With R13 in fiber to the exterior of ~R4-R5 of foam+gypsum there will be zero hours/year where the temperature a the depth where foam meets-paper that is below the dew point of the interior air.
In summer the outdoor dew point would have to be well north of 80F to have condensation at the facer, even if you set the AC to 70F indoors.
Bottom line, leave the facers on the fiberglass- removing them doesn't improve anything, and risks damaging the batts. DO take the time to sculp around electrical boxes and split the batts over wiring etc to relieve any compressions that would leave a void in the wall though.
Using 3/4 polyiso would give you slightly higher R than 3/4 XPS, and foil facers make air-sealing easier since FSK tape does a good job of it. Seal the top/bottom edges with 1-part expanding foam. It's also good insurance to lap the seams of the gypsum with those of the foam board, and foam seal electrical boxes before doing any finish work on the gypsum.
If you're actually removing and reinstalling the batts, air-sealing the sheathing would improve the overall performance. In an old house with plank sheathing this would have to be done with closed cell foam, but with plywood/OSB caulking the framing/sheathing seams and using 1-part foam to seal any wiring/plumbing/other penetrations works. If you're touching the exterior siding at all, drilling from the exterior to blow cellulose over the batts compressing them in place will also substantially tighten the stud bays, from an air-infiltration point of view. |