OK, the backstory can be found in this previous questions: Repairing leaking DWV vertical 3" thin wall PVC. Basically, found mold growth behind baseboards of a powder bathroom adjoining the kitchen. Discovered it was due to a slow condensation leak from the joint of a vertical radon vent pipe in the wall. With help from Stack Exchange, I figured out the pipe size, cut out the leaky joint, made the repair with two slip couplers, and everything was great. The area stayed dry for a good (dry) month.

However, we recently had a sizeable snowstorm, and when I checked the pipe today, the new joints were great, but there was water on the base studs next to the pipe. Looking closer, there is water coming up from the slab where the vent pipe exits the slab. From my pictures previous to the repair, I see there was some sort of sealant around the pipe at the base, which looked old and worn so I removed it.

I'm particularly concerned about a source of water underneath our foundation, as awhile ago I discovered in our kitchen a few tiles that cracked (all in a semi-straight line, covering the kitchen, and pointing towards this powder bathroom). I haven't pulled these tiles yet, but I can see the foundation under the kitchen island, and there is a smallish crack (<0.125") in line with the cracks in these tiles.


  1. Should I seal around the radon vent pipe at the slab foundation, to prevent the water seeping up? If so, what should I use?
  2. Should I be worried about water underneath our foundation?
  3. If I should investigate further before sealing the pipe base, what should I look for? Or who would you call to investigate a source of water under the slab foundation, a plumber?

One last thing, that may be important, the hot water to the kitchen sink has always been slow (about half flow speed of cold; faucet has a continual lever). Previous owner said it was a safety feature of the faucet, but I've never had a plumber check it out.

  • 1
    Where are you and what kind of slab? Is this a basement slab? Or slab on grade?
    – longneck
    Commented Jan 29, 2016 at 2:53
  • Slab on grade (no basement)
    – Michael
    Commented Jan 29, 2016 at 18:02

2 Answers 2


I prefer all slab penetrations to be sealed, especially if you are in an area where radon is such of concern to require a vent. Your local building code should spell out what is a locally acceptable material to seal the penetration but any sealant with draft stopping capabilities and made for wet areas should do the trick.

As to the water under the slab, nearly all of us have water under our slab, just some deeper than others but enough water for intrusion through the slab, crack, or otherwise in to your home is very concerning. The fix depends on a novel of conditions that could be causing or allowing this to occur.

  • Thxs, would you recommend I contact a plumber or a contractor to look at this?
    – Michael
    Commented Jan 29, 2016 at 18:06

You should also ensure good drainage around the outside of your home and that snow is cleared a few feet back from your foundation. I had trouble with water seeping under walls when I did not move snow back a couple of feet from my foundation. The snow would melt and seep under the walls and under my laminate flooring.

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