Still confused by plethora of information on insulation
I’m in North Dallas, zone 3, and here’s what I have so far (in progress pics attached).
2×6 framing for walls and 2×8 ceiling joists. Everything is 24” OC. It’s a skillion roof and I’ll route plumbing vents through the walls to avoid any penetrations through the roof. I have it decked with ½” OSB.
I am planning on 2 alternating, taped layers of 1” XPS on the roof deck (total of R-10). This would be covered by a layer of the high temp Grace Ice and Water Protection peel and stick. I will run 1×4 purlins on top of this for a standing seam metal roof. The only venting I have planned is from the 1×4 purlins – unless you recommend something else?
In zone 3 I’m supposed to have a min of R-38 in the roof assembly, but not sure how I’m going to get there without using more foam above (would prefer not to do this), or having a layer of closed cell foam underneath.
So here are my questions:
1. Should I install the WIP directly to the OSB deck and install foam above this? Or, I assumed the WIP would have to go above the foam to provide a cohesive seal across the above deck assembly and allow me to wrap to the fascia. Is this the right order?
2. Is the WIP even needed if I have 2 layers of XPS?
3. I’m confused by the guidance from GBA on the application of (spray) open cell vs closed cell under roof decking. My understanding is open cell has the potential of allowing the roof decking to absorb moisture from inside and cause a potential failure, so vapor barrier paint needs to be used. An expensive alternative would be to use closed cell spray foam, but not if there is above deck foam board insulation, since the decking has no way to vent if sandwiched between 2 layers of closed foam. What do you recommend?
4. For the exterior walls I’m planning to have ½” OSB sheathing (seams taped), 1” XPS foam boards, Tyvek wrap (seams taped), and 1×4 purlins for metal siding. I plan on having ‘outie’ windows, so keeping the wrap at the outer layer. Inside I’d like to use closed cell spray foam too, but have the same concerns as the roof. What do you recommend?
I appreciate any guidance you can provide. Love the GBA site – It’s like drinking from a firehose.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part