I have a 2 story + basement townhome with 8" concrete blocks walls between my neighbors. At the top of each shared wall is a sheet metal cap. Some of the cavities have concrete in them but many are completely open from the basement to the roof.

  • Was this a cost savings approach by the original builder in the 1970s or is there a reason to keep some of the cavities open?
  • Assuming it's safe to seal these up, what materials can be used to seal them? e.g. concrete, or a boat load of closed cell foam.

Concrete Block Concrete Block wall

  • Is this a foundation? I thought it was in reading but was not sure. My jurisdiction they are required to be filled if they are load bearing.
    – Ed Beal
    Dec 31, 2019 at 0:32
  • Unlike the picture above, the first floor sits level with the ground so the entire basement is below ground. Neither the shares walls nor the exterior wall (retaining) have all the holes filled.
    – J'e
    Jan 1, 2020 at 15:44

2 Answers 2


Just a guess, but I'd say it was purely a cost saving measure. The sheet metal cap is sloppy, IMHO, but their might be anesthetic reasons for using metal. Personally, I would have gotten concrete wall caps and sealed up the top using those caps. I'm not sure I would fill up the inside if the wall was not load bearing or a retaining wall. Before I filled them, I'd inspect the wall thoroughly for cracks.

If the wall is not load bearing or retaining, one potential risk when filling them up with concrete is that you will more than double the weight of the wall which might trigger settling. I have never worked with foam fillers, foam would certainly weigh less but might be harder to inject.

Concrete wall cap


Since it is a retaining wall it should be filled with concrete, I am surprised this was not required to get the occupancy permit or final inspection on the building permit especially with a multi family dwelling.

  • I just called the county permit office and was told that even if the house was built today, it's only really required to have every other hole filled.
    – J'e
    Jan 2, 2020 at 17:55
  • I guess I am in a seismic zone (west coast) so that may be why our standards are tougher. I waited to answer for more info. Since it is not required I would cap them so they don’t collect water and grow things that they may already be doing. I would not use expanding foam but the rectangular concrete bricks like Jim has a picture of in his answer. In my area the carpenter ants love to chew foam sub surface I am not sure about above grade.
    – Ed Beal
    Jan 2, 2020 at 20:16
  • Regarding Ed's point about carpenter ants, a contractor friend of mine insulated the ceiling in his cabin with XPS and the ants had a field day making tunnels and paths through it. Building Science also published anecdotal evidence of carpenter ants enjoying XPS when it has any moisture content. May 18, 2022 at 0:01

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