It is not normal for crawlspaces to be damp and moldy — conditions that can be caused by high water tables, leaky plumbing, bad drainage, and humid outdoors air. Mold allergens within the crawlspace can move into the living spaces of the house and cause respiratory problems in people with allergies and asthma. Additionally, the crawlspace moisture and mold in crawl spaces can lead to structural damage to the building.


Though most building codes require basis openings to provide air flow to the crawlspace, these openings might not decrease moisture levels and mold growth in crawlspaces. Actually, in some climates (such as these with high humidity), the additional humid air that they allow entering the crawlspace will more than likely, just aggravate crawlspace moisture and mold problems.


The best solutions for these problems are to:

1. Correct drainage problems: Soil will have to be graded with a slope that drains water away from the outside of the building. Additionally, missing or disconnected downspouts allow water to splash onto and puddle near the foundation causing dampness or leaks in the foundation and crawlspace. Expert assistance is also important for some drainage problems.

2. Install a ground cover to minimize moisture entry from the soil: Cover the soil floor in the crawlspace with long lasting plastic sheeting (similar to 6 mils black plastic). Overlap edges of the sheeting roughly 12 inches and anchor with gravel, boards, bricks, etc. Join the plastic sheets with special tape that sticks best to plastic sheets.

3. Repair any plumbing leaks.

If conditions 1 – 3 had been addressed, then foundation vents usually are not needed and the crawlspace walls will also be sealed against entry of moisture, air, and pests. The vents not very desirable in crawlspaces that:

  • Have ducts, pipes, or air conditioning or heating equipment
  • Are located in climates with lengthy periods of climate that’s:
  • ? Cold (it’s an excellent suggestion to place thermal insulation on the walls of unvented crawlspaces in a cold climate), or
  • ? Sizzling and humid.

In crawlspaces with unavoidable or naturally occurring air pollution (for example, radon), contaminated air will also be eliminated from the crawlspace through air-sealing and depressurizing the space using an exhaust fan that is vented outdoor. If the source of contaminated air is the soil (which is the case for radon), it is more effective to depressurize beneath the ground cover). The installed fan should use less than 90 watts, have a sound rating of fewer than 2 sones and should not move more than the amount of air it will take to meet the air flow requirements of the home.

Note: Crawlspaces that have naturally vented combustion equipment (such as a furnace) should not be depressurized through this process as it’s going to lead to the back drafting of this equipment.

An exhaust fan can be used to help dry a moisture crawlspace and as a temporary measure to keep mold allergens from coming into the home. For crawlspace moisture with broad mold contamination, an expert with experience in mold cleanup should be contacted.


For a house that will be constructed on a site that is unavoidably damp (e.g., drainage techniques won’t work), or with other hydro-geological conditions that make an enclosed crawlspace unsuitable, put the construction on piers. In this method, the floor of the building now becomes the boundary against heat loss, and air, moisture, and pest entry.

Most of the conditions described here would require the assistance of an expert on crawlspace moisture problems.