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Radon fan causing increased humidity

hotandhumid | Posted in General Questions on

Thanks everyone on the feedback concerning humidity and attic insulation. I have discovered the problem. After documenting the temp and RH in both the house and crawl space, I have discovered the Radon Fan is somehow bringing humidity ( outdoor air) into the building envelope. i noted the RH and temp in both the crawl and home with the radon fan running-and I did it in the evening hours so the HVAC or outdoor temps would not alter the findings- but the dedicated dehumidifier in the crawl continued to run. So here we go:

7:30 PM (ac just turned off) indoor: RH 56% / 74 F — Crawl: 52% RH/ 72 F—- Outside: 91% /76 F

11:30 PM ( ac remained off) indoor: RH 47%/74 F—-Crawl : 42% RH/ 74 F—- Outside 90% RH/75 F

12:30 AM ( ac remained off) indoor RH 41% / 74 F—Crawl 40% RH/74 F—-Outside 90 % RH/ 75 F

So I now know its the Radon Fan, making the house humid- but what I’m hoping some incredibly smart building advisor on here would know is why…and how to correct.

Thank you all for your time-this is such a great forum!

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  1. hotandhumid | | #1

    Forgot to mention I have a Fan Tech Radon fan installed in a crawl space...terminates just above roof line, and is sealed at entry point on house ( concrete block foundation) with spray foam. It also has a 4" pipe running underneath a vapor barrier. I am suspect of the vapor barrier, because there are gaps all along the exterior wall, as well as on the concrete block supports on the interior of the crawl space. I did notice just after the radon system was installed the humidity went up some, and warm humid air was coming down from the HVAC intakes on the ceiling ( attic above)...same for the bath fans. I had hoped removing can lights below attic and air sealing some would help. But just after attic insulation was done, the humidity got much worse.

    Any feedback or advice would be much uncomfortable !!


  2. Expert Member
    Dana Dorsett | | #2

    If there are any air leaks in the slab or the seam between the slab and foundation walls, the radon fan will depressurize the basement, pulling air into the house from where ever there are air leaks. If the slab is very well sealed at every crack seam & edge (with the appropriate polyurethane caulk) that won't happen.

    If you start sealing up the obvious air leaks in the slab everywhere you eventually get to the point where you can hear the air moving into the smaller air leaks when everything in the house but the radon fan is off. Most radon abatement contractors take a stab at sealing the truly large leaks (such as sumps and drains), but often ignore the rest, and just install a higher cfm fan than would otherwise be needed.

  3. hotandhumid | | #3


    Thanks so much-so helpful!

  4. GBA Editor
    Martin Holladay | | #4

    When a basement is depressurized, outdoor air can be drawn into the basement through the usual cracks near the rim joist. It's also possible to pull outdoor air through the soil into the region near the footing, and thence into the basement. That's why it's so important to seal cracks in the basement slab, and to seal cracks in the basement walls, when retrofitting an active radon mitigation system.

    During the winter, the type of leakage you are noting can help lower the humidity of the basement. But during the summer, this type of leakage can raise indoor humidity levels.

    One further point: it's usually not a good idea to install a radon mitigation fan in a crawl space. The fan belongs in an unconditioned attic or outdoors. As I wrote in one of my articles ("All About Radon"):

    "Radon exhaust fans should not be located within a home’s conditioned envelope; all pressurized lengths of pipe need to be outside the home’s conditioned space. (Here’s why: if the vent pipe ever develops a leak, you don’t want the fan to send radon-rich air into the home.) If it isn’t possible to install a fan in the attic, consider installing it in the garage or on the outside of the building."

  5. onslow | | #5

    To HotandHumid,

    The description you have provided of your radon mitigation installation is a bit puzzling to me. I had radon mitigation done on a house with a crawl space and the set up seems different in critical ways. First the plastic film barrier ( a very heavy material unlike drop cloth plastic) is sealed to the entire perimeter of the foundation with special mastic; as well as the pier blocks supporting the beams under the joist spans. This creates the equivalent of a jar lid over all the soil which isolates it from the house.

    Under the plastic is a network of perforated pipes which come together in a common fitting that is then directed to the outside world. Martin is correct to advise that the pipe pass through non-living space and preferably to a high point outside for maximum dispersion. Where the pipes come together and pass through the plastic ground barrier is tightly sealed, as are all joints in the vent pipe stack. The negative pressure being applied to the soil under the barrier means very little air volume actually is being moved out of the pipe, but that any air under the barrier which has picked up radon is expelled. Point being there should NOT be a noticeable amount of air exchange going on above the barrier.

    I think it might be advisable to have a third party review the installation if I am interpreting the description you provided correctly. A good way to think about the details might be to review the many diagrams available showing radon mitigation under slabs. The plastic barrier is just a substitute slab and all sealing details remain essentially the same. I am now in a new house with a basement and busy sealing the perimeter slab to wall connection and all the post penetrations through the slab to the footing points. The embedded perforated pipes under the slab collect to a common point and there will exit if the radon tests in an actionable range.

    A last point, the testing of radon levels is meaningless if you are actually pulling in outside air above the barrier. I believe the proper testing for radon levels before and after mitigation is done in a closed house without active HVAC and over a time period of at least two or three days. This is a very awkward proposition for most people and I was able to comply due to travel. Given your sudden shift in humidity I am suspicious that the venting, barrier or both are not correctly installed.

  6. hotandhumid | | #6

    Thank you to everyone-educated insight very much appreciated!!!

    We do have a crawl space, and I noticed Dana referenced a slab, and Martin referenced a basement. Roger, you clarified my next question to both Dana and Martin-basement, slab or crawl- they all need to be sealed/separated from the interior envelope to ensure the radon is working properly, as well as not introduce humid outside air in the summer.

    Thanks again everyone!

  7. hotandhumid | | #7

    I just noticed I got that backwards-meant to say sealed from the exterior, so as not to introduce humid outside air into the envelope.


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