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?
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.