Cathedral ceiling - R-38 plus 1 or 2" of taped polyiso?
I am finishing the cathedral ceiling insulation and I am trying to decide between 1 and 2" of polyiso sheets. It's deep truss design and standing seam steel roof. There is about 12" of void space above installed unfaced R-38, ridge vent, soffit vents.
Just for a reference, walls have cedar shingle siding, 1x4 strapping, 2 layers of taped polyiso, typar, 1/2" OSB, 3+" of 2lbs spray foam and 3/4" T&G pine as wall finish. Probably close to R-40 (about R-25 at framing)
Either way I will likely have to install strapping before putting the T&G pine as my ceiling finish, since 2.5" finish nails will have a bit short grab on single layer option (they are shot a bit on an angle, 3/4" T&G, 1" ISO then about 1/2" grab in the truss, which just isn't enough).
I am in zip code 53951 and wonder if I should spend the extra $$ on 2nd layer of foam on my ceiling.
There would is some air space between top of the polyiso, where it faces fiberglass, as well as below of polyiso where it faces the T&G (thanks to strapping)
If I were to install just 1 layer, I would do it reflective side down, tape, 2x or 1x strapping.
If 2 layers, 1st would be shiny side out, tape, 2nd layer shiny side down, tape, 1x or 2x strapping.
Either way I would leave a 1/2" gap on the perimeter of the polyiso (where it meets the floors, walls and peak) and foam it for airtight seal.
1 layer would add up to about R-47 and 2 layers would be about R-54.
Beside 1 vs 2 layers, would you advise 1x vs 2x strapping? Difference would be mainly air gap and finish nail hold strength (kinda doubtful)
Posted Wed, 10/30/2013 - 14:23
Other Questions in General questions