Insulation behind Shower surround
I have heard multiple ways to best insulate behind fiberglass/tub surrounds below grade. We always insulate basement remodels with 2” of XPS = R-10 against the foundation wall (sealed with canned foam all 4 sides, seams) and then frame out for the tub/shower surround.
My specific question is the stud cavities behind these surrounds and whether its best to leave open (rely on a single layer of 2” XPS) or go a step further and fill with unfaced FG, or faced FG? Were in MN and my concern is the potential for condensation on the backside of these surrounds. At the same time FG behind a fiberglass surround also makes me a bit uncomfortable with its ability to hold moisture.
Ideally I would double up another sheet of 2” XPS over the original piece for a total of 4” = R-20. However I have read articles by Joe Lstiburek that suggests only using a max of 2” of XPS as using more, reduces the perm rating, thereby reducing the walls ability to dry? What about this concern? If we increased the XPS, by how much? Lastly would you add another layer of XPS directly over the first layer? Or install the 2nd layer so it’s flush with the front/face of the studs (of course you would have a cavity with XPS on both sides). Or simply just rely upon a single layer of 2” XPS against the wall and leave the stud cavities as is.
Any insight would be most appreciated.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part