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Radon is 9.3 pCi/L even after radon mitigation — Is this OK?

hotandhumid | Posted in GBA Pro Help on

Hi. I am having elevated ( 9.3 pCi/L) radon after mitigation done. I have an encapsulated crawl space ( insulated, air sealed, dehumidifier ) with piping running under vapor barrier-which terminates to an outside radon fan connected with a 4″PVC pipe. Would like to find out if it’s Ok to install drainage ( perforated black pipe) piping under vapor barrier ( in encapsulated crawl) for radon mitigation. The piping is spilt off of a main trunk ( no continuous circle) to different sections under the crawl. I have read different opinions on radon design and using black perforated pipe vs white PVC drain pipe. I have also read the piping underneath vapor barrier ( on crawl) should be run in a continuous circle.
One last question-I have an attached concrete porch adjacent/sharing the crawl space foundation-should this have a suction access as well?? I have been told by the company who installed the mitigation company that radon levels fluctuate and this is to be expected.

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  1. user-2310254 | | #1

    Anything above 4 pCi/L is a concern. Was this a snapshot test or a 90 day sampling?

  2. hotandhumid | | #2


    It was a short term 7 day test. State issued radon test, done when daytime highs were in the 20s, no storms or snow. Test mailed to lab via express ( 2day mail), per instructions.

  3. Expert Member
    Michael Maines | | #3

    The color of pipe has no effect on its utility. Running the radon vent to daylight in theory should short-circuit its ability to generate negative pressure below the slab, though it's a pretty common way to do it and seems to work fine in most cases. The threshold of 4.0 pCi/L is not universally considered a safe level, so I recommend to clients that they vent if over 2.0, but that's just a guess on my part.

    It's unusual for you to have such high levels in a sealed and actively vented crawlspace. It sounds like you don't have a slab on the floor of the crawlspace? Is the soil below the vapor barrier gas-permeable? You might need to add a soil gas mat of some sort--basically a way for the entire area to easily vent to the riser pipe--or you could add additional riser pipes. If you haven't already, I would find a radon mitigation specialist and get their opinion.

  4. Expert Member
    Dana Dorsett | | #4

    If the measurement is taken in the crawlspace it's not a big deal. If that's the measurement in the conditioned rooms above it's worth considering further action, but still fairly low risk (a risk low enough to be impossible to measure, and thus only a theroretical risk) , even though it's ~2x the EPS recommended remediation level.

  5. hotandhumid | | #5

    Thanks everyone.
    The crawl space is sealed and unvented..we had it encapsulated before the mitigation. During the encapsulation install we had radon vent piping installed underneath the 12 mil vapor barrier as a precaution-even though we tested at 1.0 pCi/L before encapsulation. There is also a slight positive pressure in the crawl due to adding supply only from the ducts located in the crawl as part of the encapsulation. In addition, a dedicated dehumidifier was also added to the crawl space to control high humidity.

    Is there anyway possible the adjacent raised concrete slab porch ( which is not accessible from the foundation concrete block wall in crawl) could be contributing to radon??? I should mention the concrete block walls in the crawl were spray foamed-with the exception of a small clear termite strip.

    During a blower door test we found out there was a cfm differential with the radon fan on vs off. The cfm was 3428 cfm with the radon fan on vs 3678 with the radon fan on. We had the the seams and gaps of the vapor barrier re-sealed with mastic in hopes of fixing pressure difference, but we never followed up with another blower door test to see if mastic solved radon and house cfm difference.

    Really hoping someone can help-have been working on this quite awhile.


  6. hotandhumid | | #6

    OOps...meant to say the house blower door cfm was 3428 with the radon on vs 3678 with the radon off.

    Thanks again!

  7. Jon_R | | #7

    I'd close the supply only ducts into the crawlspace and allow the radon mitigation fan to also maintain a negative pressure in the crawlspace itself. Measure the pressures to verify.

  8. GBA Editor
    Martin Holladay | | #8

    Hot and Humid,
    Dana's point is important. Radon testing should be done in the lowest occupiable floor of the house. The crawl space reading is irrelevant. You want to test the air in the room above the crawl space -- the living room, kitchen, or whatever.

