You can't used foil-faced iso over the interior t & g since that would trap moisture in the t & g, but you could use as much as 2" of unfaced XPS (R10), or 4" of EPS (R15) to get there. Alternatively you could use up to 2" of closed cell spray foam (R12). There's at least one higher-perm lower-R closed cell foam out there (Icynene MD-R-200) that you could safely install in depths of up to 6" (R30). At 6" the latter product has a permeance of ~ 0.6-0.7 perms, which is about where you are with only 2" of most closed cell foams, or 2" of XPS.
If you can find a sufficiently high-perm fiber faced iso roofing insulation you can go pretty deep with it, but most are using sub-1-perm facers, you can't layer it. If you keep it to at most 2 facers in the stackup most will be OK. It'll have a permeance of less than 0.5, but it won't be much more vapor-tight than kraft facers, and it's not air-permeable, properly sealed at the edges with 1-part expanding foam to avoid convection loops from depositing room moisture on the now-colder t & g you'll be OK. (This is a must for any rigid-foam solution.) At 4" iso delivers ~ R24, at 5" it would be about R30. (If there is no insulation currently on the exterior, derate that to R22.5/R28 for mid-winter.)
With any foam insulation the interior finish surface needs to be rated as an ignition barrier. (1x t & g would qualify in most species, if you want to preserve the look.) Purpose- made foam board adhesive (available at box-store home centers) works, but using 1x furring at the edges to hold it in place while the adhesive sets will likely be necessary. To mount the finish surface it needs to be mechanically secure- code doesn't allow you to just glue it to the foam, so using 1x furring through-screwed to the original t & g 24" o.c. would give you something to mount the interior finish surface/ignition-barrier on. |