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Strategy for radon in crawlspace (piecemeal approach vs. complete and vapor barrier durability)

sippyCUP | Posted in Energy Efficiency and Durability on

Hey all, I’ll keep this short and sweet. I have read two of Martin Holladay’s seminal articles touching on crawlspaces issues – first the one about sealing crawlspace, next the one about radon. I am under contract on a 1943 home in East TN [hot/humid in summer] which has not one but two crawlspaces (first is the original foundation, 1200 sq. ft., the next is a 350 sq. ft. addition, which is vented into through a foundation vent on the original larger foundation.) I’m in the inspection period and the radon test came back at 14 pCi/L (EPA action limit is 4). Other than the radon and a few other minor quibbles, I like the house, and any other house in the area is >50% likely to have a similar radon issue.

I am sold on the vapor barrier already, it needs to be done. Seller has a half-solution in place – some plastic sheets on the crawl space that aren’t sealed together, and an active dehumidifier. I think by properly sealing the crawlspace dirt floor I can mitigate both humidity (save $$$ on dehumidifier operation) and radon.

A few questions for the audience:

(1): Talking to radon contractors yesterday, the consensus solution is to install the vapor barrier and a vented, active depressurization system drawing suction underneath the vapor barrier. Sounds great, I’m an engineer and I understand how it works. However, each scope of work is $1,500. Would it be worthwhile to do the vapor barrier first, see where radon ends up (maybe even buy my own detector), then if further improvement is required, go ahead with the active vent? Granted, the contractors probably want to do everything at once ($) and talking them into a piecemeal approach could be difficult, especially with respect to their <4 pCi/L guarantee.

In addition, for the techniques that use perforated horizontal pipe under the vapor barrier, there could be a legitimate issue with installing that under an existing vapor barrier.

(2) Within reason, I would like the vapor barrier to be a lifetime (~30yr) solution if possible and economical. I have heard of 6mil, 10mil, 20mil, etc. plastic barriers. Other than barrier thickness, any advice on installation strategies to increase longevity? Seems like I saw that someone laid down some sort of product underneath the plastic.

Thank you very much,


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  1. Expert Member
    Peter Engle | | #1

    A vapor barrier alone is unlikely to bring your radon level into compliance. Even a very well detailed passive radon system generally only brings indoor radon levels down a few points. Your'e probably going to need the active suction system.

    make sure they detail the vapor barrier very well. All seams must be taped and/or sealed with wet sealant. Use sealant and termination bars on the walls. You are shooting for as close to airtight as you can.

    If you choose to try the piecemeal approach, I would recommend digging trenches and installing stone and the perforated radon pipes before installing the vapor barrier. This will add cost, but make it much easier to just connect additional pipes and fans if necessary to make it an active system.

    For the vapor barrier, there are many more durable solutions than the typical 6-mil poly. Look up "reinforced slab vapor barriers." Most are 12-20 mils thick and have woven reinforcement to add toughness. These are far more durable than simple poly.

    Good luck.

  2. Brian Pontolilo | | #2

    Hi Eric,

    It sounds like you are planning to seal the crawlspace when you buy this house, and you have read Martin's blog on sealed crawl spaces so you know that radon levels tend to be higher in sealed crawl spaces. If so, I guess the answer to your first question depends on you finances and a discussion with the contractor about the idea of doing the work in two stages and the cost implications. As you alluded, it may end up being more expensive to have the contractor come out twice. If it were my house, I'd spend the $1500 to have the active system installed because radon is a real health issue.

    I've seen people do as little as kick any stones aside before laying the poly to pouring a slab in the crawl space to protect the vapor barrier. Some people put down a layer of thin rigid foam or just use an additional, sacrificial layer of poly. A sealed vapor barrier is a sealed vapor barrier, so I guess it depends on the condition of your crawl space floor and your willingness to spend the time and money to ensure longevity. I know some folks recommend a vapor barrier called Tu Tuff for areas where durability is an issue.

    Please let us know what you decide to do and how it all works out.

  3. Jon R | | #3

    I'm with Peter - install the stones/pipes since you are likely to need an active system. With smooth stones and no traffic, I'd expect the plastic to last forever.

  4. sippyCUP | | #4

    Guys, thanks for the responses today!


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