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Radon and drains to daylight

canadianexpy | Posted in Green Building Techniques on


Designing a new build, working on foundation drainage. Plan is to have perimeter drains run to daylight, also to have drains through footings from under slab. Question is since I’m unsure about radon I was going to install passive system .Will this system work since the area under the slab also is connected to a drain to daylight? Will stack effect draw the air up enough? The pipe will be located on a inside wall.
Thanks Dave

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  1. GBA Editor
    Martin Holladay | | #1

    There are two ways you can detail the pipes (to the best of my knowledge):

    1. Because you have radon concerns, you can omit the through-footing pipe, and keep your exterior footing drains separate from the sub-slab depressurization pipes (the perforated pipes that are part of of your passive radon mitigation system). This will ensure that the radon system is more likely to depressurize the sub-slab region (especially if you ever retrofit a fan to this system).

    2. You can keep the through-footing pipe, in which case you need to install a check valve near the outlet of the footing drain that leads to daylight. I don't like this option as much, because check valves can get sticky and can eventually fail.

    For more information on these issues, see All About Radon.

    -- Martin Holladay

  2. canadianexpy | | #2

    I'm unsure of if any radon is present in this area, but since we plan on a full finished walk-out basement, I though it might be wise to install. Alright I will omit the through the footing pipe, I though this might be my only real option.
    Thanks Martin

  3. SwitchgrassFarmer | | #3

    Dave, I have a walk-out ground floor, so a couple thoughts on what, in hindsight, I would have done differently for radon piping.

    1. Because of the walk-out our footers are at different elevations. Where the ground floor is below grade, the footer is just below the slab level. That is where the Form-A-Drain channels are that feed the passive radon pipe up through the house. Where the ground floor is at grade, the footer is down below the frost level, and so is a few feet below the slab. There isn't good connectivity between the two footer levels.

    2. Be mindful of any Form-A-Drain connectors that bridge between the inside channel/form and the outside channel/form. They are a dead short between your inside and outside sub-slab areas. We have two of them where the footer drops. Great for drainage, lousy for radon.

    Net-net, I wish we would have spent a few hundred dollars on dedicated piping under the slab, versus relying on the Form-A-Drain.

    Fortunately our radon levels are right around the 4.0 pCi/L action limit, varying above and below depending on outside weather: rain/snow/wind; I can see the change on the Corentium radon meter I sprung for. (I am 2 months in to a year long canister test to get an "average".)

  4. Jon_R | | #4

    I find it odd that all of the focus is on sub-slab depressurization and none on lower basement wall depressurization.

  5. GBA Editor
    Martin Holladay | | #5

    Radon mitigation contractors have developed a bag of tricks that work to lower indoor radon levels.

    If you have an innovative scheme -- some way to depressurize the soil region on the exterior side of your basement walls -- and if your innovative scheme is (a) cheaper, and (b) at least as effective at lowering indoor radon levels as sub-slab depressurization -- then by all means share your scheme with other GBA readers, or patent it.

    -- Martin Holladay

  6. Expert Member
    Dana Dorsett | | #6

    Radon in most areas originates as uranium or thorium decaying to radium being leached out of the bedrock by dissolving in water. Radon is a chemically inert noble gas, not nearly as soluble in water as the radium that spawned it, and migrates upward quickly. It's coming from the ground water and deep rock, not surface water or shallower surface soils adjacent to the foundation, unless the foundation required excavation into uranium or thorium bearing bedrock (which happens, but it's not the usual scenario.) Depressurizing just the walls separate buys almost nothing.

    Seasonal changes in the bedrock & subsoil hydrology do affect background emission rates. Seasonal changes in ventilation & infiltration rates will also affect the levels measured indoors. Air handler driven depressurization of basements can pull more radon in when the basement's air pressure is lower than the outdoor pressure- the air tightness of both the walls and slab matter.

    Depressurizing the slab when it is well sealed to the foundation walls brings outdoor air down the sides of the foundation, effectively purging the offending radon from the walls before it's actually purging it from below the slab. With highly air permeable drainable backfill in new construction this is the most likely air path. As long as the walls are reasonably air tight, there's little additional benefit to separately depressurizing the walls. If sub-slab gravel also has a drain system that's daylighted somewhere, that will be the lowest impedance air path, which may scavenge somewhat less gas from drainable-backfilled walls, but it'll still pull some, and it doesn't take much, volume-wise.

  7. canadianexpy | | #7

    Thanks for the advice Andrew , I'm going with a dedicated radon under slab passive system.The outer drainage at the footing will go to daylight and not be tied into anything under the slab.

    Dana I wish there was a pre-construction test for radon, but since there is not I will play it safe, and be proactive.The plan is to have tight house.Thanks

  8. Jon_R | | #8

    Gravel improves depressurization under the slab. I expect that the same applies to the footers. Ie, no gravel (or through-footing pipe) underneath them means little depressurization of the wall exterior (by the system on the other side of the footer). But with it, more wall depressurization occurs. No innovation here, just something to be aware of that effects the amount of radon reduction.

    See here (walls are an entry point covered by exterior DTD, communication is important):

  9. Expert Member
    Dana Dorsett | | #9

    Dave: If a nearby house has been (or can be) tested, that's usually a good indicator of where you would be on radon levels if you built the house with similar air tightness and foundation tightness, and whether remediation would be in order.

    People tend to worry about this a bit more than they perhaps should. The EPA's 4 pCi/l remediation level (which is a recommendation, not a regulation) is based on data from miners at orders of magnitude higher levels using a linear approximation (when it's known that health effects from radiaiton are not exactly linear in response) and an assumption that there is no threshold limit of exposure, below which it's safe. It's next to impossible to measure risks this low directly, especially when the signal is swamped by the amount of cigarette smoke exposure (primary and second-hand), so don't take the EPA comparison of radon levels to X amount of cigarettes per day literally, take it with a pound (not a mere grain) of salt.

    The order of magnitude is probably more important than where you are within that order of magnitude. At 400 pCi/l (same order of magnitude of miner studies) clearly has some risk, 40 pCi/l may have some too, especially for those with aggravating factors such as smoking, but at 4pCi/l it's literally in the noise- the risk can't be measured directly, and some large cohort epidemiological studies indicate that somewhere between 1-40 pCi/l there may be a protective factor regarding lung cancers, others not so clear, but the risk is really quite low. The European remediation levels are set 50-100% higher than that.

    If you're in single digits but over 4 pCi/l don't lose sleep over it, and don't spend thousands just to limbo under that EPA recommendation- most people have more important higher risk things to be spending the emotion & money on. In a high radon zone pre-installing a passive system in new construction can be rational, and getting it to test under 4 pCi/l may have some value a time of sale.

  10. canadianexpy | | #10

    unfortunately no houses very close to our new build and the few that are close are old and leaky. , no one in the house is a smoker. I though I'd be proactive about it Even though it plans on being a tight house. . Some maps show our area as higher risk and the soils are sandy, which I assume increase chance. I don't think the cost of adding a passive system will be very much.
    Thanks Dana

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