pls help w/ rigid foam insulation install ...

[floatee]

i'm working on my garage to studio conversion and on one out-facing wall i'm having problems installing 2"-thick rigid polyiso foam board insulation. I'm leaving the existing plywood-faced wall as is, so i'm attaching the board to plywood. only, i can't get a flat fit against the plywood. some of the 4'x8' insulation board is warped and i can't screw it flat because screws go right through the foam. even the flatter foam has some warp to it. is there a way to get it right solid next to the ply? i mean, even if i glued it, how'm i going to keep everything tight together while the glue dries?

second question: this stuff is backed with foil-kraft-foil on the heat facing side and foil on the other side. isn't that the same as having two vapor barriers? is this going to be a problem?

btw/ i am going to be covering the foam with 1/2" sheetrock.

anyway, i'd appreciate any help i can get!

[m Ransley]

Why did you buy warped boards, use roofing cap nails or screw and washer if needed glue. Drywall over foam, foam is not a good support it gives, if you want a good job and are taping you need solid support to wood like 1x2 strips nailed to plywood and foam cut to insert inbetween the furring strips. The foam itself is a vapor barrier even wthout the foil.

[louie]

First, you need to get some special screws or nails that have a large plastic washer to prevent them from being pulled through the insulation. You should be able to find where you bought the insulation. Glue is a fine idea, but the nails will help hold it in place. As for the vapor barrier, since the foam is a close-cell foam, it acts as a single barrier. The fact that it's not a breathable foam makes it a good insulator.

[Chris Lewis]

According to louie :

There's a different type of washer that might be easier to find. Old fashioned and/or rural Hardware stores will often have boxes of these things sold by the pound. It's galvanized steel, about 1" square with a dimpled centre hole.

If I recall correctly, if the stuff has a foil side, the foil should be facing _in_.

Drywall over foam board is okay provided it's the right foam board and reasonably well fastened, and that the drywall screws (don't use nails!) are properly driven into the ply (or better, studs - mark where they are so you can find them later).

[yourname]

Find 'roofing tins' really thin ~2 inch diameter circles designed to be nailed right thru. I did this exact thing almost 20 years ago. I drilled the tins in stacks[1200 sqft to nail] because the foam is soft enough you punch through.

Furring strips is a good idea, more as it adds another airspace for insulation

[spudnuty]

Use the washers as above. When I did this I also went over the studs with 2" polystyrene. This gave twice the R value but if you do this make sure to mark the stud location carefully for wallboarding. Richard

[spudnuty]

Use the washers as above. When I did this I also went over the studs with 2" polystyrene. This gave twice the R value but if you do this make sure to mark the stud location carefully for wallboarding. Richard

[Goedjn]

Since he's got plywood behing the foam, he doesn't need to worry about finding a stud. just put glue on the foam and smash it against the wall, pull it off and let it cure per the directions, put it back against the wall, and screw the sheetrock on. The sheetrock should work fine as a "washer".