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Radon Mitigation for Dirt-Floor Basement

rustyshackleford | Posted in General Questions on

I’ve bought a new house with somewhat high radon levels.   Inspectors test, placed in basement, gave EPA protocol reading of 7.4 pCi/L.   I’ve purchased an electronic meter and am getting comparable levels.   I’m handy with DIY and plan to mitigate myself.

The house has a “day basement” (partially above grade, partially buried into a hillside), and two floors of living space above that.  The basement already has a floor system of joists and plywood, slightly above grade; below that, on the soil, is a vapor barrier, but I’m not sure how good it is (are seams taped, etc).

I’m hoping I can simply ventilate the space between the vapor barrier and the plywood and achieve adequate mitigation.  My plan is to run 4″ PVC from a hole in the plywood, put an inline fan, and vent out the side of the cinderblock basement wall (where the basement is above grade).  Initially, at least, I don’t plan to pipe up to above the roofline.   There is only one window anywhere near above the place where I’ll penetrate the basement wall, and it’s on the top floor (so about 20ft above where the vent will be).   

My main question (other than is this likely to work, which I’ll find out and do more if not) is what kind of fan to use.   I simply don’t know how much suction will be required.   It depends mainly on the integrity of the plywood floor, I suppose.   I could add a makeup air vent as well, and then I wouldn’t need much suction, but I’d be introducing outside air into the space under the plywood: that would be a heating load in the winter (I hope to finish the basement out into a rec room or something) and could introduce excessive humidity in the summer.

Here’s a simple drawing:

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  1. jberks | | #1

    Well, I find this interesting for various reasons, but hey, systems design.

    Since you're planning to make it usable space, the more appropriate approach would be to do a concrete slab assembly. You don't mention it, I am not sure if you are purposefully avoiding it. If not, a more appropriate assembly would be something like a 4" layer of 3/4" clear gravel with your radon/groundwater system within it, stego (or similar min 15mil) vapour barrier, 4" concrete. Where your radon fan would create a negative pressure within the gravel layer.

    Otherwise, consider just good general ventilation if you're looking for a more immediate solution. I'm not a radon expert, but I believe Radon creeps slowly in over time and usually accumulates in unventilated areas. Certainly you want to mitigate radon from even coming in, but take this opportunity to look at your homes ventilation system as well.


  2. brad_rh | | #2

    I'm not a radon mitigation expert, but I think the way it is usually done is the way it was done in my house. Several perforated pipes are laid on the ground and are connected to the fan. A tough airtight fabric is on top of the pipes. They didn't add a gravel layer. The fan pulls a negative pressure below the layer. In my case it's in a crawl space. Search for radon fan.
    You're plan is probably worth trying since it looks like it will be hard to put the pipes under your air barrier. I would not add make up air, the ideas is to create a negative pressure below the floor, not to pull a lot of air. Seal up the plywood floor as best as you can.

  3. rustyshackleford | | #3

    Thanks for the responses ...

    It seems to me that a plywood on joist floor is a perfectly adequate flooring system for this space - I can't think of any reason to incur the expense of pouring a slab down there (and it'd be worse than most, requiring pumping the concrete).

    I realize under slab is the usual method of mitigation. And/or under vapor barrier - but getting pipes under the vapor barrier would be a huge PITA, requiring pulling up most of the plywood first. It seems to me that virtually any radon that enters the house is going to have to get above the vapor barrier, and then above the plywood. If I can vent that space between the vapor barrier and the plywood, seems like I'd get a lot of mitigation, and I don't need anywhere near 100% mitigation, my levels being relatively low.

    But I'm not sure how best to accomplish that. Specifically, what rating of fan.. Do I need one like the AMG Eagle or Eagle Extreme that can move air against 4-5" H2O, or something more modest ?

    1. Expert Member
      Deleted | | #5


  4. kbentley57 | | #4


    I might be having a dense moment, but is the plan to install a joist floor system, or does one already exist?

