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Radon system design with drain tile to daylight

AndyMN | Posted in General Questions on

We are in the process of remodeling our basement and will be pouring a new concrete slab. The house was built in the 1940’s and sits on very dense clay soil. The old concrete has just been removed and we’ve dug down 4” below the top of the footings so that 2” of stone can be placed followed by 2” of EPS, a poly vapor barrier and new concrete. A one year radon test was completed two weeks ago and the average value was 6.6 pCi/l (test completed with all old concrete in place); we will be installing a radon vent prior to pouring the new slab. We have both exterior and interior drain tile that connect to a sump pump. With the exterior drain tile system there are vertical vent stacks that extend up to the into the bottom of the window wells for the basement windows. The tops of the vents are covered with a grate and about 1” of clean rock. The radon vent pipe in our slab will be located approximately 7-8’ away from the perimeter wall of the foundation. Being that the entire drain tile system is effectively open to the exterior air through the vertical vent stacks at each window, will a powered radon mitigation system be effective? It seems like I would just be pulling in exterior air through the drain tile and doing little to create negative pressure under the slab. It was suggested by a radon mitigation company that a PVC swing check valve be installed on the horizontal inlet pipe where the exterior drain tile enters the sump pit. I’m not sure how effective this would be in sealing the system as the valve could potential swing open when the radon system fan turns on. This may not be a concern, but I am not sure how much vacuum the fan is capable of creating. I also don’t want to introduce an unnecessary valve that may fail/clog and be impossible to access at a later date. Any advice would be appreciated.

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  1. user-2310254 | | #1


    Just to clarify, you are not connecting the radon pipe and the perimeter drain. The exterior footer drain ties into the window well drains and then drains to the sump. Is this correct? Is there any way to keep the exterior drain on its own loop and drain it to daylight without needing the sump?

  2. AndyMN | | #2

    No, the radon pipe will not be connected to the perimeter drain, it will be placed 7-8' out from the wall, which means it will be around 6' from the interior drain. We'll have a continuous bed of stone under the slab so I'm assuming there will be good air flow. I wish I could drain to daylight but our lot is much too flat and there is no place else to go except the sump pit.

  3. GBA Editor
    Martin Holladay | | #3

    To the best of my knowledge, if there is any chance that a radon mitigation fan's attempt to depressurize the soil under a basement slab we be defeated by the presence of a drain to daylight, the usual solution is a swing check valve in the main drain pipe.

    You may not like it, but I think that's the standard solution.

  4. Expert Member
    Dana Dorsett | | #4

    With the poly vapor barreir sealed to the walls all around the perimeter and all slab penetrations of plumbing etc sealed with an appropriate polyurethane caulk, the odds are pretty good that it won't even need slab depressurization to duck under the EPA recommended remediation level. But you have to assume that it might need it, and design in the hooks for it. A slab poured over a vapor barrier is surely going to be tighter than the old concrete slab, and even passive convection through the rock and perimeter drain up the radon stack will interrupt and dilute the soil gases at the most likely leak points, the slab perimeter.

    Putting it in perspective, at 6.6 pCi/l you're already under what many European countries would accept. A level of 6.6 pCi/l is about 245 bq/m^3, well below the 400 bq/m^3 remediation level used in Spain and several other countries, if somewhat above the 200 bq/m^3 (~ 5.4 pCi/l ) limit in the more stringent countries. Just sealing the old slab better might have gotten you do the EPA recommended remediation levels or 200 bq/m^3 Euro-levels even without a passive radon stack.

  5. Jon_R | | #5

    You will get some depressurization/mitigation even with these air leaks. But traps in the window well drains make sense to me - if they are low enough to not freeze and sized to resist the air pressure.

  6. AndyMN | | #6

    Thank you everyone for the helpful advice. I'm leaning towards installing a check valve at each of the drain tile extensions in the window wells instead of one valve where the tile enters the sump pit. My thought is that this would provide a better depressurization as the fan would need to open 5 valves instead of 1 and if there are any future problems the check valve are readily accessible in each window well.

  7. Jon_R | | #7

    The water flow that you want to allow and the air flow that you want to block flow in the same direction. So why a check valve? On the other hand, a spring loaded relief valve that blocks at low pressures and allows flow at higher pressures (water from above) could work.

  8. AndyMN | | #8

    Jon, I would like a spring loaded valve, it's a matter of finding the right pressure balance. I located a 2" valve with a 2 psi opening pressure. My math says I would need 6.3 lbs of water to open this valve (Area = pi(1 in)^2 = 3.14 in^2; times 2 psi = 6.3 lbs). With a 2" diameter pipe I'd need a height of 55" to get enough water weight to open the valve; 4" would require 14" height. A valve with a .5 psi cracking pressure would probably be fine, require less digging and would stay closed with radon fan pressure.

  9. Jon_R | | #9

    Perhaps just set a 4" hollow plastic ball in a 6" funnel at the top of the 2" drain pipe. It will float upward when water is present. Or a commercial equivalent (no idea what this is called).

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