Sealing basement drain tile to mitigate mold/odor
October 27, 2021 3:14 PM   Subscribe

Small house in the Midwest, built in the 1950s, concrete slab and cement block walls. A few years ago, I had drain tile (where the floor meets the wall) and a sump pump installed. This apparatus does what it says on the tin, collecting water that rises up through the foundation. However, the system intentionally leaves a small gap between the floor and the walls, such that water which might trickle in down the walls is caught by the drain tile. I need to seal the system so it is airtight. Special snowflake details inside.

Unfortunately, while this opening will capture water that runs down the walls, it also leaves an opening to sub-basement air, and to standing water that sits in the drain tile system, collecting mold. My home has mold and odor issues, and I strongly suspect that part of the problem is this drain tile open to the air.

See this image for example:

I'm trying to brainstorm ways to completely and seal the system. My first thought is to use a right angle piece of vinyl trim which meets the floor and the wall, and apply caulk such that it forms an airtight seal. I'd like your ideas and suggestions.

posted by 4midori to Home & Garden (4 answers total)
If there's not actually any water trickling down the wall, what you propose sounds like it would work. If there is water trickling in, you could address that separately, (a) with a dehumidifier in the basement, (b) by sealing up any cracks in the wall, preferably from the outside, and (c) by grading the soil surface outside so it slopes away from the foundation in all directions. And maybe (d) by installing gutters that move water well away from the foundation. The dehumidifier might even solve the moldiness problem all by itself, depending on how much standing water there really is.

You might want to do a radon test (before implementing your solution), because with that trough of pebbles basically your basement is fully open to incursion of radon gas if it exists in the soil. So your solution would help mitigate that as well, but certainly would not fix it, since radon will travel right through the concrete floor and walls.

Also check out this video.
posted by beagle at 4:04 PM on October 27, 2021

I wouldn't do just that, because I don't believe it will be sufficient and it would leave you open to flooding.

Concrete is porous. The whole wall is a wick releasing moisture into the air. In a new build, there is waterproofing applied to the outside of the wall and the drain tile is installed outside, so the wall stays dry. In your case, your wall will always be "wet", so the water/moisture-proofing must be done on the inside. The ideal is 2-3" of EPS or 1-2" of XPS foam along the wall from floor-to-ceiling, appropriately tied into the drain tile opening at the bottom. This has the additional benefits of allowing your basement to be warmer. You mustn't use thicker foam than I stated as a little moisture must be allowed to escape. This is the best practice currently; old approaches were plastic sheeting, wood wall directly on concrete, or very thick foam, all of which have issues.

On top of the foam you can build a stick frame (wood or metal) wall and drywall the top. Alternatively you can apply the drywall directly to foam if you are tight for space. Either way, the foam must be covered with a fire-retardant layer.

Similarly the floor is likely wicking some moisture up, but much less because the drain tile should prevent much moisture there. Here paint could be good enough.

Beagle is right about radon and the fan. Adding a radon fan to the drain tile (plus the above fix) will result in a small negative air pressure in your basement's highest moisture area.
posted by flimflam at 11:01 AM on October 28, 2021

Response by poster: Thanks, all. To clarify:
* To the extent that a small amount of water trickles down the wall, I'm not worried about that, it will flow to the drain or be evaporated by the dehumidifier.
* I have addressed the other best practices, including outside grading and gutters.
posted by 4midori at 11:59 AM on October 28, 2021

I know lots of people with open drain tile and none have this mold/moisture issue. Usually the drain tile solves the moisture issue, actually.

If you ever had sewer back up through a toilet, you wouldn't think the answer would be to pour concrete in the toilet bowl.
posted by flimflam at 2:59 PM on October 28, 2021

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