  9. Expert Member
    Michael Maines | | #9

    Dana, I'm disappointed that you would downplay the risks of radon exposure. It's true that it's not possible to be perfectly accurate when assessing risk, any more than climate change scientists can predict exactly what will happen in the future, but as far as I can tell there is scientific agreement that it is a significant risk, as the second leading cause of lung cancer.

    According to the National Cancer Institute, "scientists estimate that 15,000 to 22,000 lung cancer deaths in the United States each year are related to radon." notes that "A family whose home has radon levels of 4 pCi/L is exposed to approximately 35 times as much radiation as the Nuclear Regulatory Commission would allow if that family was standing next to the fence of a radioactive waste site. (25 mrem limit, 800 mrem exposure)." Further, "Most scientists agree that the risk of death for radon at 4 pCi/L is approximately 1 in 100. At the 4 pCi/L EPA action guideline level, radon carries approximately 1000 times the risk of death as any other EPA carcinogen. It is important to note that the action level is not a safe level, as there are no “safe” levels of radon gas."

    The EPA states, "Radon is responsible for about 21,000 lung cancer deaths every year. About 2,900 of these deaths occur among people who have never smoked." That's less than 15% of those killed by lung cancer from smoking, but it's still a significant health risk.

    It's colorless, odorless and there are other environmental and behavioral factors that contribute to lung cancer, so it's common enough to downplay or not understand the danger radon poses. But to state that 9 pCi/L is not worth worrying about is, well, worrying.

    My mom is an epidemiologist and was Maine's anti-smoking program manager, winning national awards for reduction in smoking, and she is also a cancer survivor, so I may be more sensitive than most. (Of course our illustrious governor took the grant money she earned and applied it to other uses, forcing her out of a job, and our smoking rate quickly rebounded. Thanks Gov.)

  10. hotandhumid | | #10

    Thank you everyone. We tested at the lowest living level ( living room), not the crawl space. We don't know what to do-being told my mitigator they've done everything. The crawl space is encapsulated, closed, with a dedicated dehumidifier and 3 small supply vents ( from HVAC ducts in crawl)-no return.
    Since there is a positive pressure from the supply ducts, I will try what Jon recommended and close them. Still, a little worried since we're more than double on EPA guidelines.

    we had another certified Radon mitigator look access, and he recommended adding another fan since the footprint is large and the current piping is running on a trunk. They wanted to run two separate systems, with tubing running in a continuous ring. Original mitigator is concerned adding additional fan could cause negative pressure on house-since we had that problem with the first/orignal radon fan.

    Thoughts/ideas appreciated???


    Thanks all!!

  11. Expert Member
    Michael Maines | | #11

    Hotandhumid, it's hard to fully visualize your situation but it seems unlikely that radon is entering through the walls or the raised patio area. There are various ways to run the radon pipes, but best practice is a continuous loop within 12" of the foundation perimeter, and you can add a central run as well if you want. It's not uncommon on retrofits to need more than one riser, but they usually run to a single fan.

    If the soil below the vapor barrier is not porous (usually sand or crushed stone) then you might need to install something to keep the vapor barrier from sealing to the ground, as you want to draw from the entire floor area. A soil gas mat such as this: would do the trick, but may not be necessary if the soil is porous.

    You mentioned a 15-mil membrane; do you know what product? Thickness does not automatically correspond to resistance to radon. Look for one that lists a radon diffusion coefficient, such as this:, and make sure all seams are carefully sealed.

    The fact that you still need a dehumidifier is a clue, in that your crawlspace is not well sealed.

  12. Jon_R | | #12

    Measuring under-plastic pressures at multiple points will tell you if you need to improve under-plastic distribution (more pipes, gravel, etc).

  13. SwitchgrassFarmer | | #13

    H&H, it might make sense to purchase some real time instrumentation.   I have found it to be incredibly instructive as to when levels are high or not.  You may well find, as I have, that making assessments during the winter might lead to over reaction. For example it has been raining and it's very windy, levels are high in the ground floor of my home today, pix from just now attached.

