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What to do about gravel crawlspace floor?

Insular | Posted in Green Building Techniques on

Question is about a house in the northeast–cold winters, humid, raining summers. It was built around 22 years ago.

Most of the house is above a 500 sq foot crawlspace, about 5.5 feet in height. The rest is a connected walk-out basement that was added as part of an addition to the upstairs. A gas boiler (for heat) and electric water heater are located in the crawlspace. The crawl space has a couple of vents (I haven’t closed them off because of the gas boiler).

The floor of the crawlspace is loose stone gravel-rather chunky pieces not the usual stuff you see in yards.

Underneath the stones is a old layer of clear plastic.

During a rainy period in the summer, the humidity in the basement/crawlspace soared to over 80%. The gravel floor in the crawlspace had a large blotch of wetness, almost a perfect square around 4′ on a side.

I put in the largest portable dehumidifier I could find, which brought the humidity down to 50% after 3 days of running constantly on high.

What should I do about the floor of the crawlspace? Does the stone gravel serve any purpose whatsoever?

The most logical course of action seems to be to (i) remove the stone gravel (not an easy undertaking) and (ii) install a new plastic vapor barrier on the floor that can be walked on.

What do you think?

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  1. GBA Editor
    Martin Holladay | | #1

    You are correct that the stones don't do anything (except to provide you something to stand on other than plastic or standing water).

    To reduce the amount of water entering your crawl space, you need to be sure that the grade on the exterior of your foundation slopes away from the house in all directions. This may require excavation or shovel work. You should also make sure that there are gutters on your roof eaves, and that the gutters are connected to conductor pipes that convey the roof water to a location far from your house.

    If these measures don't help, you may need to install French drains on the interior of your crawl space. These French drains can be connected to a sump; you'll need a sump pump that moves the water out of the crawl space.

    It's a fairly hopeless task to try to address the problem with a dehumidifier, especially if your crawl space is vented. What you are trying to do, essentially, is dehumidify the entire neighborhood. You'll never succeed with that approach.

    For more information on crawl spaces, see this article: Building an Unvented Crawl Space.

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