Sealed Crawl Spaces

Sealed Crawl Spaces are becoming the Norm in Crawl Space Design

By Mark J. Donovan

A sealed crawl space provides advantages over a traditional unsealed crawl space. A sealed crawl space is comparable to a basement in that it is a reasonably dry and warm space that can be used for storing temperature sensitive items. A sealed crawl space also provides warmer overhead floors and prevents the risk of freezing pipes during winter months. Finally, a sealed crawl space will have fewer moisture problems in comparison to a ventilated crawl space.

For about 50 years building codes have required crawl spaces to be unsealed to enable moist air to escape from the crawl space.

Moisture enters the crawl space via the earth floor. Moisture literally permeates up through the damp earth floor into the crawl space area.

Crawl space vent systems, however, have never really performed an adequate job of dissipating moisture laden air from crawl spaces. Often they create more of a moisture problem. For example, during summer months crawl space vents let warm, moist air into the crawl space rather than remove it.

If you would prefer to seal your crawl space you need to incorporate three elements into it. First you need to install a vapor barrier (aka vapor retarder) on the earth floor of the crawl space. Second, you need to add some insulation. Finally, a minimal amount of heat needs to be introduced into the space. Note that these three elements are typically found in basements which normally do not require ventilation.

To create an unventilated crawl space begin by installing a vapor barrier on the floor of the crawl space.

Use a 6 mil sheet of polyethylene plastic for the vapor barrier and make sure you tape any seams with duct tape. In addition, make sure the plastic laps up the sides of the crawl space walls by about 12 inches.

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Use duct tape to attach the plastic sheathing to the crawl space walls to seal the moisture barrier. Lastly, apply a couple of inches of sand over the vapor barrier to allow you to walk/crawl around the crawl space without the threat of damaging the vapor barrier. Alternatively you can also install crushed stone over the sand.

Next, install insulation in the crawl space. More specifically install batt or rigid foam insulation on the exterior walls of the crawl space and in-between the floor joists near the band joists.

Next provide some low level heat to the crawl space area. In some cases you may be able to open an existing basement door or window to the crawl space area to provide adequate heat.

In other cases you may need to provide a separate heat zone or small heating system to the crawl space. Check with a local HVAC contractor to see what might work best for your crawl space.

Crawl space vapor barrier

Lastly, seal the existing crawl space vents and insulate the doorway entrance. By introducing these three key elements to a crawl space and sealing, the existing crawl space vents, your sealed crawl space should be warm and dry year round.


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