We have a basement kitchen which was dug out about 25 years ago and wasn't insulated. Behind the kitchen units there is a void up to the retaining wall on one side and an external wall on the other. I'd like to add some insulation but I'm limited in what I can do given the size of the space and the gaps into it. I've attached photos of the void

What's the best thing I could install?

Void between units & retaining wall

Void between units & external wall

  • Is there waterproofing between wall and soil?
    – Martin
    Mar 27, 2023 at 11:03
  • 1
    Define "best". Lowest cost? Greatest R-value? Least VOC emissions potential? Easiest to install? What are you looking for?
    – FreeMan
    Mar 27, 2023 at 11:12
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    Please do some investigation and research for your area, then come back with a more specific question about your work. The question is too broad as it is.
    – isherwood
    Mar 27, 2023 at 12:49
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    Before you insulate you might want to fix up that very sketchy looking wiring.
    – KMJ
    Mar 27, 2023 at 15:00
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    Why was a walk/crawl space left there at all? If it was to let you insulate, that should have been done before bringing the cabinets in. If it was for some other reason, you need to think about whether what you're doing now will compromise that goal.
    – keshlam
    Mar 27, 2023 at 17:28

2 Answers 2


It's hard to tell for sure, but it looks like the smallest depth is ~2ft from the brick wall to the back side of the wooden wall.

Since you're interested in maximum R-value, filling a 2' gap with spray foam will give you great R-value! Won't be cheap but it'll be warm. Since it's a fairly narrow gap, that would probably also be the easiest to install since it would only require sticking a spray gun and an arm into the gap and wouldn't require getting a body and/or fixed size materials into the gap.

Of course, if the gap is bigger than that, it opens up other options.


If you have less than infinite money to waste on this project, 4-6" of rigid foam is probably the point where the payoff (from more R-value) doesn't meet the expense of buying more insulation. Assuming the walls are indeed below grade (since we seem to be looking below the countertop level of a basement kitchen) likely the thinner end of that, as it won't be as cold outside a buried wall as it is aboveground in winter.

It would appear that you can cut long sheets to fit the height and shove one into the corner, apply foam adhesive or spray foam to the end of another sheet cut to fit on the other side, and shove it in so the two join at the corner without having to crawl in there from the wider side.

You could also choose a rigid rockwool product which will be considerably safer in a fire, though it may also cost quite a bit more.

  • 1
    For fire safety, sheet foam usually needs to be covered with a less flammable surface -- wallboard or half-inch plywood.
    – keshlam
    Mar 27, 2023 at 17:25

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