Revisiting the Basics of Crawlspace Remediation


June 26, 2015


Ground cover should both do the remediation job and improve the appearance of the crawlspace. Photo: Paul Hardy

As homeowners react to the need to save energy, tighter construction has become the norm in nearly all new structures — more insulation and better-sealed walls, windows and doors. The tighter the house gets, though, the more the crawlspace becomes a problem because it leads to moisture problems such as fungus, mold, mildew, insects, foul odors and structural damage.

Crawlspace remediation, when done properly, is often the best way to solve these problems.

Dehumidifiers to the rescue
First, look at what the requirements are to prevent excess moisture, which leads to higher-than-desired relative humidity. As a rule, moisture less than 20 percent inside the structural timbers in the crawlspace, and humidity less than 60 percent, won’t cause mold or fungus rot. The best setting needs to be 10 percent to 20 percent moisture, and 40 percent and 60 percent humidity year round.

Thanks to improved construction practices, some type of dehumidification is recommended in all areas. Some of the newer central air conditioning units offer built-in dehumidification, provided the unit is large enough to include the volume of the crawlspace.

Closed crawlspaces also can cause a problem when groundwater comes in the crawlspace or a leak accrues, allowing water to collect on top of the ground cover. With no way to dissipate the excess water, inactive fungal spores will germinate, and the problem begins anew. This is why it’s recommended to install a dehumidifier and a water alert system in every closed remediated crawlspace.

Ground cover options abound
Ground cover is more than just a piece of polyethylene. The objective isn’t just to cover, but to seal the space completely so rising humidity doesn’t enter the space.

As a rule, the thicker the ground cover, the better. Currently, the millage runs from 6 mil all the way to 24 mil, with 8 to 12 reinforced being the most common. Options include clear, white and color variations in-between and with or without under-the-cover liners.

Joints have to be sealed with at least 1 ft. of lap, although 2 ft. is recommended. Sealing can be done with adhesive spray or double-sided 4-in. tape. Piers also need to be covered and sealed similarly.

Prepare foundation walls
Regardless of type — brick, block, rock or poured concrete — the foundation wall needs to be sealed. Leave an inspection area of 4 in. at the top and bottom of the wall for termite and other inspections. Also ensure there are vents, but seal openings around utilities entries and foundation cracks. The smallest opening will allow hot outside air to enter the crawlspace, usually bringing water with it.

Also consider insulating the interior of outside foundation walls with foam insulation board. There are several products on the market today treated with borates for the control of insects. When installed correctly, none have been shown to increase subterranean termite populations.

Don’t forget to treat the wood
Wood treatment is a vital part of the crawlspace remediation program. Mold spores should not be left on the substructure. They’ll go dormant when moisture is reduced. But if the moisture returns to 20 percent or more, the spores will reactivate as they were before the remediation. After cleaning, the wood needs to be treated with a fungicide and water repellent to control any existing fungus and prevent it in the future.

Finally, a follow-up inspection is needed to verify the space is dry, and that humidity and moisture are at expected levels.

Crawlspace remediation saves energy and improves the quality of the whole structure. It’s a service your customers will appreciate in the long run.

Paul Hardy is a PMP contributor and can be reached at


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