Replies

1. 
   Piffin | Dec 28, 2002 04:06am | #1

   foil faced foam board ( trade name Thermax in my area) is rated at R7.2 per inch of thickness. You can get it up to at least two inches thick.

   Run tarpaper over the decking to keep from getting water stains while in construction. Then place the foam and secure it with furring strips and deck screws long enough to reach the framing under by an inch. ( You have some strict wind codes down there so a local building inspector or engineer might possibly need to help you figure out the fastening scehdule and layout span) Then install plywood over that, followed by the roof underlayment and shingles.

   Up here in the coldnorth, I would eliminate the furring ( 1x4 ) and place the plywood directly over the foam but that will reduce the lifespan of the shingles by letting them overheat too easily and too often.

   Homasote makes a product which has foam and sheathing all in one but it is not commonly stocked so it might be expensive for a small order.

   .

   Excellence is its own reward!

   "The first rule is to keep an untroubled spirit.

   The second is to look things in the face and know them for what they are."

   --Marcus Aurelius

2. 
   Mongo | Dec 28, 2002 08:58am | #2

   Thinnest?

   What piffin said. Polyiso is currently r7.2 per inch. However, they're changing the blowing agent used in making the polyiso, as well as changing what r-value they can actually claim for the polyiso. It will be less.

   That said, ask your inspector if you can use 1" of foil-faced polyiso laid directly on the decking, a la le piff-meister. 7.2 plus 1.9 = not quite there. If for some reason he lets you add in the r-value for the air space, 1/2" ply sheathing, and shingles, that's another 1 + 0.63 + 0.44 =11.17, but that's for the whole sammich, from asphalt shngles to the interior face of the decking.

   If none of that floats, bump the thickness of the polyiso up to 1.5" thick and you're home.

   In CT I can get foil-faced polyiso under the names of TUFF-R and ULTRA-R. Manufactured by celotex and homasote.