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Community and Q&A

How to prevent mold in unvented basement crawl space?

rshearer | Posted in General Questions on

I have an in-law addition (800 sf) which has an unvented crawl space which is approx. 2.5 ‘ high X 800 sf with a 3.5′ X 2’ opening to the main house basement. I have put down poly to reduce moisture release to the crawl space. However, I am thinking of installing a small low volume fan in the upper corner of the opening to the main house basement just to keep air moving. I would like this to be safe so as to run continuously just to improve air quality and hopefully minimize mold and mildew that might occur. I welcome thoughts of adequacy of this idea and suggestions on where I might find a suitable fan which I’m not going to have to worry about a fire hazard. Thanks, Bob

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  1. GBA Editor
    Martin Holladay | | #1

    Unless you have mold right now, you may be worrying unnecessarily. Make sure that the exterior grade around your home slopes away from the foundation in all directions. In some homes, it is advantageous to provide a small amount of conditioned air to your crawl space, especially if your home has air conditioning, or to use a fan to move air between your upstairs conditioned space and your crawl space. For more information on reducing crawl space moisture problems, see Building an Unvented Crawl Space.

  2. Expert Member
    Dana Dorsett | | #2

    I'd call a 7 square foot access hole into the conditioned basement plenty of ventilation for a crawlspace volume of ~2000 cubic feet without needing any help from a fan.

    If you seal the poly ground retarder to the foundation and insulate the foundation walls in the crawlspace with some rigid foam you'll be in even better shape. Depending on climate it may be worth insulating the floor of the crawlspace too.

    In my own home (in US climate zone 5) there is a ~13' x 12' x 2.5' crawlspace under the back half of a bedroom with about 2 square feet of open hatch into a conditioned basement mechanically dehumidfied to <60% RH. I retrofitted the crawlspace with a 10mil poly ground vapor barrier and R20 foam on the foundation walls/band-joist a handful of years back and it's far drier than it ever used to be, doesn't smell, and while not exactly warm in winter, it's OK. Some day I'll probably insulate the floor in the crawl too, but it's pretty far down the priority list on energy upgrades for this particular house. The crawlspace upgrade to date has proved to be plenty from a mold-control point of view, which used to be something nose-detectable during the muggy part of the summer.

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