Replies

1. GBA Editor
   Martin Holladay | Oct 26, 2011 04:26am | #1

   Pete,
   Yes, it's possible to add polyisocyanurate to your existing tongue-and-groove board ceiling. Since you noted, "I don't know how to attach them to the ceiling," you might want to hire a contractor for the job.

   Here are the necessary steps:

   1. Attach one or more layers of polyiso to the existing ceiling with cap nails.

   2. Seal the perimeter of the polyiso with canned spray foam and seal the seams with foil tape.

   3. Install gypsum wallboard with long screws, or install furring strips, 16 inches on center, with long screws, followed by drywall. The screws will help hold the insulation to the ceiling, so be sure you have enough of them.

   4. Tape and finish the drywall.

2. 
   toffee | Oct 26, 2011 10:46am | #2

   Thanks Martin. I was thinking of adding 4"-6" of polyiso, is that advisable? With that kind of thickness, I don't think screws are long enough, how can one attach them?

3. GBA Editor
   Martin Holladay | Oct 26, 2011 01:05pm | #3

   Pete,
   It's certainly possible to buy 8 inch or 10 inch screws. If you can't find them locally, buy them from a website.

4. 
   toffee | Oct 26, 2011 04:43pm | #4

   wow, 10" screws are awesome. What size should I use? like a 10?

5. GBA Editor
   Martin Holladay | Oct 26, 2011 04:49pm | #5

   Pete,
   Obviously, it depends on the thickness of your foam, and whether or not you intend to install furring strips. Do the math!

   If you are screwing into 5/4" or 2-inch boards, you don't want your screws to extend beyond the boards you are screwing into.

6. 
   albertrooks | Oct 27, 2011 08:56am | #6

   Pete,

   Make sure that you know what's behind the face. Drill a small hole at an edge of one of the roof bays through the T&G to find out how thick they are. If there is an electrical box in the roof, open that up to see what you can find.

   The big thing would be to not run a screw through your roofing. Don't guess!