I’ve just ripped out our tiny master bathroom (~35 sq ft), which is on an outside wall. We live in Minnesota and the house was builty in 1985 with 2×6 walls. The bathroom is on the top floor – the attic has about 18″ of blown fiberglass insulation which appears to be in okay shape

My primary goal is to improve the comfort in winter of what is to be a mostly tile bathroom. When ripping out the walls, I noticed a couple of mouse trails through the outside insulation along with some voids in the wall. I also saw one or two trails at the attic vapor barrier when I ripped down the ceiling. I’m sure I can improve the mouse resistance with sealing all the cracks, but solid foam wall insulation seems to be better at mitigating air movement through walls, particularly with a big temperature gradient.

I’m wondering if it is worth the 10X insulation cost to install foam insulation in the outside wall compared to fiberglass. Don’t get me wrong – I believe in the benefits of foam insulation. I remodeled my basement and had the outside walls and rim joist insulated with ~4″ of closed cell foam. Also, I am considering having foam insulation installed in my attic, under the blown insulation, but the install costs seem pretty steep (~$5000/1200 sq ft attic). However, I wonder if there is any value in having foam insulation in this one section of an outdoor wall when I have no intention of ripping out the walls, along with the rest of the fiberglass wall insulation and replacing with foam, unless costs dramatically decrease or rebates improve..

zi sppreciate your enagement in my conundrum.
Best regards,
mark