Vents installed in the crawl space walls should be closed to prevent outside moisture and humidity from entering the space. Exterior metal and plastic vent covers are available, or you can plug the openings with rigid foam board. If you are concerned about aesthetics, use material that matches the existing wall.
Small gaps, ¼ inch or less, are also notorious for allowing air penetration, especially around the band joists and between the sill and foundation wall. Acrylic or urethane caulk will provide a proper seal. Larger openings, up to 3 inches in diameter, can be filled with an expanding spray foam product. Although you can use fiberglass batt to insulate the wall cavities, rigid foam board will yield the best results. Caulk around the perimeter of the board to enhance the insulating value even more.
Water goes into the ground around the house and eventually seeps up into the crawl space from the dirt floor in the form of moisture vapors. There are basically two ways to remedy this problem: pour a concrete floor or use a polyethylene liner to cover the floor surface. Concrete is difficult to pour in a crawl space. If you choose this solution, and don’t mix the concrete yourself, have the concrete pumped into the space. Install a liner. It is much easier and will serve the purpose.
Avoid using 6mm plastic sheeting, which is the standard material chosen by most people when sealing crawl space floors. Over time, punctures and tears become an issue and you’ll have to continually make repairs. Instead, go with a thicker, more durable 20mm polyethylene liner. It is flexible and very durable. Make sure that the covering resists mold and mildew.
If a combustible appliance, such as a heating unit or hot water tank, is installed in the crawl space, there must be a source of air for the combustion process. The least desirable option is to leave the wall vents uncovered to allow for the proper air flow. You can install vents leading to the first floor. Or, a better choice may be to wire a combustion air supply fan to the appliance’s burner circuit. The fan opens a damper, which allows air in when the appliance is in operation and closes the damper when the appliance shuts down.