We have high radon levels (~10) and I would like to add a mitigation system. There is a sump pit in the basement, but there are no drains flowing into it (i.e. it's getting water directly from floor of pit.) I'm concerned about sealing this for the radon vent because of the vacuum that would be created when the sump pump runs. But if I don't seal it, then I don't think the vacuum necessary for radon mitigation to work will be possible. What do I do in this situation?

More Details:

House is 110+ years old -- when I reach to the bottom of the sump pit all I feel is soil. The center of the house has the basement but it seems there have been multiple addons -- about half of the house is on top of crawl spaces which are accessible from the basement.

The floors of the crawl spaces are all just dirt.

  • Is your house modern and with tight seals? What are you doing for makeup air when the furnace or dryer is ejecting air out of the house at high volume? Commented Sep 24, 2022 at 20:56
  • No house is 100+ years old Commented Sep 24, 2022 at 21:02
  • So I’m not doing anything for those things Commented Sep 24, 2022 at 21:03
  • Where are you going to try to extract the radon gas from? From the basement? Or from the ground under the floor (concrete, I hope) of the basement?
    – SteveSh
    Commented Sep 24, 2022 at 23:03
  • When I reach to the bottom of the sump pit I just feel dirt -- so that's where I assumed the venting for radon system would begin. Is that incorrect? Commented Sep 25, 2022 at 0:22

1 Answer 1


If your sump pit extends into a layer of crushed stone, rubble and whatever else is under the concrete of your basement slab, then I have what you have. Except my radon was 55 pCi/l before remediation. Now it is zero, i.e., no detectable radon. The remediation was done by a certified radon remediator before I moved in.

You have to seal the pit or you won't be able to maintain negative pressure in it. My setup has a square of 3/4" marine plywood glued down to the concrete with a polyurethane sealant, completely covering the pit, plus it's caulked with the same stuff all around its perimeter. In addition, all seams and cracks in the concrete in the vicinity of the pit are sealed with a self-leveling sealant. The blower's suction tube passes through the plywood and is sealed with the same goop.

There's a square hole cut out of the center of the plywood that's big enough to pass a sump pump through. That hole is covered with a square of Lexan with a hole in it to pass the sump pump's pipe and power cord. The Lexan is screwed down to the plywood and it's sealed all around with RTV silicone. The pipe and power cord hole are also sealed with silicone. In the event that the sump pump needs service, you unscrew the Lexan, peel off the silicone and remove / repair / replace the pump. Then you seal it up again with more silicone.

There is a manometer attached to the blower's vacuum pipe -- a U-shaped tube with colored liquid in it. It shows about a 1-inch-of-water differential between the sump pit's standing pressure and the basement's pressure. That negative pressure extends through the interstices in the rubble to maintain negative pressure below the entire slab. The whole deal works trouble-free.

The sump pump doesn't care at all that it's pumping from a slight vacuum. It completely overwhelms the negative pressure, since its head pressure is measured in feet, not inches. There's a check valve in the sump pump line. If not, when the pit runs dry the pump would admit air through the pipe into the pit.

There is no measurable change in air pressure in the pit when the sump pump runs. The water level drops and air from under the slab replaces the missing volume. So no cause for concern.

  • This sound right, so long as you have a house built with the basement poured over a layer of crushed stone that can collect the radon gasses. But, OP's house is 100 hears old. His house may have well been built with a dirt floor basement and then, sometime in the past, a concrete floor was poured over the dirt. If that's the case, then I'm not sure trying to create a negative pressure in the sump is going to do much.
    – SteveSh
    Commented Sep 25, 2022 at 0:13
  • @SteveSh Exactly right, that's why I qualified the answer with my first sentence: "If your sump pit extends into a layer of crushed stone, rubble and whatever else is under the concrete of your basement slab, then I have what you have." I'm willing to edit and clarify further if this is unclear. Suggestion?
    – MTA
    Commented Sep 25, 2022 at 0:18
  • I think your answer is fine - that's why I gave it an up-vote. My comment was not meant to be a criticism.
    – SteveSh
    Commented Sep 25, 2022 at 0:34

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