Insulate inside cathedral tongue-and-groove ceiling?
Hello GBA Forum,
Unfortunately, our first priority when we moved into our new house was to replace the 1979 wood shake roof (we are also in a forest fire area) on the NW Cali/NV border (Climate Zone 5 – our average snowfall at the lake is 125inches, AC isn’t’ needed and it’s not humid here). So as such, insulating the ceiling from the from the shingle side is no longer a viable affordable option. And after our first winter last year, we realize we are in dire need of more insulation.
Here is the current composition going from exterior to interior on the roof:
Composite shingles (installed 2017)
Water guard HT (bottom half of roof to prevent ice damns installed 2017)
30LB roofing paper (upper half to roof to peak installed 2017)
Plywood (from original build)
2 inch rigid foam with Rmax (From original 1978 build)
Tongue & Groove
You can’t cut the tongue and groove out as they are overlapped on the main beams. See picture for idea. So there is no space above the tongue and groove to insert any more insulation, since in several gaps in the tongue and groove you can see the reflective rmax. So our option is interior only.
The spans between main beams are 3.5’ and the depth of the beams are 11”. We would insulate from the underside of the tongue & groove with ridge foam, drywall it and then leave a bit (inch or two) of the main beams exposed and add recessed lights. I realize we loose the look of the T&G, but we are focused on economical solutions and keeping warm. Plus there is enough wood elsewhere. I was thinking of 4” of XPS (Extruded Polystreen) or ISO (Polyisocycantrue) and then drywall between the main beams.
My questions are:
1. Do I need an air gap between the inside T&G and the ridge foam? (the other side of the ridge foam is 2” Rmax and there are no eves or soffits)
2. Which foam would you use? Would I want one that doesn’t use a backing materials so moisture isn’t’ stuck between the rigid foam and the inside of the T&G (if that would even been a problem)?
3. What distance do I need between the bottom of the ridge foam and the drywall, if any?
4. Would you cut receded lights into foam, or drop drywall ceiling lower so you have a full 4” of rigid foam and then start the recessed lights?
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