Basement and Crawl Space Moisture

Most home inspections start with the basement or crawl space. It is also here where we find a disproportionate number of problems in homes. While most homeowners would prefer to concentrate their attention on more "interesting" portions of the home, it is in the basement and crawl space that we find most of the structural components, major elements of the plumbing and electrical systems, most central heating plants, and one of the main avenues into the home for water and moisture.

The last point is worth repeating: wet or moist basements and crawl spaces are one of the primary sources of moisture problems in structures and such moisture often results in damage to all areas of the home even the roofing!


  • All basements and crawl spaces should be considered as having potential moisture and water problems. The most common causes for these problems are found on the outside of the house. By addressing these basic and often preventable causes of basement and crawl space moisture, it is possible to create basements and crawl spaces that are 95% dry 95% of the time. More expensive methods can be used to increase these numbers by a few points, often at great expense. But, a 100% dry basement is a pipe dream, and any system with a "lifetime guarantee" should be viewed with great skepticism.

  • Most of the water which enters a basements or crawl spaces comes from the outside, usually from roofs and surface water drainage. Yet, most of the "stories" about wet basements and crawl spaces blame "underground springs" and "that new construction up the hill". Even in those cases with changes in area drainage patterns, roof and surface water drainage is usually the primary source of the H20. Such roof and surface water drainage may also be the only solvable problem for most homeowners.

    Poor Drainage Example Public Enemy #1! The intentions were good: a splash block, nice landscaping and very little soil touching the siding, but here is a perfect example of how to keep your basement wet. You collect all of the roof drainage water and dump it right next to your foundation wall.

    Basements and crawl spaces will stay dryer if the are built and maintained in the following manner:

    • Yard Drainage - Slope soil around the house in such a way as to divert water away from the house.
    • Roof Drainage - Diverting all water from the roof and gutters away from the house. Don't allow roof drainage to saturate the soil around the foundation of the house. See: Down spout Drain Systems.
    • Gutter systems - Need regular cleaning and maintenance.
    • Tree limbs - Also need maintenance but this work needs to be done without damaging the trees. Tree limbs that overhang the roof and gutters make roof and gutter care much more difficult and contribute to drainage problems and wet basements.
    • Gravity drainage is preferable to pumps. Pumps require a lot of maintenance, a constant supply of power and more frequent repairs.
    • Drainage System Maintenance - Maintain the roof, gutters, down spouts, and drain systems to assure proper operation.
    • Window Wells - Maintain the window wells so that the soil level in the window well is six inches below the top of the concrete foundation wall.
    • Driveway and Stair Well Drains - Install, maintain, and repair any drain system that prevents water from draining into the basement, or when a driveway for a basement garage slopes toward the house.
    • In New Construction - Carefully seal the exterior of the foundation; install a quality perimeter drain system, and a separate drainage system for gutters and down spouts.
    • Splash Blocks - They usually don't work, don't bother getting any. The don't move the roof drainage water far enough away from the house. The same can be said for those "roll up" down spout drains! They look great in theory but they don't work.

    In addition to proper maintenance on the outside:

    Crawl Spaces will remain drier if the following precautions are observed:

    • "Head room" A minimum clearance under beams of 12 inches, and under floor joists of 18 inches. Larger crawl spaces tend to remain drier; they allow for more air circulation and easier maintenance and repair.
    • Vapor Barrier - A four mil black visquene (plastic) should cover the entire soil area.
    • Storage in Crawl Spaces - Most crawl spaces are too small to be used for storage. In addition, the storing of materials in crawl spaces often reduces vitally needed air circulation and creates conducive conditions for rodent and other pest infestation. Very large "crawl spaces", spaces with 5' or more of head room, may be fitted out with vented storage shelves.
    • Venting - There is a considerable amount of dispute regarding the role of exterior venting in keeping crawl spaces dry. In some climates such vents may add moisture to crawl spaces by allowing entry of warm and moist exterior air into the crawl space and increasing moisture through condensation. Most building codes do call for screened crawl space vents with 1-1/2 square feet of venting for every 25 linear feet of exterior crawl space wall.

    My experience in the Pacific NW has convinced me that crawl spaces should be vented - the more the better!

    Basements will remain even drier if the following precautions are observed:

    • Floor Drains - Floor drains must be kept in good repair.
    • Sump Pumps - If you have a sump pumps, it must be checked and maintained twice a year.
    • Cracks in Concrete Walls and Floors - Cracks in walls should be sealed with a concrete caulk.
    • Interior Drain Systems - One of the best ways to control basement and crawl space moisture is to build foundations with an interior and an exterior footing drainage system. Such drains are cost affective and become long lasting components of a crawl space and basement moisture control system. This is one more reason to build things right from the get-go!

