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I live in a partially below-ground finished basement of someone else's house. I have recently tested for radon and found the level to be elevated.

There is a 2-foot deep sump pump pit in the basement that the homeowner says has not been used for over decade and the pump is not even plugged in.

I have discussed radon mitigation with some professionals, who recommend sealing the pit with a cap and running a pipe through a hole in the exterior wall to an outdoor fan for venting out the air. However, the homeowner is wary of putting a new hole in their exterior wall and would prefer to just fill in the sump pump pit.

Would this be a reliable way of mitigating the radon and, if so, what fill-in technique would you recommend? Complete concrete, sand, foam, plastic capping, combination, etc?

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No, it probably wouldn't be. Radon can come in through many ways, not just via the sump pit. It can come in through cracks in walls or the foundation, as well as other ways. See http://www.dea.human.cornell.edu/sites/default/files/pdf/radon_illustration.pdf. If you do fill in the sump pit, the radon will probably find another way into the basement.

If you wanted to try it and see what the impact is, I would suggest just buying a premade radon sump pit cover and installing it. Make sure you caulk around the perimeter. I would suggest against filling in the sump pit itself, were I the homeowner, unless I'd done something to mitigate against whatever water infiltration the sump pit and pump was put in there for in the first place.

  • I agree, but I wouldn't buy anything to do the test other than maybe some foam tape. Lay scrap plywood over the pit and place some weight on it. – isherwood Jul 6 '18 at 13:44

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