We recently discovered that our downstairs bathtub has a large hole in the slab beneath it where the tub drain and a few other pipes go into the dirt. We had a radon mitigation system put in a few years ago to deal with elevated levels, but our monitors show that it still gets up to around 3-4 pCi/L in the winter. (Current 7 day average is 3.78 pCi/L). Granted, that's (barely) below the EPA's recommended "action" level, but I work from home and my office is in the basement, so ... (cough, cough) ;) We're thinking that that hole in the slab is potentially how a lot of the radon is still getting in, and wondering if it would be worth it to somehow fill it in and seal it, and if so, how. Thoughts?

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It would be cheap and easy ( my favorites for any home improvement) to spray urethane foam under the tub. You can get it at any lumber/hardware store. It would certainly reduce the amount any gas that might enter that way. It makes a "closed cell "foam which is relatively impermeable.

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