I have a vented cathedral ceiling that is as follows. I am in climate zone 5 (dry) so I assume this is an R-38 assembly:
1. Asphalt Shingles
2. Roof decking
3. 12" I-beam joists full length with styrofoam vent channels stapled to the underside of the decking
4. 10.5" fiberglass batts in between I-beams
5. Visqueen sheeting stapled up underside of I-beams over entire underside of roof.
I want to finish some of the space under one side of the roof. I am planning on putting up rigid foam board to the underside of the joists to add a bit of R-value and prevent thermal bridging. I will then install furring strips, and half inch drywall . My questions are this:
1. Should I leave the visqueen in place, and put the rigid foam over it?
2. Should I use PolyIso or EPS? EPS is cheaper, so I would prefer to use it.
I am planning on using outward facing foil board. I will get about a 1/2" air gap between the board and the sheetrock to increase the R a bit.
3. If EPS is OK, is 1 inch EPS OK? I would use the Lowes RMax stuff.
4. What am I missing, if anything?
Thanks for the advice.