Best method to insulate/ air seal a rim joist in a basement with or without spray foam.
I’ am currently in the process of air sealing and insulating problem areas in my basement and when it comes to rim joists it seems there are many different methods. I plan on residing the house and will add 2+ inches of exterior insulation (EPS or GPS) so I’m a little wary of simply spray foaming the rim because it might impede drying potential and I would like to keep VOCS to a minimum. I’ll have vapor retarders on both sides of the rim in that case. What other methods are recommended? Could I use a liquid or spray applied type air sealer like visconn and then stuff the space with mineral wool? How about cut and cobbling with eps or gps foam instead of xps? Would the newer wood fibre panels be too vapor permeable for a basement application?
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Replies
DWest,
What climate zone are you in, and what are you using as your primary air barrier on the walls?
If your exterior foam is thick enough you can use permeable insulation in the bays, and if you tape the sheathing as your air-barrier, you don't need to worry about sealing the individual bays.
Malcolm,
Thanks for your advice. I am in climate zone 6 in coastal Maine. Walls are existing 2x4 construction. The plan (as a multistage retrofit) is to work on air sealing and insulating the basement and other high-priority areas first, and then the exterior insulation would come much later. l just don't want to set myself up for failure by creating a foam sandwich on the rim board when I do eventually get to siding/ foam.
DEWest,
In climate zone 6, with 2"x4" walls you are safe if you have R7.5 exterior insulation.
https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing.
Delaying the exterior insulation means using permeable insulation in the bays at the rim-joist is a bit risky in a way it won't be once that foam is in place. My suggestion would be to use batts, and air-seal each bay with a square of poly on the understanding that you should keep an eye on them periodically until you get around to doing the exterior work.
My experience is that if you never did get around to it the rim-joists would be fine. Are foam boards or spray foam better? Sure, but sealed poly over batts is the standard detail here and doesn't seem to experience problems.