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 ?

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    You could start with this question: diy.stackexchange.com/questions/246639/…. See the comment about radon mixing attached as a comment to the answer. Commented Apr 14, 2022 at 6:18
  • "Can I successfully access the space to install the plywood, by walking on the joists to avoid stepping through the vapor barrier?" Can it be done? Yes. Can you do it? Dunno. I spent a stint doing construction and walking joists was a pretty common occurrence. Doing it on slick, plastic covered joists will be more difficult. Start at the door, lay your first sheet of plywood. Stand on that to lay the next. Also far too many questions in one.
    – FreeMan
    Commented Apr 14, 2022 at 12:32
  • @FreeMan - Good point, no harder than walking on attic joists when doing work up there, except easier to repair a mistake than if you step through drywall, and less chance of breaking a leg. Commented Apr 14, 2022 at 16:07
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    @FreshCodemonger - you're talking about this comment: "even considering its large atomic mass, there is very little tendency for radon that is in the air to "settle out" and sink." Interesting, thanks. Commented Apr 14, 2022 at 17:02
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    All due respect, I know you're a rather handy guy and it's no insult to your abilities or ingenuity... but amateurs should not be designing radon systems for liability reasons. Lawyers love cancer, and the last thing you want is to be on the witness stand saying "But I thought if I..." If a pro designs the system, liability is on them not you. You did your part. That is especially true if using active systems e.g. a fan. Commented Apr 14, 2022 at 19:54

1 Answer 1


I suspect just installing the fan to vent the space would eliminate the problem. Do you detect 7.4pCi/L at the living space area or is this a reading from inside your crawl space.

Either way I'd start cheap, install the fan, check the results. I wouldn't worry about it going out the roof. Once mixed the gas won't separate in an appreciable sense.

What is your radon detection device? I use the AIRTHINGS Corentium Home which gives 1 day, 7 day and long term averages. I have a crawl with a vapor barrier (6 mil poly with taped seams) and a slab on top of it. I don't live in an area known for high radon. Without ventilation the readings have varied in the crawl from 1.x to 4.x over the course of a year (2.4 is recommended mitigation). If I ventilate (.33 ACH) the crawl space the reading drop below 1.

  • We just bought this place and had the home inspection guy do it - I'm pretty sure he did it in the basement. Yes, I just bought one of those AIRTHINGS units, and will be interested to see how it reads when we go up there in a couple of weeks. Commented Apr 14, 2022 at 16:09
  • If I just try a fan as you suggest, to see if that's sufficient, would you just vent the basement, or the area under the plywood ? Would you have an intake as well ? Commented Apr 15, 2022 at 7:13
  • I'd vent the crawl space. Air intake won't matter too much... if it is really cold outside it might be better to have air intake directly into the crawl otherwise you pull your conditioned air into the crawl and outside air into your conditioned space. Commented Apr 15, 2022 at 15:52
  • If the house above is well built and sealed, the lack of an intake would result in negative pressure in the basement and effectively suck more radon out of the walls. An intake is very important as you can make somewhat of a guarantee about its freshness, particularly if it's coming from a HVAC system. Commented Apr 15, 2022 at 17:34
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    Thinking about it a bit more, if you have any CO emitting devices you'll need some kind of makeup air so you don't backdraft CO into your cabin. So probably just have a hole at the crawl level (below basement joist floor) equal to the size of your output hole to allow for intake. Commented Apr 15, 2022 at 17:42

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