Rigid insulation – above or below deck???
I’m looking for a little guidance here in western NC. We’re under construction, and planning an unvented roof, mostly because valleys would prevent us ventillating about 1/2 the roof anyway. The ceiling is cathedral throughout, no attic. I’ve read Joe Lstiburek’s article ‘A Crash Course in Roof Venting,’ and he illustrates two methods for detailing an unvented roof assembly — one showing the rigid insulation above deck (to keep the deck above dew point) and the second showing the rigid insulation on the interior, just above ceiling (presumably to prevent moisture from entering the roof / ceiling assembly???).
http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting
The reason I ask is that it would be much easier to install the insulation to the interior. We have deep overhangs, and would need to either continue the insulation over the overhang or build it up to match. And it’s a 12/12 pitch, so I don’t want to spend any more time up there than necessary.
I should add, we’re doing R30 blown cellulose between 2×10 rafters plus 1 1/2-2″ of poly iso foam board and the finished roof will be metal.
Any thoughts?
Replies
Cellulose is a big papery sponge, I'd think about making extra sure it didn't suck up moisture. That would have me placing the foam on top of the deck - it acts as a thermal break to keep out that heat as well. Tape the seams to make it a waterproof barrier.
Depending on the details of the metal roof, it may require a specific layout of furring to attach to. Attach the furring through the foam to the rafters, then if it is a flat metal roof, fill the gaps between the furring with more foam board so it doesn't sound like a hollow can when it rains.