I have a gas pipe that is going through the foundation underground, it's going through a hole that is about a 1/4" bigger than it that has been cleaned out, there is no sleeve.

Don't ask how we got here, this is a very old house. I know many places don't even allow gas lines that penetrate the foundation underground, but this is what I have and we can't afford to completely run a new gas line. Fortunately it doesn't freeze here, and the temp changes aren't much at all.

So the question is - what is the best way to seal the gap between the pipe and the foundation that will protect the pipe firstly, and then protect the foundation secondly so we don't leak any water, and will last as long as possible.

I'm thinking that some sort of rubberized epoxy could seal it up and possibly allow for some thermal expansion, but I'm not clear on what the best product choice would be.

  • As long as the pipe is not in contact with the concrete you don’t have a problem, a simple can of spray foam wits a straw like nozzle can do the job.
    – Ed Beal
    Feb 5, 2022 at 8:24

2 Answers 2


Urethane foam, foamed in place. Adhesive, closed cell so water resistant. It is injected into existing walls for insulation; it would be best to find a contractor who does this. Alternative is an aerosol can sold for caulking. You would need to experiment to find a hose to fit over the nozzle so you could get liquid deep into the annular space. Unfortunately the aerosol can stuff immediately begins to expand as it leaves the can. The insulation contractors can get it with different gases that expand faster or slower to give time for the liquid to get where they want it.

  • Are you talking about something like this? ositough.com/en/products/view-all-products/… It's closed cell, but can't tell if it's "adhesive" or not. I can hit it from both sides, so the straw would probably be enough. I presume this has the "flex" I need to absorb the temperature expansion differential?
    – daveola
    Feb 5, 2022 at 6:16
  • Their website claims: "OSI QUAD is composed of elastomeric polymers and high quality synthetic resins. This product offers the superior adhesion characteristics and unmatched elasticity found in high performance sealants. It yields a tough, rubbery seal which resists outdoor weather-related elements" Don't know if that qualifies as an adhesive or not, but if this would work, that would solve my problem.
    – daveola
    Feb 5, 2022 at 6:18
  • OSI is one perfectly good option; you’ll also find “Great Stuff” on the shelves of the big box stores. I’d opt for ‘window and door’ (more flex) as opposed to the ‘big gap filling’ formulation. Try to get it evenly on all sides so the foam doesn’t push the pipe. Feb 5, 2022 at 21:03

A plumber may chime in and say what the professionals use to make a seal, but I was in your situation (except my climate had freezing weather) and I made a seal using ordinary potter's clay. I dug down to the pipe next to the foundation outside and built up a boss of clay around the pipe several inches thick where it entered the concrete. I worked and kneaded the clay until it was pliable and applied golf ball-sized lumps to the pipe and wall, pressing them firmly in place. When complete, I backfilled with soil, well tamped. The hole never leaked water again in the remaining years that I lived at that house. The moisture in the ground kept the clay soft and pliable, so it never shrank to leave a gap. And the clay was literally dirt cheap, which was a plus.

  • An interesting idea, but one concern I have would be the moisture that you mention kept the clay soft, also wicking into the interior of the foundation.
    – daveola
    Feb 5, 2022 at 0:46
  • @daveola Soft, damp clay is widely recognized as impermeable to water. It's used to line landfills to prevent leachate from reaching the water table. There's a layer of clay in some earthen dams to prevent water under hydrostatic pressure from penetrating the dam. You have nothing to worry about. Just pack it firmly around the point where the pipe penetrates the wall. Or consult a plumber for their take on it.
    – MTA
    Feb 5, 2022 at 2:21
  • I must confess that I am confused as the how it can be damp and yet impenetrable to water. Does that just mean that once the side gets damp it doesn't let any water through it? Also, how do you think it compares to urethane foam? I like the idea of clay if I can access the concrete from both sides, but I'm going to be filling dirt back in on the outside of the wall and won't ever be able to access that side again, so I'm tempted to go with the foam for that reason...
    – daveola
    Feb 5, 2022 at 22:00
  • 1
    @daveola You have an interesting question about clay that's answerable at Earth Science SE. I've mentioned impermeable damp clay used to seal landfills, earthen dams and in my own experience with a pipe through a foundation wall. Once placed and backfilled, that's the end of it. No further access required, and no access from the inside is ever required, since the clay is applied only from the outside. If you're more comfortable with a high tech approach, by all means use urethane foam. I have no experience sealing a pipe hole with urethane, so I can't advise. But clay works.
    – MTA
    Feb 5, 2022 at 22:22
  • I liked both answers so I upvoted both of them, but I went with urethane because my foundation is 16" deep and it was a heck of a lot easier to fill it with something that expands coming out of a tube... :)
    – daveola
    Feb 6, 2022 at 4:26

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