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How to address gap between stud wall and concrete foundation wall

rdugan | Posted in General Questions on

I’m wondering how to (or if I need to) address a variable depth gap between a stud wall and a concrete retaining wall below grade.  I’ve glued up 1.5″ of foil faced EPS over the concrete, but still, due basically to contractor incompetence, I am left with anywhere between 1″ and 5″ of space between the EPS and back of the 2×6 stud framing (stud bays to be filled w/ Rockwool batts).

I understand in general the concept of filling any gaps to avoid convection loops, but am wondering if that is really a concern in my case, given a.) the use of EPS, b.) the radiative properties of the foil facing, which as i understand, requires some amount of gap to function anyway, c.) the wall is no taller than ~3ft from bottom plate to joists, meaning max ~2-2.5ft below grade, and d.) I am in climate zone 4C.  I will also be installing fireblocking at the top plate and horizontally every 10ft or less, which should further limit loops or air movement to some degree.

If it is still critical to close these gaps, my best option AFAICT would be to build up the batts (i.e. R30 @ 7.25″ + R15 @ 3.5″ in the worst case), but this would be extreme overkill for our climate from a total R-value standpoint, not to mention costly.

Would love to hear thoughts on necessity of closing up the gap, as well as alternative solutions.  Thanks!

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Ryan.

    You have a concrete foundation wall with EPS rigid foam insulation installed tight to the concrete. In from of that, there is a stud wall. The distance between the face of the EPS and the studs varies. Am I understanding this right?

    If so, the first thing to do is to make sure the EPS is air sealed well at the perimeter and the seams. With that done, and if the basement is dry, you could add any insulation you'd like in the stud bays. Some builders won't install fibrous insulation in basements, others will, cautiously in dry basements. If that's your situation, compressing oversized batts may be the easiest thing to do, given the irregularities in space. You could also choose a blown in option, which will more consistently fill the spaces behind the studs.

    1. rdugan | | #2

      Hi Brian,

      Thanks for the response. Yes, your summary of my situation is correct. The EPS is sealed at the seams and the bottom - I've yet to determine how to address the top (that's a different topic.)

      But my main question is whether I really need to fill the space between the EPS and the stud wall at all, given the somewhat unique circumstances of my build-up. Will free-air flow or convection loops really be a concern, given the short wall height and installed EPS?

      The main concern is cost - for that reason, I'm tending to ignore blown in foam altogether. But even the extra batts needed to fill the space would mean hundreds of dollars extra (vs just installing a standard R23 between the 5.5" studs.)

      1. Expert Member
        MALCOLM TAYLOR | | #3


        Unless someone can point to something showing convective-loops or wind-washing are more than a minor concern, I wouldn't worry about it.

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