Replies

1. GBA Editor
   Martin Holladay | Jul 31, 2011 07:18am | #1

   Bill,
   If you want R-50 using closed-cell spray foam, you'll need about 8 inches of foam.

   However, that's going to be expensive, and the spray foam won't address the problem of thermal bridging through your rafters. You'll get better thermal performance if you put all of the insulation on top of the roof sheathing.

   You can install 8 inches of polyiso foam over the sheathing, or SIPs, or nailbase. These options may be less expensive than spray foam, and they will certainly perform better.

   If you do install closed-cell spray foam between the rafters, you don't have to worry about moisture issues on the interior side of the cured foam. Closed-cell foam is an effective vapor barrier.

2. 
   n9GH4QN97x | Jul 31, 2011 07:38am | #2

   I never even considered adding insulation to the top of the sheathing, I know this is a common practice if you have finished area within the cavity of the home, wouldn't this create other issues by adding 8 more inches to the top of the sheathing with the increased fascia hieght? What about strapping the top side of the rafters with 2 inch strapping to significantly reducing the thermal bridging? When you say expensive to spray foam what do you think it would cost to spray approximately 4000 square feet of roof area with 8 inches?

3. GBA Editor
   Martin Holladay | Jul 31, 2011 07:51am | #3

   Bill,
   You're right, of course, that adding 8 inches to the height of your roof requires a plan for detailing the cornices and rake trim in a way that is elegant and aesthetically pleasing. If this is beyond the abilities of your builder, talk to an architect.

   You are also correct that installing strapping or purlins above the rafters will reduce thermal bridging, so that's an option.

   The only way to know the cost of installing spray foam in your area is to call up a few contractors and get bids.

4. 
   bxxKT22MVd | Jul 31, 2011 01:08pm | #4

   Don't forget actually fastening R50 foam board to the roofdeck. Foam board R50 will take 9" or so.
   Better compromise might be 4" foam board outside and 4" closed cell inside. Just an idea but would still take exterior detailing consideration.
   4000 sq ft * 8" thick is approx $30k for closed cell foam. Only hire a licensed, insured, EXPERIENCED, foam specialist for this application. Beware of companies that install FG, cellulose, gutters, garage doors, gar fireplaces, and yes we just added a foam truck! We can do everything!
   Did i mention to get an EXPERIENCED foam professional?
   Closed cell foam should cost $1 / bd-ft. +/- depending on the job. Thicker does not equal cheaper. Since 8" thick will have to be done in 3 or 4 passes. Thicker = dangerous to me. That's why I might have mentioned to hire and EXPERIENCED foam PROFESSIONAL with certified sprayers.