      After a home is already built, interior perimeter drain systems maybe a necessary part of a drainage retrofitting project. But such projects are expensive and will not work without the exterior drainage considerations listed above. Interior drain systems should not be installed until after it is determined that properly installed and maintained exterior systems were found to be inadequate.

    • Sealing The Walls - Concrete walls can be painted on the inside with a moisture sealant. However, as with interior drain systems such solutions are relatively expensive and unlikely to work unless the more important and basic drainage and moisture control issues are addressed.

    Homes in Areas of High Water Tables and Flood Planes: There is very little that can be done about water intrusion into basements and crawl spaces that are located in flood planes and/or areas with high water tables.

  • Good drainage systems are critical for your environment:

    • Poor drainage is the primary cause of wet basements and crawl spaces.
    • Forested land has very few drainage problems: trees and forest floors absorb rain and allow much of the rain to re-evaporate into the air. The water that does flow into the streams does so very slowly. So, forests and forest like landscaping practices promote good drainage.
    • Buildings, roads, driveways, compacted earth, and even lawns cause water to run off quickly and require quality drainage systems. The best construction and man made drainage systems have the drainage characteristics of forests.
    • Wet basements and crawl spaces cause high moisture levels in the home. These high moisture levels damage wood (wood rot and insect damage) and other building materials. So, by keeping basement and crawl spaces dry, houses can last much longer and thus fewer trees will be cut down for home repairs and replacements.
    • Trees should be planted so that they will not interfere with the house. Trees that are planted too close to the house cause gutter problems and more moss growth on the roof. So, properly planted trees can live longer, will require less pruning and will not interfere with you roof drainage system (and that will help keep your basement and crawl space dryer).

    So here is a chance to help the environment, reduce runoff and sewer overflows, keep your downhill neighbors happier, while at the same time reducing moisture problems in your own home.

  • Gravity drainage systems are preferable to sump pumps. However, sump pumps are the only solution when a home is located in an area where gravity drainage system can't work. The following should be kept in mind when a sump pump is required.

    • The water leading toward the sump must be filtered through gravel and drain tile in order to minimize the silting up of the system.
    • The sump must be located in the lowest spot of the basement or crawl space.
    • The sump pump must be installed on its own GFCI circuit breaker.
    • The water pumped out of the system must be drained into a storm drain or on site drainage system that prevents back flow toward any structure.
    • The sump pump must be maintained regularly - twice a year.
    • The sump and sump pump must be kept covered at all times.
    • Sump pumps can't be used to lower water tables.

    Note: a sump pump is used for storm water, a sewer ejector is for waste water. See: Glossary

  • Wood will rot when it is exposed to moisture and in contact with soil or other material which prevents the wood from drying out. The only (partial) exception is ground contact, pressure-treated lumber. However, even such "treated" lumber is usually not fully impregnated with wood preservatives and subject to some wood rot damage.

    It is therefore very important to maintain a separation of six inches or more between all wood and any soil, gardening material, or firewood. All foundation posts must sit on concrete piers, and the perimeter foundation or skirting must be constructed out of concrete or pressure-treated lumber. All wood that has previously been in contact with soil, but which remains in relatively good condition, should be cleaned, allowed to dry out, and treated with a wood preservative. Wood that has deteriorated due to soil-to-wood contact must be replaced, and future soil contact prevented.

  • Crawl spaces are not generally designed for storage. Stored material tends to reduce air circulation, trap moisture and be conducive to pest activity. In very large crawl spaces, it maybe possible to install some shelving with good air circulation on all sides of the stored material.

    Even in a relatively dry basements, it is recommended that storage be done in such a way as to allow for some air circulation around and under the stored material. This can be done by placing the stored material on palates or simple shelving that is a minimum of four inches above the concrete slab, and not touching concrete walls.

  • The following guidelines are intended for any contemplated basement usage or remodeling.

    • Assume that the basement will have moisture problems at some point in time.
    • Install any water heater, washing machines etc. near a floor drain or on a special pan which collects water from any leaks.
    • Use pressure-treated lumber for plates and other material that could become wet or have to be in contact with concrete surfaces. Maintain a minimum of 1" air space between any other framing and concrete walls.
    • Leave a 1" plus air space between the bottom of drywall walls and floor surfaces. This will help reduce water from wicking into the drywall. This gap can be covered with a base trim.
    • Use paperless drywall in areas subject to moist conditions. In shower and tub areas don't use any kind of drywall. See: Tub and Shower Walls
    • Any carpeting should be installed without glue or tack strips so as to facilitate removal and drying if/when necessary. Carpet which becomes wet must be dried out ASAP, preferable by having the carpet removed and professionally dried.
    • Store items on shelves that allow for some air circulation all around the stored material.
    • Forced air heating systems tend to provide uneven heat in basements. The air at the top of the room is hot, at the floor it is cold. Zone heating such as electric wall heaters or gas fireplaces work well in such areas.