BobsYourUncle wrote:
Consider rigid insulation such as SM or similar. It is available in various thicknesses including 1 1/2".
Cut to fit the voids, and for any gaps or going around wiring etc, use a bit of expandable spray foam.
Super light weight and it has an R value of R5 per inch, so R7.5 for your application. That's double what you'll get from regular batt insulation.
Don't use just any rigid foam board, there are multiple types of faom board, some have better R Values than others..
Typical foam board is expanded polystyrene, basically the same as your favorite throwaway foam coffee cup.. Has a low R value just a notch better than fiberglass batting..
The highest R value foam board is Polyisocyanurate or "polyIso" and has a typical R value of 6.5-7.0 per inch. Also has a reflective foil backing on both sides.
Downside is cost, it costs considerably more than fiberglass batting and other low R value foam boards.
Typically available in .5", 1" or 2".
Easy to determine if you have PolyIso at the home center, it will have a light yellow color to the foam, lesser R value foam boards will have white beads (expanded polystyrene) or green color (extruded polystyrene) might find extruded polystyrene in pink or blue colors in 1/4" thickness.
For OP, you can use 1" and add a .5" layer of PolyIso rigid insulation which will maximize the RV value assuming R6.5 (1") and R3 (half inch), that gives you R9.5.
Ideally if you had enough space, you can gain 1 or 2 R factors by leaving a dead air space between the foam boards.
Make sure you really have 1.5" to work with, shaving off foam board thickness would be a real pain and a big mess.
You need a good tight fit between wall studs.
Be aware, foam boards do a terrible job at sound control when compared to fiberglass batting.
