Crawlspaces and Moisture

By Frank G. Ross

 

Crawlspaces can be dark, dank, dirty and a home to spiders (of which I am not fond).   It is an unfortunate fact that crawlspaces are prone to moisture problems.  Crawlspaces with dirt floors are tremendous sources of moisture. The surface might appear dry but most often moist earth isn't too far below. Moisture in a crawlspace affects not only the flooring right above it but can affect the entire house. Traditionally, the most common method of dealing with moisture in crawlspaces — and the one longest endorsed by codes — has been to ventilate them.   However, non-vented crawlspaces are becoming more common.

A non-vented crawlspace is usually contained by a solid concrete or masonry foundation wall that extends down to footings just below the frost line, the depth to which the ground freezes during the winter.  It would also need to have a moisture barrier on the floor and be properly sealed.  Insulation on the walls would be another feature of a conditioned crawlspace. 

Research and experience has shown that non-vented, conditioned crawl spaces provide better moisture control than vented crawl spaces and behave similarly to houses constructed with basements.  In our area of Utah, many times a full basement makes more sense than a non-vented crawlspace.  Talk with your home improvement specialist or builder to see what is best for your situation. All systems should be professionally installed and inspected by local code enforcement.

Vented crawlspaces are by far more prevalent than non-vented types. A number of vents (the exact number is spelled out in the building code) provide adequate cross ventilation, the sub floor of the home is insulated along with the pipes in the crawlspace to prevent freezing, and there may be a vapor barrier on the ground.  Ideally the vents get closed in the winter time. A small heater or heat tape is used to help keep the pipes from freezing in addition to the insulation on the pipes.

Few builders take ALL the precautions necessary for keeping water out of a crawlspace.  Moisture in the crawlspace provides the perfect environment for mold growth and other moisture problems such as structural damage from dry rot, potential indoor air quality problems, as well as termites or other pests attracted to the presence of moisture.

Natural air flow in a house is from bottom to top.  Air from the crawlspace will naturally flow through gaps in the flooring, etc. to the living space above.  This phenomenon is called the chimney effect.  It’s a good reason to keep an eye on the condition of the crawlspace, regularly inspecting it and noting any situations requiring attention. 

Conscientious homeowners will also take measures to keep moisture out of the crawlspace.  Following are some suggestions, most of which apply to vented crawlspaces.  The first two will solve about 95% of water problems at the foundation.

Surface water drains away from the foundation.  All surface water should be directed away from the home. A swale may be necessary if your home is built on a slope. The foundation can have significant structural damage caused by water along the foundation that freezes in the winter.  Be careful when watering lawns and landscaping.  Don’t allow water to hit the house.

Rain gutters and downspouts.  Either provide working gutters and downspouts, or install generous eave overhangs coupled with positive surface drainage.  The downspouts need to shed water away from the home at least 6 feet or more or to underground pipes that lead away from the home.

Vapor barrier.  Install a full plastic membrane or a concrete slab on every crawlspace floor. The plastic membrane should be 6 mil thick poly, overlapped and sealed around columns and the walls. This will help to keep the ground moisture vapors from rising up into your framing. Most homeowners will add gravel on the top to help prevent damage.  This is the number one preventive measure that in most cases is much more useful than ventilation. 

Adequate ventilation.  Don’t forget to open vents that were closed for the winter. A crawlspace needs a minimum of 1 square foot of ventilation for every 150 square feet of crawlspace floor.  Shrubs and other plantings; low decks and other types of construction above or against vents can block the vents making them useless.

Sump pump.  When ground water is the source of moisture, it may be necessary to create a drain system with a sump, equipped with an automatically controlled pump to remove water from the crawlspace.

If you have a sump pump installed, make sure it is at the lowest point in the crawlspace and all areas easily drain to it. The pump should be inspected regularly. Nothing can take the place of a properly installed perimeter foundation drainage system by the builder but a sump pump is the next best thing. The pump should be in a plastic cylinder in the ground and eject water outside, away from the building and not into the septic or sewer system.

Added moisture.  Do not locate vent fan exhausts, dryer exhausts or condensate drains in the crawlspace.

Avoid air leaks.  Seal and insulate heating/air conditioning ductwork in vented crawlspaces to minimize energy losses and prevent leaks in the system’s return ducts from distributing crawlspace air into the house.

Storage.  Avoid using the crawlspace for storage.  Stored items may be damaged by moisture.  Also organic materials may make a good food source for mold, mildew or pests.

Please remember that a crawlspace is an integral part of your home and should be kept neat, clean and easily accessible. Inspect it often looking for signs of moisture or mildew and consult a professional for any repairs. Regular maintenance can save many a homeowner a costly and inconvenient repair.

Frank Ross is a Certified Utah Home Inspector for Pillar to Post Professional Home Inspection and a full member of the National Association of Certified Home Inspectors and a member in good standing with the Utah Better Business Bureau.  Readers may contact Mr. Ross at (435) 867-6400 or frank.ross@pillartopost.com or www.pillarposthomeinspection.com