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Crawl Space Encapsulation & Radon

pico_project | Posted in General Questions on

We’re about to have our crawl space encapsulated with closed cell spray foam on the wall and rim joists (R15) and a 20 mil vapor barrier on the floor.

During our inspection we had a 48 hour radon test done and the numbers ranged from 1-2 pCi/L on our main floor (single story ranch). This was with all the windows closed and all the crawl space vents closed (not occupied).

Should I be concerned that with the encapsulation we will be increasing our radon levels?

We are having an ERV installed and we could potentially exhaust the crawl space if that makes sense.

Overall I just don’t want to start a radon issue from fixing a different issue and am curious to what people have measured in a similar scenario. Also best practices as far as providing air to the crawl so it generally doesn’t come up to the living space.

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  1. charlie_sullivan | | #1

    There's a decent chance that the encapsulation will reduce radon, but there are too many variables to be sure.

    Continuous radon monitoring is more accurate and cost effective than it used to be. I've heard that AirThings monitors are good, although I don't have solid data on that.

    Exhausting the crawl can actually suck more radon into it. Balanced ventilation is less likely to create a problem and might help. But if monitoring shows you have a problem, a real radon mitigation system is much more likely to work and work consistently.

    1. pico_project | | #2

      I have an air things monitor at our current house and was going to bring it over next week to start getting a longer data set.

      I’m trying to avoid paying to have the crawl space encapsulated then find out a few months later it needs to be corrected with mitigation.

      Really just looking to see if anyone has had experience with a low level (<2 pCi/L) that started to get higher as they sealed up their basement/crawl space. And if mitigation upfront would make sense even though it’s not needed now.

  2. Expert Member
    Akos | | #3

    No matter what, I would plumb up a largish stale air pickup to your ERV. The pickup should be sized to supply the code required air flow for the crawlspace.

    This would serve a dual purpose of providing conditioned air flow to the crawlspace, exhaust any smells and at the same time exhaust any radon that makes it in.

    See #3 here:

    1. pico_project | | #6

      Thank you! I was looking for a post like this but couldn’t find one. Seems like balanced in/out is the way to go and then follow code for CFM?

      We’re going with a Renewaire Premium L for a 1,600 sq/ft house so we have plenty of CFM.

      1. Expert Member
        Akos | | #9

        You only want a stale air pickup in the crawl. The fresh air supply from the ERV is never quite at house temperature or humidity, so you don't want that air in the crawlspace. The fresh air should go to the house above.

        With a correctly sized floor transfer grill, the crawl space ERV stale air pickup will draw in conditioned air from the house above which is what you want.

        Putting the stale air pickup down low close to the floor is best for radon.

  3. walta100 | | #4

    You may want to measure the radon level in the crawlspace and install or not install a radon system based on the crawlspace numbers.

    I don’t think you will find the words “encapsulated crawlspace” in the code book. I think code compliant crawlspaces are either vented or conditioned.

    The quotes I have seen from the code books seems to require the crawlspace be connected to the conditioned space with vents or a fan that seems likely pull air mostly from the conditioned spaces and more or less conditioning the crawlspace.

    When I see the word “encapsulated” I think someone is looking for a free lunch. If you can convince yourself to call it a “conditioned crawlspace”. I suspect only then will you be willing to spend the money needed to keep the air in your crawlspace more or less the same temp and humidity as the rest of your home.

    The way I see it leaving the crawlspace to find its own temp and humidity is playing Russian roulette with mold and rot. Yes, some do win at this game and other ignore the smell and hope to move before rot set in.

    I do like the Idea of insulated walls and a heavy-duty vapor barrier covering the ground and if you have the head room to install the below grade part of the radon collection system in 4 inches of clean gravel before seems like a better time to do it than after.


    1. pico_project | | #8

      The company that is doing our HVAC has an insulation sister company. Pretty small and specialized team. They seem very good. At least best in the area.

      They want to put close cell foam on the rim joists down the block onto the 20 mil vapor barrier.

      I asked if they could do EPS on the block instead and then just spray the rim joists with foam.

      They are going to quote both ways. EPS seems more like an access issue. But we’re redoing the floors and we might be able to sneak though some sheet through the subfloor by removing a few boards.

      I plan on leaving the crawl space access on the interior closet open. So for sure it will be similar to our living space. That’s why I’m worried about increasing radon.

  4. user-5946022 | | #5

    Radon can vary over time.

    Your best bet is to install the portion of the radon mitigation system that goes under the crawl space sealant PRIOR to having the crawl space sealed. Very easy to do ( a few hours effort for one person?) and alot less work now than after the crawl is sealed.

    Just dig a 1' deep x 1' wide x 6-8-10' long trench near the location that you can run a vertical vent stack.
    Buy a few bags of #57 stone and put a thin layer across the bottom of the trench.
    Get a 6-10' long 4" pipe with holes in it, and cut in half.
    Get a 4x4x4 tee. Put 1/2 of the 4" pipe on either side of the tee, with the third part of the tee pointing up.
    Stick a short 1' or so piece of 4" pvc into the top of the tee and put a cap on the other end.
    Insert contraption into trench lined with #57
    Place balance of #57 over the pipe in the trench until it is flush with surrounding crawl grade.

    Install vapor barrier in crawl. If you ever need to install a radon system you take off the cap and attach it to the 4" PVC

    1. pico_project | | #7

      Thank you! Very helpful.

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