Our sub-slab ductwork (sonotube and sheet metal, no transite) is in terrible condition, so were switching to mini splits. We need to figure out how to safely and effectively abandon the current system, but were having some trouble finding knowledgeable contractors to do the job because we live in an area where slab on grade is very uncommon (southern Ontario).

One foundation company says they can pump concrete into the registers, but that they wont be able to get the concrete to spread enough to completely fill all the ductwork. Will this be a problem?

Alternatively, it seems like a lot of people also DIY it by filling the areas around the registers with spray foam, and sealing the top with self mix concrete. Would that also work?

I have two concerns about leaving the ducts unfilled. First, that leaving a void could be bad for the foundation - but the ducts have already existed as a void for 50+ years so I'm not sure thats really a problem. Second, that bugs, mold, and fungus will fester under there and eventually find a way up. Are sub slab ducts usually treated before filling to prevent that? I saw one online commenter suggest spraying with borate before sealing.

Any advice would be greatly appreciated. It seems like no one is really sure what to do here, and we are getting a bit desperate.

  • Flowable mortar is feeling like a really good option to me. I abandoned in-building ducts with fireblock foam and hardware cloth, to keep pests and grossness out.
    – KMJ
    Commented Nov 21, 2023 at 20:55

1 Answer 1


Turning my comment into an answer.

I'm no expert, but I'd think that if you haven't have any issues with the voids for the last 50 years, it's unlikely that you'll have any in the next 50, unless you're planning on massive weight shifts on the floors above.

Additionally, once the register openings are sealed with concrete, even if something were to manage to grow inside the duct, it's not going to get out.

If it were me, I'd mix up a thick mixture of concrete and pour it down each register. By making it thick, it won't flow very far, meaning you may only need 1 or 2 bags of concrete mix for the whole house. Pour it until it stops flowing, then float the surface for a nice even finish with the existing concrete, then pretend the ducts don't exist.

As an alternative, you could attempt to use a "big gap filling" spray foam to create a plug an inch or two down the duct as a blocker, then fill it with concrete as above. You could even shove some old rags down there, then cover that with the foam then concrete. The goal being to plug up the elbow to securely support the bit at floor level, but not worry about filling the entire pipe for however many feet under the floor it might run.

I suppose that if it makes you feel better, you could get a very thin mix and pump many cubic yards of it to fill all the duct work. Start at one register and pump it in until it starts coming out the other registers. Once full, float all the tops for a nice smooth finish.

Again, though, if the concrete over the ducts hasn't collapsed yet, there's really no reason to suspect that it will in the future under any sort of normal use. (All bets are off if there's an earthquake.)

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