    See the end of this GBA article for some of my comments on such real time metering, long term lab tests etc.    (I am 11 months in to a 12 month long set of lab tests.)

  14. Expert Member
    Dana Dorsett | | #14

    The problem with the radon risk analysis is the model that assumes no threshold, and a linear risk with dose, which is KNOWN to be inaccurate, but in lieu of a better model they are using a linear risk model based on miner data at dosage levels in the hundreds of Pci/L. If there were data to actually support the EPA recommendation it would be a REGULATION,rather merely a recommendation. Estimates of risk based on the linear model are almost certainly wrong, and the error bars are large.

    There are large cohort studies and smaller ones out there that indicate radon levels up to 50 Pci/L might even be protective, others that don't show that effect, but NONE that show a dose-response risk when down in single digit or even double digit Pci/L levels.

    Somewhere near the bottom line, the radon concentration in the crawlspace is irrelevant, since people aren't spending many hours of their life there. The concentration in the conditioned space might be relevant, but there is no known risk at those levels, only a theoretical risk based on cohort studies at extremely high doses, not well controlled for the risk from other factors found in mine (like dust, or cigarette smoking.)

    The European radon standards are stated in Bq/m3 rather than Pci/L (1 Pci/L= 37 Bq/m3 ) and are also usually in the form of recommendation rather than regulation, with remediation in some countries recommended/regulated if the levels exceed 200 Bq/m3 (5.4 Pci/L) in others at 400 Bq/m3 (10.8 Pci/L).

    So if you just move your crawlspace to one of those countries in Yurp that draws the line at 400 Bq/m3 you're already good to go, eh? :-)

    Those recommendations & regulations are also based on a linear model from miner studies at doses 100x the EPA remediation level or more.

    I'm not saying there is no risk, only that the risk is both unknown & unclear, but that it's low enough that nobody should waste any time worrying about it (or money for remediation) if the highest concentration is in the crawlspace, and measures only in high single-digits. If it's that high in your bedroom you might want to do something about it, but in most homes you still probably have bigger things of KNOWN and MEASURABLE risk more deserving of attention & investment. A single-room HRV can usually cut the radon levels by half or more, if it came to that.

    Also note- radon levels in the crawlspace will usually vary by season. At my house the radon levels rise after weeks of wet weather as moisture leaches more radium and uranium out of the bedrock into the water table only a foot or two (or sometimes -3" ) from my basement slab, and falls during stretches of dry weather. In the basement it's exceeded 10 Pci/L (as measured by a cheap not really direct measurement electronic monitor), but the monitor in the bedroom upstairs only rarely trips the 4Pci/L alarm for long-term averages, even though in the short term readings it can hit the sixes and sevens. My lifetime risk of sleeping in that environment is at worst still orders of magnitude lower than the health risks my daily commute, or my skiing hobbies, or the health risks of couch time spent watching K-dramas & Hindi movies.

  15. Jon_R | | #15

    "...investigators found a statistically significant association between radon concentration and lung cancer, even when the analysis was restricted to people in homes with measured radon concentrations below 200 Bq/m3."

  16. hotandhumid | | #16

    Thank you Jon, very good information from WHO....trusted and knowledge source. I will follow up with pressure testing ( home and crawl/HVAC on and off), as well as additional sealing as needed.Great advice since we had pressure differentials with radon fan off vs on!!

    Michael-I appreciate the recommendation to move vent piping ( underneath vapor barrier) 12" from foundation and running in a continuous loop -thank you!! The vapor barrier we currently have is 12 mil Guardianliner brand with Anti-Microbial... not sure if it's considered a radon barrier?? I will look into


  17. KeithH | | #17

    First, I'm not a pro. But I've successfully used a pro company to mitigate 1 house and my own work to mitigate a second. Maybe these experiences can help.

    The first house went from 7.0 to 0.2 pci/L using a membrane over dirt crawl space with black plastic perforated irrigation piping under the membrane and an radon fan. These readings are despite a relatively poor job by the contractor on the plastic membrane seams.

    The second has a rat slab crawl space. The maximum observed radon was ~14 pci/L, the final radon was 1.2 pci/L. That crawl space was mitigated with a spray sealant, a vapor barrier on the walls, and a large sump basin with sealed lid attached to an radon fan. Both crawl spaces went through encapsulation during the mitigation process.

