Is it okay to spray foam the gap between drywall and a basement slab?

Submitted by jaaassshhh t3_11byqna in DIY

I’d probably end up using Great Stuff spray foam.

Reasons:

  • I can feel air movement out of that gap
  • we get the occasional basement critter
  • since we’re replacing the carpet and baseboard seems like an easy thing to do to help seal up the room

Concerns:

  • it might wick moisture up into the drywall (though I think this is mitigated by the fact that Great Stuff is closed cell(??) - so it’s a vapor barrier itself)
  • anything other reasons why this would be bad?

Thanks!

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Comments

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zedsmith t1_ja0o15o wrote

I would ask myself the question “why is there air movement there?” first.

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ezbake_fpv t1_ja0s7m9 wrote

If you are doing this to keep drafts out, I think you should do just that. No reason it shouldnt work. I would use the Great Stuff for windows and doors... minimal expansion... Then mask off any areas nearby that you dont want the finish ruined, and don't touch the overflow until it has dried completely. Then just clean it up with a sharp knife. If your primary motivation is keeping critters out, it may or may not work. A few insects and rodents, if determined can dig or chew through it. And If you are hoping to keep moisture out, you would be better off with a good, quality silicone sealer, but you wont be able to paint it.

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blacksewerdog t1_ja0z7x5 wrote

I had some gaps and mixed up some hydraulic cement.Worked great and is water proof.Dries very quick though.Do in small batches

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jaaassshhh OP t1_ja19549 wrote

Thanks for all the replies!

Picture of the gap: https://imgur.com/a/W2xzyTF

I was thinking spray foam becuase it's anywhere from 1/2" to 1+" across the whole wall - and was thinking of doing the whole basement. So it'd be a LOT of caulk.

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iamkiloman t1_ja1s4ag wrote

Our builder left a gap like that so that if there's ever some moisture intrusion into the basement - leaking plumbing, water coming in through the wall or slab, etc - it doesn't immediately wick up into the sheetrock.

Maybe just put some trim over it?

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jspec t1_ja3gyq9 wrote

This is exactly what it’s for. You just need to put your trim of choice down to the concrete that will act as the final barrier.

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bobjoylove t1_ja4q2mg wrote

You could pack it with something porous like rockwool.

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M_Me_Meteo t1_ja2w8mt wrote

Looks to me like this gap is intended to allow air circulation behind a panel where there is direct access to earth/dirt. If you seal it, the moisture from that dirt may not have anywhere to go.

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fizzwig t1_ja1boqv wrote

I doubt that you would seal the actual source of the air leak.

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nakedavenger22 t1_ja1eu82 wrote

I’d do that - I’d shove the straw way inside & fill that cavity

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CommanderAGL t1_ja3j82z wrote

Have you considered just stuffing chewing gum in there?

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sonicjesus t1_ja1tzjs wrote

I'd avoid doing so. The concrete will always be colder than the wall, and you're just inviting condensation.

Every contractor I have worked for always leaves half inch gap from the floor, no matter where it is in the house.

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ezbake_fpv t1_ja25o00 wrote

Drywall is installed 1/2" off the floor, but it should be fastened tight against the bottom plate. There should never be unsealed openings into the wall space. Concrete and walls will be different temps, but the foam is a thermal insulator, so excessive condensation shouldn't be a factor.

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skippingstone t1_ja14ljm wrote

Backer rod and Big Stretch caulk.

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New_Engine_7237 t1_ja17h7b wrote

See if you can stuff some insulation in first than the backer rod. Do you know if there is insulation in the walls??

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nhorvath t1_ja1m3se wrote

If you're going to need more than one can of foam consider getting the pro gun. 1000x easier to control application and you can actually use a little and save it for later. Also keep acetone on hand for clean up it dissolves uncurred foam.

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Heavy-Attorney-9054 t1_ja1s1uz wrote

I used foam to seal a gap between the foundation and the sill plate. Took about 20 cans for roughly 170 lineal feet. Massive change in draftiness.

I decided to use individual csns because they allowed me to work for 10 or 20 minutes at a time and then stop and not have to clean up anything. I looked at the cost of buying cylinders and it wasn't much different given that I might lose some and I'd have to spend more time cleaning up the equipment.

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vIv_Cobra_vIv t1_ja27jem wrote

yeah Great stuff is a moisture barrier just don't get crazy with it. And if you are worried about critters stuff some brillo into the hole so the foam adheres to it. Critters hate the stuff because it cuts their mouth up but the will chew through foam alone.

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anon702170 t1_ja4ek0c wrote

I'd put a small bead of foam in the gap and then install baseboard.

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Doctor_Frasier_Crane t1_ja57hp0 wrote

Great Stuff will work. You're correct that it's closed-cell and will NOT wick water. The "Windows and Doors" type will expand less, but once you get the hang of the regular stuff you'll be able to tool it out in just the right amount. Any overspray and you can just cut it back after you've let it dry for a couple hours.

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dahadster t1_ja2wtia wrote

Had an exterior wall in a ground floor condo that had a slight gap (normal) between drywall and concrete slab. When it rained, thin black worms would come inside and dry out - 50 to 100 along a 12 foot stretch. Used foam before re-installing baseboards and quarter round and my problem was solved.

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g_st_lt t1_ja3o9np wrote

It sounds like the "gap" between the drywall and the floor is not the issue. Is the air coming through the concrete? Or between the concrete and the bottom plate of the framing? Or from above the bottom plate?

It seems more likely you need to seal the drywall to the framing.

Others have mentioned moisture from the other side of wall. Hopefully there is a barrier between the framing and the wet dirt around you.

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brandon0228 t1_ja3sfrd wrote

Depends on where you live. If you’re in Colorado you have to leave a gap so the slab can move with the soil swelling.

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routerg0d t1_ja3tgkc wrote

It’s a basement. You’re seeing the bottom but likely there’s a second 2x4 the drywall is attached to. The gap is for movement so if there’s any it does not break the drywall unless it’s extreme then you have other problems. The air movement is probably because cold air sinks and the slab behind is cooling the air that’s getting in via other intrusions and thus you have some airflow. Trim is fine.

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Mildly_Angry_Biscuit t1_ja3u0uv wrote

Should be perfectly fine to use - but I would recommend the foam that has the critter-repellent in the mix to keep the critters from making foam Habi-trails behind the drywall and through the foam.

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RicksterA2 t1_ja3ynje wrote

Don't use Great Stuff - way too sticky and hard to work with.

Use DAP Texplus. You can clear (with water) the pipette to use again and things clean up way better.

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BuffaloBoyHowdy t1_ja4anjr wrote

I had an exterminator friend tell me that mice enjoy chewing spray foam, so only use the kind with fiberglass in it. Not sure which brands, but not regular Great Stuff.
You might want to consider laying a piece of 1/2" vinyl board along the floor and nailing it into the plate. Not sure if it comes in 1/2" but you could rip some 8' 1x's into 1/2" strips. Waterproof and quite chew resistant.

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gdilalo t1_ja2q6a7 wrote

Before you do this you should confirm you don’t have a French drain. That would be very bad.

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Indicamnesia t1_ja0mk9e wrote

Personally I would use a mix of Elmer’s glue and sand but that would also work

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