• Make sure the height of the space exceeds local regulatory requirements (usually 4 feet or 1.22 m)
• Install plenty of perimeter drainage and some central floor drains in the crawl space
• Slope the floor of the space towards the floor drains
• Lay down at least two layers of six-mil (0.015 cm) thick polyethylene plastic sheets as the VDR. Overlap the seams at least 2 feet (0.61 m). Extend the plastic up the foundation walls (avoid covering the vents in the event that you may need to open them again in the future).

These measures allow for emergency drainage and ventilation. As an option you can also pour two inches (51mm) of concrete over this to protect the polyethylene VDR from damage.

Vapor Retarders in Existing Buildings
Except for extensive remodeling projects, it's difficult to add materials like sheet plastic as a VDR to an existing home. However, many existing homes don't really need a more effective VDR than the more than likely numerous layers of paint on their walls and ceilings. These multiple layers are quite effective as a VDR in all but the most extreme northern climates.

"Vapor barrier" paints are also an effective option for colder climates. If the Perm rating of the paint is not indicated on the label, an alternative is to read the paint formula. The paint label usually indicates the percent of pigment. To be a good VDR it should have a relatively high percent of solids and thick in application. Glossy paints are generally more effective VDRs than flat paints and acrylic paints are generally better than latex paints. When in doubt apply more coats of paint. However, it's best to use paint labeled as a VDR and follow the directions for applying it.

In any case, the key to controlling unwanted water vapor movement is the careful air-sealing of gaps in the structure and not the VDR alone.