    If it doesn’t exist, you should do the math. Even with a pump truck, you’re probably going to come out cheaper, and better, with the slab. It can be counter intuitive. Not to mention the height loss.

    On the radon front, I’ll have to check my code book, but I’m nearly 99% certain that a radon mitigation vent must be placed at least some large distance above the ground, so that a child doesn’t go immediately stick their face in the pipe, and get a lunge full of 10k pC/L dose of radon gas. It’ll happen, probably with 1/3 of adults too. It needs to be higher.

    Most radon fans that you see have a few different modes. There’s really no good way to tell which you need, other than by buying one, installing it, and seeing if it drops the measurements down. Get one with good ratings and a good warranty and go from there.

  5. kbentley57 | | #6

    From, section 601.2
    and From , section AF103.5.3,

    Paraphrasing by me - "No less than 1 ft above the surface of the roof (doesn't imply that it has to penetrate the roof) and no closer than 10 ft to any window, air intake, or other air input into a conditioned space".

  6. brad_rh | | #7

    I don't think you'll need more than 1" WC, probably less.
    Until you sell the house you probably don't need to meet the code that Kyle quoted. It's going to dissipate outside.
    Around here everyone seems to be forced to get radon systems when the house is sold

  7. rustyshackleford | | #8

    Joist system and plywood covering - and vapor barrier underneath of unknown quality - are already in place. (Sorry, somehow that got cut out of OP, I just edited it).

    I am not really concerned about code. I guess I could run it high enough a child couldn't stick their face in it though. It will definitely not be within 10ft of any windows - like I said, there's one that's somewhat directly above the place I'd put the exhaust, but about 20ft up.

    Described the situation to one of the fan vendors, and they suggested a $100+ one that's good for about 2" WC. So I'll try that first.

  8. Expert Member
    PETER Engle | | #9

    Essentially you've got an inaccessible crawl space under the floor. An exhaust-only approach to venting that space can work for Radon, with the added advantage of venting moisture and odors out of the subfloor space. However, the constant fan operation will draw air in from other areas, potentially increasing your energy bills. If those "other areas" include penetrations in the exterior walls, the space itself can get very cold, making the floors above it uncomfortable.

    You haven't mentioned your climate zone. In cold weather, the floors above may become colder. In a hot/humid climate, exterior air in that space can cause condensation and mold growth. You mention that you aren't worried about meeting code, but you should still consider it. Both local codes and EPA radon mitigation standards apply. The codes are there to protect you as well as future buyers of the house. Radon is a carcinogen and kills more people every year than most other controllable factors. At least consult with a local licensed or EPA certified radon mitigation contractor to see their cost and approach. If you install a non-conforming system, you may still have to install a conforming system when you sell the house, so why not have the benefit of having one now?

    FWIW, one radon contractor I used to work with has been moving towards using ERV's to vent that sort of space. Same idea as yours, but balanced ventilation and minimizes the risks discussed above. Ductwork is simple, with one inlet at one end of the space and one outlet at the far end of the space. The equipment is certainly far more expensive than a single exhaust fan, but the other benefits are significant.

  9. rustyshackleford | | #10

    Thanks, Peter, for your comprehensive remarks.

    Climate zone is 4000ft elevation in North Carolina mountains. I'll have to look up heating degree-days etc.

    I feel I can insure my result is safe (as far as radon), having purchased one of those Airthings meters - so I can continue to monitor levels, longterm and in various locations. It's a good point, about resale, but radon testing is not a requirement in this state - and all we did was have the presale inspection test it and find out the level was high; if a buyer chooses to test it, and finds out the level is acceptable, I hope they won't be too concerned with the remediation scheme not meeting code.

    I don't think it's going to take much movement of air thru my "inaccessible crawlspace" to solve the radon problem. So I'll just do a small fan and see if it looks like those ancilliary risks you mentioned, are significant. But it looks like some of the ERV units aren't terribly expensive, so I'll check that out too. Not quite sure how the ductwork will work with that though... can you say a little more ?

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