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!

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.

According to 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.

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).
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.

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.

m Ransley wrote:
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.



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

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

On 6 Dec 2005 07:03:21 -0800, wrote:

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

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".

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