My home was built in 1927 and at some unknown point a previous owner added fiberglass to insulate the rim joists. This has provided a wonderful landscape for mice to live. I’m removing it but need a plan for replacing the insulation.
Spray foam is only quick if your basement is already empty so I’d like to use an insulation board material. I live in Wisconsin so I’m aiming for R-20 or R-15…I’ve read different things in different places. XPS looks like a good option but I’ve read that it doesn’t meet fire code. Thermax would be the alternative but I’ve read it doesn’t perform to its R rating in colder climates. Then there’s the addition question as to whether its best to use two layers of rigid foam or go with one and add a layer of fiberglass. I think I’d just assume do two layers of rigid foam.
Anyway, hoping to get some advice from others who have experience navigating this maze of decisions.
Thanks!
Replies
I cut and cobbled XPS (cut foam board ot fit with a margin gap around, spray foam against the rim and the perimeter and install the foam board. Can do one or more layers depending on depth. Spray perimeter to finish). Then I installed 3 1/2" Roxul mineral wool. There's an article somewhere in the FH archives about doing this and as I recall the code allows that small amount of foam to be used without a cover but the author, if I recall correctly, recommended the mineral wool due to its fire resistance properties. Tip: for the spray foam, get a 12" or so length of clear tubing to put on the end of the spray foam tube that comes with the can. You can hold the can upside down easily and the tubing can then be used to get all the nooks and crannies.
one such article: https://www.finehomebuilding.com/2013/09/12/insulating-rim-joists
Rigid mineral wool would be perfect here.
Mineral wool is air permeable, I need something that seals.
Do a layer of 3/4 in. foam followed by the mineral wool.
Thinking about this, are you worried about winter condensation? I wouldn't be in the basement. The stack effect tends to pull air in through the basement, so I doubt condensation from interior air penetrating the insulation and hitting the rim would be an issue. As to sealing outside air, run a bead of spray foam around the perimeter of each bay before installing the wool.
I never heard about this.