I've seen a drastic improvement after sealing all cracks in the floor. The method of construction was to pour the basement walls, then later pour the floor. That left about a quarter-inch gap around the entire perimeter. I used a radon sealing caulk (from Menards) and sealed the entire perimeter. Since doing that, my levels are rarely above 2 or so. Usually in the 1 range. I think I'm almost nearing outdoor levels for most days.
Between the fan and the sealing, I've gone from almost 30 down to 1. Pretty awesome.
Just because I was curious, when we were out of town for 10 days, I turned the radon fan off. I anticipated not much increase, but boy was I wrong. Even with the sealing, it jumped to 17. Needless to say, keeping the fan on all the time now.
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