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Community and Q&A

Foundation Wall – Interior Insulation – XPS ‘and’ Spray Foam

stuartd | Posted in General Questions on

Climate Zone 6
On the interior side of foundation wall (wall below grade), I have 2 inches of XPS (the blue stuff) between the studs. Is this ‘good enough’ even though it is not a vapor or air barrier, or should have another inch or two spray foam in addition, or some other material, like batts? No worries, not going to use any plastic vapor barrier on inside.

How about on the inside of the exterior walls only at the rim joists (2×6 wood stud plus zip sheathing). These areas also have 2 inches of XPS so are not spray foamed at the moment. The other parts of the exterior wall will have 2 inch spray foam and xx inches of fiberglass batts. In the planning stage, I wanted all insulation to be spray foam.

In other words, is it better to have spray foam on top of blue foam vs. only blue foam (I know it’s more expensive)


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  1. Expert Member
    BILL WICHERS | | #1

    Does the XPS in the basement continue behind the studs, or are the studs right against the foundation wall with the XPS cut to fit between the studs? Ideally, you want the XPS directly against the *entire* foundation wall, with the studwall framed over the XPS, which allows the XPS to act as continuous insulation without thermal bridges.

    In the rim joist area, it is probably going to be safer for you to have spray foam only and no XPS against the rim joist. The reason is that you want to avoid having any voids where moisture could potentially accumulate here. Spray foam only will (should) avoid having voids, but if you apply spray foam over the XPS panels, there could potentially be voids in and around the backside of the XPS where spray foam wouldn't be able to reach.

    I would avoid using spray foam in any above grade exterior walls as it really don't help you much there. I would avoid using batts in the basement, although it is possible to make a safe assembly using them if you have sufficient rigid foam or spray foam between the batts and the foundation walls. My own preference would be to put rigid foam against the foundation wall, spray foam the rim joist and use the spray foam to seal and "tie in" the rigid foam in that area, then use exterior rigid foam and mineral wool batts in the above grade walls. I would not frame out a full studwall in the basement, I would only fur things out enough for electricals and would use the rigid foam to do all the "work" of insulating the basement walls.


  2. stuartd | | #2

    Hello Bill,
    The XPS is continuous 'behind' the studs on the below grade wall.

    Are you suggesting I pull out XPS from the rim joists (rim joists on above grade wooden wall, and spray foam?

    I was not clear, the house has a walkout (daylight) basement ... it is on a slope. The interior insulation on the exterior wood walls is spray foam, except for the rim joists on that exterior wood wall.
    Exterior wall in photo -- XPS at rim joist, spray foam below

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