How apply wallboard to sloping foil-faced polyiso?
In a Green Building Advisor discussion titled “Proper wall to attach rigid foam to the inside of the wall?” on Nov 17, 2014 asking about applying drywall on the interior side installed foil faced polyisocyanurate Martin Holladay responded:
. . . Most drywall contractors hate installing drywall directly over foam, for the reasons you mention. One way to improve the situation is to install horizontal or vertical furring strips, 16 inches o.c., between the rigid foam and the drywall. If the rigid foam is foil-faced, this air space will have an R-value of R-2 or R-3. The furring strips are not necessary, however. I have installed drywall over 2-inch rigid foam without furring strips. You may get a few more screw pops than usual with this method. Q. “Because of a very low profile roof and attic living space, I have almost no choice but to spray foam the lid. I’m hesitant about the high GWP blowing agents. Are we seeing any good alternatives in the market yet?”
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I have a related question. I recently had a closet behind a kneewall retrofitted with insulation. 2″ foil-faced polyisocyanurate was installed to the bottoms of the roof rafters and to the wood studs of the exterior vertical wall. Cellulose was blown in behind the poliso and all the edges and holes were sealed and the screw locations were all taped as well.
My question:
I would like to finish the insulated sloped ceiling (12:12 roof slope) and the vertical wall with drywall (or alternatively some thin panelling). Can the drywall (or panelling) be installed directly to the sloped ceiling? I was thinking of using 3 1/2″ long screws to penetrate through the wallboard and thought the polyiso to attach directly to the rafters. Also it would be easier to adhere the wallboard with adhesive prior to screwing the wallboard. Is construction adhesive compatible with the foil facing or will it damage the product.
Alternatively, I could screw 1×3 wood furring strips perpendicular to the rafters and then nail the wallboard or panelling finish to the furring, but I’d like avoid that if it’s not necessary.
I’d appreciate your advice on the best approach.
[I know where the rafters are because I can see where the screws are (covered with foil tape) on the surface of the poliso].
Thanks very much!
Anthony V
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Replies
Anthony,
Q. "Can the drywall (or panelling) be installed directly to the sloped ceiling?"
A. Yes.
Q. "I was thinking of using 3 1/2" long screws to penetrate through the wallboard and thought the polyiso to attach directly to the rafters."
A. That will work.
Q. "It would be easier to adhere the wallboard with adhesive prior to screwing the wallboard. Is construction adhesive compatible with the foil facing?"
A. Yes, but the adhesive cures too slowly to secure the drywall without some method of applying pressure. Just hold the drywall in place and screw it.
Thanks very much Martin.
Another idea has occurred to me for finishing the foiled pollso sloped ceiling, and that is applying wallpaper.
Self-adhesive wallpaper made for textured walls would solve the aesthetic issue. (I think slight irregularities caused by dimpling at the screw locations might make thinner wallpaper impractical).
Would the adhesive damage the foil inner surface of the poliso? Would there be a problem with adherence to the foil?
Thanks for your help on this.
Anthony
Anthony,
Most building codes require rigid foam to be covered with a layer of 1/2" drywall for fire safety. I advise you to install the drywall rather than wallpaper.