    Several things you say don't make a lot of sense:
    - I'm not surprised the radon changed after encapsulation but it makes no sense for it to go from 1.0 to 9.0 pci/L. Passive rim vents just don't work that well for mitigating radon. For example, the first house we mitigated didn't change at all when we went from vents to sealed vents.
    - It makes even less sense that an actively vented systems would increase so much.
    - As others say, a crawl space that is encapsulated and has a sealed floor should not need a dehumidifier. The first house that we had mitigated had pretty high humidity before the membrane went down. After the membrane, all the musty humid crawl space smell was gone.

    Some ideas:
    - Is there any chance that the radon fan is running in reverse. I don't know if radon fans in general or yours specifically are 3 phase motors, but if it is and the polarity is reversed, it will run in reverse. A simple test is to get on a ladder and put your hand in front of the discharge. Or your fan may have rubber connectors to the PVC venting up the side of the house that you can temporarily undo (more involved by presumably no ladders).
    - The supply duct shouldn't hurt anything. The first house we had mitigated had a duct into the garage which we eliminated. That leftover supply was left as a register in the crawl space to help eliminate pipe freezing. It doesn't work that great for that but the radon is functionally zero.
    - I assume the fan is outside the crawl space or at least very close to the exterior of the house? It's not recommendation that you have a lot of positively pressured pipe in the house/crawl space.
    - How about a proper termination? The termination is supposed to be above the roof line and some distance from operable windows. If you just dump the discharge next to the house, you may not get good results.
    - Is your membrane a single piece? If not, are the seams well-glued or well-taped? Your membrane is likely a different material but we were pretty successful using Dow weathermate (reasonable affordable) tape to reinforce site glued seams that had come apart. Honestly, leaky seams shouldn't hurt much in terms of radon performance (you'll likely lose more heat though)
    - If you have an attached crawl space under a porch that air-communicates with your crawl or house, you'll need to mitigate it. If it is a porch on a slab, it shouldn't be hurting you too much. Example: our first house we mitigated had an attached garage that shared two sides with the house and crawl space. We didn't mitigate the garage slab at all and the radon is still very low. We did observe slightly higher values in the two bedrooms that abut the garage than the crawl space. If I was doing it again, I might put a sealant on the garage slab and caulk/baseboard the interior expansion gap.
    - If you are really battling with this, a Radon Safety Siren Pro monitor is well worth the $130. It isn't instant but ~3 days to get a good stable reading in a new location (longer if the value is changing a lot, like from no system to active system). It can be nice for measuring in the first floor, in the bedrooms, in the crawl space etc.

    Well, I hope those ideas help some. I know it sounds goofy, but my first stop would be to check the fan is operating in the correct direction.

    Last thought: in my area (Colorado), no reputable radon company would walk away from a mitigated crawl space that was producing 9.0 pci/L. Most offer a 4.0 pci/L mitigation guarantee. I recommend getting a second opinion from a company that offers such a guarantee.

  18. GBA Editor
    Martin Holladay | | #18

    Thanks for taking the time to write such a detailed, thoughtful comment.

  19. Expert Member
    Michael Maines | | #19

    H+H, I can't find a radon diffusion coefficient for your liner, but they list the water vapor permeance at 0.023 perms ( The product I linked to is 0.0086 perms ( They use different testing procedures (ASTM E96 vs. F1249) and I don't know how closely permeance tracks radon resistance, but they are an order of magnitude apart, which suggests that yours does not block radon molecules as well as other products. In any case, if the air below the liner is sufficiently depressurized, as others have suggested, should take care of the problem--most vapor barriers/retarders are closer to your specs.

    Dana, thanks for the additional information. I tend to trust European governments more than our own, but considering the amount of information from reputable sources urging levels at or below 4 pCi/L, I prefer to err on the side of caution. I also avoid flying, tobacco and excessive sun exposure, and ingest lots of antioxidants, as I'd really rather not get cancer. But your research puts me at ease to some degree. On one project we used an HRV to cut radon levels roughly in half in a finished basement, as you mentioned, because our client did not want a separate system, against our recommendation, but the HRV appears to have done the trick.

