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A few unvented crawl space questions

user-1105327 | Posted in General Questions on

I am detailing an unvented 600 sq. ft. crawlspace that is 4′ high. The exterior side of the concrete wall is insulated with 5″ of EPS and has been damp-proofed.

My plan now is to insulate the floor with 3″ EPS and cover with two layers of poly that will be mechanically fastened with horizontal battens and acoustic sealant to the side of the footing. to boost the R-value in the wall i will add 1.5″ XPS and then build a stud wall to cover with gypsum wallboard.

My plan is to notch the XPS around the floor joists so that the xps comes into contact with the underside of the floor sheathing above. i would then do the same with the GWB. How does this detail sound?

Also, do i have the HRV supply/exhaust the crawl space? also, i put a section of 4″ PVC pipe through the footing and tied it into the perforated footing drain. this perimeter drain eventually connects to solid pipe and drains to daylight. the idea was it would drain out any water trapped under the ‘slab.’ the plumber thinks i should close this off. should i?

thanks again, gba..

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Erik,
    It sounds like your crawl space will have above-average details. In case you haven't seen it, there is an article on the topic here: Building an Unvented Crawl Space.

    The question of whether or not you need some type of air exchange or ventilation in your crawl space is controversial. Code requirements call for a low-cfm exhaust fan in the crawl space rim joist or a forced-air register delivering hot air in the winter and cold air in the summer. Both options require the installation of a floor vent connecting the conditioned space upstairs with the crawl space below.

    Some observers worry about contaminating the indoor air with radon and other undesirable gases present in the crawl space. If your local code allows, you might skip the fans for a year and monitor the humidity in the crawl space. If it's dry, you might be all set without the floor register and fans.

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