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Community and Q&A

Resistance to Closed Crawl Space in Zone 4C

Gordon Taylor | Posted in General Questions on

Our house project in Zone 4C is nearing the point where we are going to sign a contract. I am planning the equivalent of a Pretty Good House: 60-40-20-10, with a closed crawl space. In our county, radon mitigation is required. After soliciting foundation and framing bids, the developer is giving me resistance on this. He has written to me, saying this:

“Being temperate moist climate, the concern is that moisture control is an issue and (in our county) radon is another potential concern. The conditioned crawl space creates a potential health hazard because of inability to ventilate affordably radon gas which comes naturally from the earth.”

Bottom line, he thinks it’s going to cost too much money to do radon mitigation with a closed crawl. I, however, don’t see how you can have a tight house with an open crawl space. It seems to me that if you turn on a blower door with an open crawl, a thousand tiny fountains of air will come up from below.

Could I have some advice on this, please?

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    Did you see my recent article on radon? Here is the link: All About Radon.

    If you are building a new house in a county where radon mitigation measures are required, your builder's opinions are irrelevant. The required crushed stone and perforated piping don't cost much, and help keep the foundation dry in any case. Any new home should include these measures, in my opinion, no matter what your builder thinks.

    On the question of the advantages and disadvantages of a sealed conditioned crawl space, I agree with you. It's your house, so you should be able to choose the specifications you want.

  2. User avatar
    Dana Dorsett | | #2

    It's true that in zone 4C a vented crawlspace with a ground vapor barrier will purge more moisture than it takes on in summer, (unlike zone 4A where the summertime dew points can dwell well above the crawlspace temps.)

    But in that climate vented crawls are not a huge benefit from either a radon-mitigation OR moisture purging point of view, and not worth the energy-use hit. With 10 or 15 mil poly under a rat-slab in a conditioned crawl you have a pretty serious radon retarder. If that plus ventilated gravel isn't sufficient radon mitgation, it's not as if a vented crawlspace would have made the difference anyway!

    Ducted active ventilation of the conditioned space with an HRV (the typical ventilation solution in a tight "pretty good house") would also render any active radon-abatement requirement for the crawlspace moot, especially if you use the crawlspace as the exhaust plenum of the ventilation system. (That way ventilation air only moves from the fully conditioned space to the crawl, and not conversely.)

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