The Best Concrete or Gravel to Put on a Crawl Space Floor

Concrete

• What type of concrete you use when constructing your crawl space can greatly impact the effectiveness of your moisture control strategy in the crawl space. Look for insulating concrete, a mixture of concrete and an insulating aggregate that is designed specifically to equalize temperature differences between concrete and soil moisture that cause condensation to form on the concrete. If insulating concrete is not available in your area, use air-entrained concrete with a 28-day minimum compressive strength of 3,000 pounds per square inch, a maximum slump of 5 inches and a minimum cement content of 520 pounds per cubic yard. A mixture of this nature will help reduce condensation and moisture problems.

Vapor Barriers

• Installation of a vapor barrier is necessary to prevent unwanted moisture from making its way into the home through the crawl space; vapor barriers also affect what kind of gravel or other materials you use to help control moisture. A typical vapor barrier for a crawl space consists of a layer of polyethylene film or asphalt-laminated paper laid out to cover 2/3 to 3/4 of the crawl space area. If your home uses all carpeting or vinyl flooring, you can cover all of the crawl space area with the vapor barrier layer.

Gravel

• To assist the vapor barrier in resisting soil moisture, homeowners often add a layer of gravel underneath the vapor barrier. The gravel simply gives some physical distance between the moist soil and the crawl space above the vapor barrier, so the type of gravel used does not actually matter, so long as enough gravel is used to create separation between the vapor barrier and the soil below. Similarly, a layer of sand can be added to the top of the crawl space vapor barrier to help prevent condensation of moisture on the underside of the vapor barrier and to absorb moisture on the top side of the barrier.

Other Moisture Control Tips

• Choosing the right type of concrete and using gravel are important moisture control steps, but they are only effective if combined with other moisture control strategies in the crawl space. Venting a crawl space to draw in fresh air was long thought to be the best moisture control strategy for crawl spaces, but homebuilders now believe that removing or sealing vents and insulating the crawl space tightly are much more effective ways of preventing the problematic buildup of moisture inside the crawl space.