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Community and Q&A

French drain with radon concerns

syakoban | Posted in General Questions on


DIY remodeling and focusing on the basement now. Last year I did a 48 hour(?) radon test and it came back a little over accepted limits. We also need to install a french drain to mitigate moisture issues. I’ve studied the french drain under the basement slab process and am ready to get cutting on the slab, but there seems to be a contradiction with radon mitigation.

The recommended french drain procedure is to place a dimpled plastic membrane about 6″ up the wall, and spanning across the footing into the trench to allow water from 1/2″ wall weep holes a path to the gravel/pipe drain. The problem is that also creates an air vent for air and radon under the slab to circulate into the occupied airspace. I can’t find any reasonable way to seal the top edge of the dimpled membrane. The one manufacturer said to use caulk on the 5/8″ gap, which sounds like a bad idea for multiple reasons.

I’ve also seen mention of attaching the radon vent to the french drain system to pull the air out that way, but with 160+ feet of perimeter, that’s a huge air intake that I wouldn’t trust to work properly.

What is the way to solve this so that fixing a moisture problem doesn’t create a radon problem? Is there any top sealed dimpled membrane that would work?

Thank you.

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  1. Expert Member
    Dana Dorsett | | #1

    The drain won't create a radon problem, but if it isn't well sealed the radon fan will depressurize the house, not just the slab. Hooking up a passive stack to the perimeter drain would still pull some room air under the slab, depressurizing the slab only slightly, but keeping contaminated soil gases from coming up through the dimple mat, since the net flow would be in the other direction, on average.

    If the test came in only a little bit over the 4.0 pCi/L recommended EPA level in the BASEMENT it's not a huge concern, and slab depressurization would be the last resort. Balanced ventilation (HRV or ERV) will usually cut the radon levels by half or more.

  2. Expert Member
    Peter Yost | | #2

    Hi syskoban -

    there are a lot of different ways to test for radon. Any test you perform subsequently needs to be reported during any sale of the home.

    I have been testing radon in my own home for more than 15 years. Radon levels vary widely and for that reason I would not trust any short term test to be really representative of radon levels. Go with a longer term test and make that part of the record.

    Couple or so GBA blogs on radon:


  3. syakoban | | #3

    Thanks for your replies Peter & Dana.

    I know about reporting testing results at the time of sale. I did read the articles you suggested Peter and they do provide a better understanding. Thank you. Like you're article states, I didn't do a long-term test because of having to wait for results for so long, but I can see where it still should be done.

    I don't fully follow why the french drain won't create an increased radon problem since the dimpled membrane connects the below slab air to the room air, presumably allowing circulation the same way an open sump pump pit does. It feels like leaving that pathway is asking for increased radon levels and we do plan to spend a lot of time in the (to be) finished half of the basement. Can you explain?

  4. Expert Member
    Dana Dorsett | | #4

    >"I don't fully follow why the french drain won't create an increased radon problem since the dimpled membrane connects the below slab air to the room air, presumably allowing circulation the same way an open sump pump pit does. "

    It's because a passive stack will reverse the air flow using stack-effect to modestly depressurize the slab. Passive radon systems of this type are standard for new-builds in some radon prone areas, with fans added only if they test higher than the 4.0 pCi/L recommended remediation level. As long as there is a stack pulling on the French drain under the slab it will depressurize the sub-slab relative to the basement room air. The gas movement will be from the room down the dimple mat to under the slab then up the stack. With the high cross sectional are of the dimple mat's air leak, adding even a small radon fan to the stack would depressurize the basement a LOT, drawing outdoor air into the house through the most available leak. But that same cross sectional area makes it more likely for the passive stack to just work.

    You still have to seal all other penetrations to the slab though, since pulling air through a low impedance path along the perimeter of the slab won't necessarily pull hard enough to fully draw through cracks or holes out in the middle of the slab unless there is a pretty good layer of clean gravel under the slab.

    If the passive stack doesn't cut it, a single set of Lunos e2 (paired ductless HRV) running 22 cfm on opposite sides of the basement will likely cut the levels by half or more, and deliver better overall air quality into the basement year round.

    If the radon tests STILL come in too high, before adding a fan, add a no-moving parts venturi-type cap such as the Empire SV04A (rated 65 cfm in a 4mph wind, if it on a very short stack on an open attic rather than a tall radon stack) to enhance the amount of pull without overly de-pressurizing the house, driving excessive infiltration.

    A wind powered venturi type cap on the radon stack proved adequate for bringing my next door neighbor's house under the EPA remediation level even without an HRV. YMMV.

  5. Jon_R | | #5

    Air leaks from the room to the depressurized space will cause increased airflow, energy loss, increased basement humidity, etc. But with enough flow, it can still depressurize and reduce radon.

    > I can’t find any reasonable way to seal the top edge of the dimpled membrane.

    Perhaps paint applied to the concrete and then a good tape?

  6. FluxCapacitor | | #6

    My house has a French drain system just like what you described, with no sealing at the top of the plastic collection membrane. Its about 100 ft long and was there when we bought the house.

    Radon test results were below the federal recommended action level but I decided to put in a Radon mitigation system anyway.

    The radon mitigation company stuck a pvc pipe in the French drain system, put an air tight lid on the french drain sump pit, and vented this “design” with a typical radon air blower.

    In my case this this French drain venting system was extremely effective in removing Radon. I went from 2-3 pCi/L to under 0.5 pCi/L.

    I retest every couple of years to verify levels.

    As always, your mileage may vary.

    Note: As pointed out above this system is far from ideal in regards to heating and cooling efficiency.

    1. Expert Member
      Dana Dorsett | | #7

      Have you tested to see how well it works on stack effect alone?

      Or is the stack not as tall as the house? To work without a venturi vent cap has to be at least a few feet above the attic floor's level, better yet a few feet above the highest ridge, otherwise there are competing stack effects. My neighbor's radon stack is 3' above the ridge, ~8-10' above the attic floor (vented attic), and above all plumbing stacks, but still a foot or two lower than the masonry chimney used by the boiler.

      1. FluxCapacitor | | #8

        Hmm, interesting.

        My radon stack is about 6 feet above attic floor and level with top of ridge. Although so level with the two plumbing vent stacks.

        In my neck of the woods the highest radon exposure is usually in spring.

        Maybe I’ll try disconnecting the radon fan and do a spring test.

        Good tip.

  7. syakoban | | #9

    After some time researching and planning, I'm finally in the midst of the french drain project.

    Thank you very much for the extremely helpful support here.

    I have found a replacement for the mat that would have been open to the basement, that will have the same function, but with this material, concrete covering the trench can seal over it to the wall. So with that and a new sump pump well w/sealed cover, we will have a reasonably sealed system that is both french drain and radon accumulator.

    So based on advice from this forum, I am going to passively vent the combined pipe up through a conditioned space, through the attic, and through the roof to a height exceeding the ridge.

    My question is about the route of the vent pipe. There will be a little jog to get up the basement wall and through the conditioned space. I will then need to slope it in the attic up towards the ridge where it will turn vertical again and exit through the roof. Is it OK to not have a straight through pipe but have what I described?

    1. TheTodddd | | #11

      I am attempting to do a similar French Drain system as you and had the same concerns. I was curious, what is the mat product you used?
      Thanks in advance.

  8. Andrew_C | | #10

    A completely straight run would be great, but unusual, especially in a retrofit. Like most other plumbing, you want to minimize turns and make any turns gradual, not 90-degree turns. Sharper turns significantly increase the effective length of the pipe from a flow resistance standpoint.

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