    Keith, radon fans are usually (or always) simple, single-phase motors, so reversing polarity would not change the direction it's blowing. But it's worth checking, in case the unit was installed facing the wrong direction. I've seen stranger mistakes. And good point about the location of termination.

  20. Jon_R | | #20

    Don't let them guess about the crawlspace vs interior and under-plastic vs crawlspace pressures - measure both, with and without the HVAC running. A very slightly negative house pressure (vs exterior) in the Winter is generally a good thing. Better air sealing between the crawlspace and the interior/exterior reduces the CFM needed to maintain a given negative crawlspace pressure (ie, is more energy efficient).

    Also make sure there are no leaks in any return ducts that may be in the crawlspace (they would suck in crawlspace air).

  21. hotandhumid | | #21

    HI all,
    I followed up on another post, but thought might be better to follow up here instead.

    Had additional blower door test done on home ( not crawl) with HVAC off;

    2554 cfm Radon fan on
    2780 cfm Radon fan off

    pressure test done on radon piping in crawl space. Pressure points nearest to start were sufficient, but only for the first 1/3 of the piping/sytem-the later 2/3 did not have sufficient suction.

    noted multiple tiny holes/pin pricks ( due to gravel beneath 12 mil clean space poly liner) throughout crawl space. see attached photos

    Question: I know several of you on here ( and previous post) recommended looking at sealing issues. Are the pressure issues related to it not being sealed and radon levels??

    Thanks again

  22. double007 | | #22

    I have been testing for radon for many years. When I first got my certification there was a great deal of conflicting information as to the risks but for all the legal reasons they all say there is no safe level of Radon. This link has a lot of interesting yet a bit tedious reading. Back in the 80's the cancer society had arguments as to who could claim more deaths, the anti smoking side and the radon side. At that time I could not find ANY research that could determine through an autopsy that the lung cancer was caused by radioactive decay (radon 222) Today the world health organization seems to be trying to take over the issue which scares me that we give away our jurisdiction outside our borders. Anyway there's my 2 cents

  23. 730d | | #23

    Your Radon fan needs to suck from under the lowest slab in the house as its primary suction point.
    Yes in my experience the slab adjacent to the house can collect gas and direct it into the house. A porch or patio or driveway.
    If you have a block foundation if all else fails put a small suction on the blocks. Close up the tops of the blocks tight.
    Agreed, if your crawl space is still damp you may have bypass into space and it is still not sealed tight enough. The most critical issue is drawing from the lowest level.
    In your pics it appears there is no vacuum under your poly. There should be some evidence of it sucking to the dirt. If you don't want to solve this yourself find a different Mitigation company. Very disappointing the one that started has walked away from it. Was the first company licensed ?

  24. user-5618764 | | #24

    The difference between the 2 blower door tests tells you that there is a lot of communication between the house, the crawlspace and below the vapor barrier. Whether or not 9 pc/l is going to kill you, having crawlspace air getting in the house is not ideal. There is some low hanging fruit that can improve the situation.
    First take a critical look at the vapor barrier and seal up any holes you find. I do think the pin holes matter in a situation like this. Consider a send layer of plastic in high traffic areas. I would also check the tops of the column wrap. This is a detail that is commonly missed.
    Next, seal off the living space from the crawlspace. Seal any penetrations in the crawlspace ceiling (plumbing, wires, etc). Remove the supply in the crawlspace and have the duct system sealed as much as possible. The supply in the crawlspace will positively pressurize the crawlspace as you said but it will negatively pressurize the house which will draw air from the crawl back into the house. Having a dehumidifier in the crawlspace is a much better approach to conditioning the air since it is directly addressing the main concern.
    Finally add supply only ventilation to the house. This helps in 2 ways. With slight positive pressure on the house and negative pressure under the vapor barrier you can hopefully cut down on the air coming in through the crawlspace due to the stack effect. The other way it helps is to dilute the radon in the house to keep the levels low.
    Monitors with real time feedback are available and helpful. It is interesting to see how it changes.

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