you can do it.
Thanks form your quick response. Yes, "under rafters", looking from interior of the house. By co-planar, do you mean "in-line"? I read something about dew-point and using 1" foam or more, but since it's insulated (R-21) do I have to be concerned w/that? Or the insulation already took care of that issue? I'm asking because I may need to use 3/4 inch foam. My concern is more for reducing summer heat up there than keeping it warm for winter as most heat from the 1st flr will flow up there.
Jeffrey,
If the bottoms of the rafters are co-planar, that means that they are all in the same plane, and that the rafters don't have any bows, sags, or size differences. You can verify whether the rafter bottoms are co-planar with a long aluminum straightedge (at least 4 feet long).
Dew point calculations don't really matter when it comes to interior rigid foam, so you can use 3/4-inch foam if you prefer. Of course, your roof assembly ends up with a low R-value.
OK thanks. Yes they are in the same plane. Also, can I add the foam also to the interior walls up there, over the studs, as w/the new dormer construction there's alot of large framing where new const meets old and I'd like to reduce thermal bridging. I have R 15 in 2 x 4 walls.
Q. "Can I add the foam also to the interior walls up there, over the studs?"
A. Yes.