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Can a foundation drain to daylight double as a radon vent?

user-5516607 | Posted in GBA Pro Help on

Ok this is probably a dumb question, but I’m looking at putting in an addition this spring, and was looking at ways of draining it and stumbled onto radon mitigation. So now I’m not sure how it would all fit together.

If we have a 4″ perforated foundation drain running around the outside of our new footing and drained to daylight, and connect it through the footing to a 4″ perforated drain running along the inside of our footing (in a 6-8″ bed of gravel, with 2″ rigid insulation on top of that and then a vapour barrier and then the 3″ basement slab), is there a good reason not to let that be my radon vent as well?

One would think that since its 8x heavier than air, the radon would take the same easy route that water will take and run through the pipe and out the other end, and then there would be no need for another vent stack running up through the house.

But then, I also want a basement floor drain, which would hook directly into that system, and i suppose that would make the stack effect pull the radon into the house instead of out the bottom. It nicely bypasses the wonderful sealing effect we created with the vapour barrier, and while great for water (no sump pump required, ever) its no good for gas.

What if I countered that with a U-trap under the floor drain so the water blocks the radon from escaping the system (would it even do that?) Or a U-trap AND a roof vent, so theres no easy access to the basement but the stack effect pulls it up the vent, and can draw it through the entire drain system? Or am I just breeding mosquitoes?

I know I’m over thinking this, i dont even know if i will have radon… But I would like to do this thing right the first time, and these are all easy and cheap to implement if you do them at the start and not when it comes up later.

Any thoughts?

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  1. GBA Editor
    Martin Holladay | | #1

    Some components of your footing drain system can be considered part of a passive radon system. But things get complicated fast if your footing drains are drained to daylight.

    You're right that a passive radon venting system depends on the stack effect. The outlet is the pipe that penetrates your roof, not the drain to daylight.

    You need to get these details right. A P-trap or U-trap won't work, because footing drains usually are dry (and water in a trap would evaporate).

    For more information, see All About Radon.

    And here is a link to a Q&A exchange on a similar topic on the Fine Homebuilding web site -- with an answer I wrote to a Fine Homebuilding reader: Radon mitigation with interior perimeter drains?

  2. user-5516607 | | #2

    So in your answer to the FineHomebuilding reader you said they would need a check valve at the termination end. Is the point to pressurize the drain system so that the only outlet is the stack vent? So no air gets through the outlet, and presumably none through the floor drain either (another check valve?), leaving only the stack?

  3. GBA Editor
    Martin Holladay | | #3

    A passive radon mitigation system includes a vertical pipe (the riser). The lower end of the vertical pipe is in the crushed stone layer under your basement slab, and the upper end of the pipe penetrates your roof. Air that rises up this pipe due to the stack effect depressurizes the volume of air under your basement slab. That depressurization is desirable; you want any radon that is hanging out under your slab to rise with the air that is pulled up the vertical pipe.

    If you forgot to install a check valve on the drain that leads to daylight, then outdoor air would be pulled into the vertical pipe, and your crushed stone layer would never be depressurized.

    1. Deleted | | #19


    2. charles_99 | | #21

      (Forgive me for responding to an old post, but this is a new problem for me).

      Where a passive radon system is used in conjunction with using a drain-to-daylight to control water levels under a basement slab, you warn that a check valve must be installed on the drain that leads to daylight to maintain depressurization under the slab. This would be an air check-valve (allowing only one-way air flow) rather than a backwater check valve (allowing only one-way water flow). Correct? If so, do you know of any vendors that sell such air check valves for drains? I've tried Googling without any luck. Is there a special nomenclature for these types of drain valves?

  4. user-5516607 | | #4

    If you have a check valve at the bottom should i assume you also put a check valve on the floor drain?

    So to be clear, there would be an open vent stack through the roof, a check valved floor drain from the basement leading to an inside and outside foundation drainage system which is check valved at the outlet to daylight. The only open-to-air escape is through the stack, though water can escape the basement if it flooded and escape the drain at the bottom.

    Check valves are just a standard home depot or equivalent backwater valve? I didnt even think of using these, but i bet it would double as a great deterrent for muskrats crawling up your pipe and making a nest!

  5. GBA Editor
    Martin Holladay | | #5

    In general, the best passive radon mitigation system won't include a drain to daylight or a floor drain.

    If you are going to deviate from the usual details of a passive radon mitigation system, I suggest that you consult a radon mitigation contractor.

  6. user-5516607 | | #6

    Ok. Frustrating that the best floor drain system makes for the worst radon mitigation. Thanks for the help anyway.

  7. GBA Editor
    Martin Holladay | | #7

    If your basement is well-designed, it doesn't need a floor drain.

    As for the footing drain to daylight, it isn't ideal (from a radon perspective), but you can follow the advice I gave in the Fine Homebuilding answer and still make your radon system work.

    As I noted in that answer, "You can integrate a radon-mitigation system with your basement floor, but the details might be tricky. The two systems you describe—an interior drainage system designed to collect liquid water and a radon-mitigation system designed to collect and remove radon gas—aren’t completely compatible, so compromises and special details are required to make sure that both work."

  8. Chaubenee | | #8

    I would also be careful about installing a floor drain that leads to daylight because your building inspector may not approve due to water pollution guidelines and regulations. The idea is that chemicals, heating fuels, household cleaners and soaps might end up downhill and untreated into the water bodies downstream. Be aware of that issue.

