I need to replace the insulation on the back and side of the bathtub. The walls face the exterior of the house (poured foundation). What type and rating should I use? I can't tell what the existing insulation rating is or if it is a vapor type.
The best would be sprayed-in-place foam. Second would be foam board, cut to fit the space and then sealed in with spray can foam. EPS foam is fine, but if you are willing to pay for poly-iso you will get a better R value. I would go for a couple of inches thick of foam.
You could also do fiberglass, but you won't get the same level of insulation and you could get condensation in the wall.
Eric G is spot-on regarding spray foam. More specifically, the best insulation for the walls behind the tub (and/or shower) is ccSPF (closed cell spray polyurethane foam) insulation. It's the best for a few reasons:
- ccSPF has the highest insulation value at R-6.5/in
- it's vapor and water impermeable
- it will seal up any gaps, cracks, and hard to get at corners
- for a DIY, spray foam is mad fun to apply (the first couple times)
The disadvantages to ccSPF are cost and wearing personal protective equipment when spraying it. In the small cans (the size of a propane tank for your grill) it costs about $1 to cover 1 sq/ft 1" thick. I used ccSPF in my bathroom ceiling because it was much easier to seal around all the pipes and wires.
My second choice is XPS, because it too is closed cell. If water gets into the wall, XPS won't absorb it. When my shower/bath eventually leaks, I don't want my walls absorbing it quietly. Better to reject that water back into the room where it'll get noticed and then fixed.