What is the best method/order of insulating rim joists with EPS+batts?

Steve J Published: July 18, 2019, 3:53 p.m.Last updated: July 30, 2019, 2:24 p.m.

I'm trying to insulate Rim joists /cripple walls in basements have read about three methods using EPS & batt with slightly different orders. Plan to insulate concrete walls with 2" of EPS, with 2x4 framing (filled with rockwool batt), and then drywall. Which one of these approaches should I use? 

1) Sandwich multiple EPS pieces againt rim joists/cripple wall cavities and build it out to EPS mounted against foundation wall.
2) Single EPS pieces flush against rim joist, horizontal along wood sills and then connected to vertical EPS against concrete. Batting added after framing and inside rim/cripple wall cavities. 
3) Sealing & batting first, then 2" EPS. EPS continues vertically straight from slab to below subfloor.

I have a 2-storey brick home (35 yrs old). My basement wall construction is poured concrete foundation, then a full-home perimeter cripple wall, and then joist that sit atop of it on a double wood sill. (no walkout). Foundation wall is approx 7 feet high, cripple wall height is approx 12 inch, then floor/rim joists. FYI: cripple wall has no lateral support (only backed with drywall...!?!)

Which of the three methods is best when insulating the cripple wall and rim joists?

Responses (1)

Ecohome July 30, 2019, 10:49 a.m. Reply


Hi Steve,

Of the methods you list, myself I would go with the rigid EPS and see if you can make it continuous with the wall insulation below it. If you install several layers of EPS it will function just like a SIP wall, the foam boards will act as a vapour barrier and the thermal barrier. For an air seal, you may want to actually do that first, which could be a simple matter of applying caulking or acoustic seal to the rim joists to stop air leakage while you still have access to them. If you were going to install batts in the cavities before rigid insulation boards, I would go with Rockwool since it handles moisture way better than fiberglass.