  9. Expert Member
    Michael Maines | | #9

    Radon systems are for getting rid of a gas. Floor drain systems are for getting rid of liquid water. It shouldn't be any more frustrating that they are best kept separate than thinking that copper water lines could also be used to carry electricity, since they are both installed in the walls and made of copper.

  10. JTyler | | #10

    I am in a similar position to the OP, with a foundation going in this spring. I have a question along these same lines.

    If I'm understanding correctly, an interior perimeter drain to daylight can serve as the under-slab component of a passive radon mitigation system by simply adding a check valve and stubbing up a riser off of the perforated pipe and routing it through the roof.

    My questions is: if the home still has high radon levels, could an inline fan be added in the attic to convert this to an active system?

  11. GBA Editor
    Martin Holladay | | #11

    Again, as I wrote before, from the perspective of an effective passive radon mitigation system, it's best if your footing drains (the pipes that lead to daylight) remain outside of your footings, and you don't try to connect the crushed stone on the exterior side of your footings with the crushed stone under your slab. Keep these systems separated, without any easy way for air to flow between them.

    If air can flow between these areas, you definitely need a check valve on the footing drain outlet. A check valve complicates maintenance, however, and is not ideal.

    As far as I understand, a passive radon mitigation system that includes a drain-to-daylight pipe with a check valve can be retrofitted into an active system by adding a fan in the attic -- but again, I strongly advise you to contact a certified radon abatement contractor to nail down these details rather than listening to me.

  12. user-5516607 | | #12

    Joe that would be hilariously ironic, because up until i redid all the plumbing in the house (which is 180 yrs old and with all the nonsensical additions you would and wouldnt expect), the only water that went to the septic system was from the shower and the toilet. Everything else - laundry, sinks, floor drains, etc went straight to the ditch through a plugged up barely workable clay tile. so whatever i have now is head and shoulders above what used to be there (not that the inspector would have a clue what was there before). And yes there was a floor drain in the main house, but as the clay tile was plugged it mostly didnt help. Now its connected to 4" pvc pipe and the stone basement (which also has a well INSIDE it!!) should never flood again.

    I'll talk to a radon/air quality guy i know about the addition and see what comes of it. Most likely i will just go with the foundation drain outside only and waterproof the hell out of it and ignore the radon and the idea of floor drains, but I'll let people here know what happens.

  13. Ericsundial | | #13

    Hi Terry,

    I have for some time enjoyed the information available from GBA. This is my first post (should I be concerned about the potential for addiction?).

    I have a fair bit of experience dealing with radon mitigation (in Winnipeg Canada) as well as general building performance issues and have a couple of thoughts related to your post. As some others have suggested, it would be ideal to have the floor drain separated from the sub-slab-soil/drain-tile system and connected directly to the septic system. I understand that if there are not any other sub slab septic services being installed that connecting the floor drain to the septic system may take more effort than desired. If you are able to connect the floor drain to the septic system you will want to consider an in-line back flow valve to prevent the septic system from ever backing up into the basement of the addition from the floor drain (this may be a local building code requirement). If a floor drain is to be connected or open to the sub slab soil/drain tile system then a floor drain cover that allows liquid to flow down but prevents air (radon) from travelling up into the living space should be used. Dranjer Corporation specializes in drain covers that do just that and are designed to provide the desired seal even when the water trap is dry. I hope this is helpful. -Eric

  14. GBA Editor
    Martin Holladay | | #14

    Thanks very much for your helpful comments.

  15. Expert Member
    MALCOLM TAYLOR | | #15

    What a fortuitous post. I was just looking for a back flow valve drain. Thanks!

  16. Chaubenee | | #16

    Eric, do you have a source for air-tight sump pit covers as well? My plan was to add a 4"tee and riser for a passive system that could be turned into a power vented system but we set up a daylight drain as well under the slab in ten inches of gravel. I can add an Oatey back flow prevention valve inside the sump pit on the outgoing side or at the foot of the pipe downhill. In order to de pressurize under the slab I would need an air tight cover.

  17. GBA Editor
    Martin Holladay | | #17

    Jackel, Inc. makes airtight sumps and airtight sump lids. Here is the link:

  18. Ericsundial | | #18

    Hi Joe,

    Yes, Jackel does makes a nice air tight retrofit sump lid cover. Their lid seals to the floor slab which is handy in situations where the sump pit is not very well sealed to the floor slab or sub slab membrane which is often the case.

    We have also commissioned the fabrication of our own version of the airtight sump lid which includes machining to accept the Dranjer JN6. This is quite handy when there are condensate drains routed to the sump pit and also makes a handy inspection point. A while back I put together a slideshow showing some floor drain and sump lid installations which I have shared below.

    It might be worth noting that we would rarely recommend using the sump pit as a suction point for sub slab depressurization if the soil drainage system (weeping tiles) terminates at the sump pit. A sub slab depressurization system is designed to create a low pressure condition under the slab. Using the sump pit as a suction point causes the low pressure field to be extended to soil surrounding the foundation often resulting in higher than desired airflow requirements meaning the need for a higher power fan and resulting energy penalties particularly in cold climates. However there are some situations with challenging (dense) sub slab soil conditions where the sump pit does provide the most effective sub slab pressure field extension.

    1. arts327 | | #22

      Thanks so much for those pictures! (7 years ago). I just stumbled on this and am curious about the sump cover drain trap you have installed and highlighted in one of the images - do you have a source for that item? Much appreciated!

  19. Deleted | | #20


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