Are there any problems with installing XPS insulation over framed walls with batts to the interior?

Question

Location: Western New York

Project: Insulating Attached Garage

Details:

• 2x4 walls with no insulation
• garage attached to house via one wall
• double garage doors
• living space above garage

I am looking to insulate the garage but want more than ~R14 in the walls (3.5" Rockwool = R14). I could build out the walls to 2x6 but would prefer not to.

I am thinking of doing 3.5" mineral wool, then an inch or two of an XPS foam board on the inside of the garage. My rationale for the XPS board is is that it would give me a full insulation envelope and mitigate thermal bridging from the studs. XPS also functions as a vapour barrier so it eliminates that cost. The XPS would then be drywalled over.

Question: Is placing XPS board on top of batt insulation an approved and recognized building practice?

Answer 1

Overlaying rigid foam is a common strategy in my northern region, both for retrofits and new construction. It's most commonly added outside the framing, but there's no reason you can't use it inside as long as you have a reasonable moisture management strategy.