I bought a vacation home in the mountains and have found that it has a dangerous radon level (7.4pCi/L). The house has an 800 sq-ft unfinished basement, including a rough plywood floor over treated joists. I'd like to address the radon problem and simultaneously start to finish the space.
My initial thought was to pull up the plywood, install a vapor barrier on top of the joists, install a fan to suck air out of the sub-floor space and vent it out the side (with or without a make-up air intake), and replace the plywood (more securely and perhaps with better plywood). But I'm concerned moisture might get trapped between the vapor barrier and the underside of the plywood, and the plywood fasteners could compromise the integrity of the vapor barrier.
The usual sub-membrane method is to lay the vapor barrier on the soil. It might be tricky to get the plastic underneath the joist and to access it for taping seams. A bigger problem might be that there's no longer that air space to trap and vent out the radon. This would require installing sumps or corrugated pipe to collect the radon and vent from there.
I prefer to put the vapor barrier on top of the joists. The space between the joists and soil is a ready-made sump of sorts. Putting the vapor barrier on the soil creates the added complication or needing sump(s) or corrugated pipe.
Is it ok to put the vapor barrier on top of the joists ?