Re: insulating a 2x3 constructed shower enclosure/exterior wall
If you can stand to lose 2-1/2" in the shower, insulate the existing wall with foam board or spray foam. Then build a second wall with 2x3 (which are 2-1/2" wide) with the studs staggered from the existing ones. Insulate that wall, too, with foam board. This way, you reduce the heat loss through conduction that would normally happen through the studs, and you get 6" of insulation.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.