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Extension > Environment > Housing Technology > Moisture management > Floor coverings for basements and below-grade space

Floor coverings for basements and below-grade space

Dee Ginthner, Assistant Professor, Design Housing and Apparel
Wanda Olson, Professor, Design Housing and Apparel
Neil Carlson, Public Health Specialist, Environmental Health and Safety

Basement moisture problems

Basement floor coverings must be suitable for the intended use of the space and for installation in spaces that are on or below grade. In below-grade spaces, high moisture conditions can lead to mold growth and damage to moisture sensitive floor coverings.1 Moisture problems must be solved before finishing a basement. Molds can cause health problems when mold spores that enter the air are inhaled. Over time, mold can damage the building and its furnishings. Molds can contribute to allergic reactions, asthma episodes, infections and other respiratory health problems. Chart 2 identifies basement conditions and potential risks for several types of floor coverings.

Chart 2. Assessment of basement system conditions affecting moisture level of concrete below-grade floors

Conditions --------------------------------------------------Level of risk--------------------------------------------------
Lower risk Higher risk
Distance of basement floor from water table more than 4 feet (HUD requires 4 feet or more less than 3 feet (USDA Rural Dev. requires 3 feet or more)
Exterior drainage issues
type of backfill free-draining compact
exterior drainage pea gravel or drainage mat, drainage tile around foundation soil around foundation
slope/grading ground slopes (at least 6" per 10") away from the foundation ground is level or slopes toward foundation
getters/downspouts house has gutters/downspouts with extenders/splash blocks no gutter/downspouts or installed without extenders/splash blocks
Interior condensation and drainage issues
concrete floor temperature warm because of insulation below concrete; heat installed in floor (seasonal) or sleeper floor cold because lack of insulation below concrete or lack of heating of concrete.
foundation wall temperature* warm because of ext.insulation cold because of int. insulation**
relative humidity (RH) conditioned space-heat, dehumidify, or air condition uncondition space
winter conditions less than about 30% RH above 40% RH
non-winter conditions less than about 60% RH above 70% RH
interior drainage no water entering basement water entering the basement continuously or seasonally
* If wall foundation surfaces are cold or single pane windows are present, condensation will occur and collect on the floor.
** Works sometimes; many moisture and indoor air quality problems have occurred with this installation.

Some moist basement conditions can be improved by adding a sump pump (sealed) or other basement system. Dehumidifying, air conditioning, ventilating, and increasing the airflow within the space will help eliminate dampness on surfaces. Other strategies to reduce damage caused by moisture include storing boxes and other items off the floor, using floor coverings with a low sensitivity to moisture, or using a floor covering that can be easily removed (such as an area rug) while the moisture problem exists. While indoor/outdoor carpet is not moisture sensitive and can be sanitized, it may not be desirable for the intended use. Avoid installing carpet in a high traffic area because a soiled carpet holds more moisture than a clean carpet. Soil and moisture contribute to mold growth and an increase in the dust mite population.

Floor heating

If radiant floor heating is used, the floor covering should have a low R-value to allow heat from the heating system to pass through into the room. For carpet, this is a total R-value for carpet and pad. The maximum R-value of a floor covering is determined by the required floor output and should be calculated by the heating contractor.

Types of floor coverings

Floor coverings below can tolerate a wide range of floor conditions, although excessive moisture could be a problem. All have a low R-value and are suitable with radiant floor heating.

Installation conditions for different types of moisture-sensitive floor coverings are shown in Chart 1. All except carpet/pad have a low R-value. For specific R-value information (carpet/backing/pad), check data specification sheets or contact the heating contractor. Some approximate R-values are: 1/2-inch synthetic carpet, (1.4); 1/4-inch slab foam rubber pad, (0.3); and 3/8-inch prime urethane pad, (1.6).

Chart 1. Installation Conditions for Moisture-sensitive Floor Coverings

Floor coverings Comments from specification sheet
wood install over a sleeper floor/moisture barrier
high pressure laminate install over polyfoam moisture barrier
vinyl-backed sheet flooring less than 3 lb/1,000 ft2 x 24 h (calcium chloride test2)
vinyl comosition tile less than 5 lb/1,000 ft2 x 24 h (calcium chloride test2)
felt-backed sheet flooring less than 5 lb/1,000 ft2 x 24 h (calcium chloride test2)
carpet less than 3 lb/1,000 ft2 x 24 h (calcium chloride test2)

Carpet must be installed over a dry subfloor. High moisture or high pH can cause product or adhesive failure. If there is a moisture problem, molds will grow unless the carpet is removed, sanitized and properly dried. There are carpet floor coverings with water impermeable backing which is secured to the floor using a double-stick tape around the edges of the carpet and "Velcro™-like" strips. Even with this product, mold may form below the backing. This product is suitable on or below grade and if there is a temporary moisture problem the carpet could be removed, sanitized and replaced. Antimicrobial carpets are also available. They offer resistance to mold, but routine cleaning is needed to maintain effectiveness.


1 Potential sources of moisture in a basement are rain; melting snow; groundwater; plumbing leaks; sewage back-up; external humid air used for ventilation; crawl spaces; and internal sources. Water as vapor can move from saturated soils through concrete.
2 Two procedures for measuring moisture emission from concrete slabs are the calcium chloride test (long prescribed by the floor covering industry and with results listed in pounds per 1,000 ft squared per 24 hours) and the hygrometer with plastic sheet test. One procedure for measuring moisture present in concrete slabs is to use a pinless moisture meter for concrete. These procedures will give an indication of moisture conditions at the time they are conducted but cannot predict long-term moisture conditions with on-grade or below-grade slabs. ASTM is developing "Practices for Determining Moisture Related Acceptability of Concrete Floors to Receive Moisture-Sensitive Finishes."


References:

Carmody, J. and Anderson, B., (1997). Moisture in Basements: Causes and Solutions. St. Paul: University of Minnesota Extension Service.

Minnesota Department of Health (1997). Mold in My Home: Is Mold a Health Concern? St. Paul: Minnesota Department of Health.

McGowan, Maryrose, (1996). Specifying Interiors. New York, NY: John Wiley & Sons, Inc., pp. 155-157. Radiant Panel Association, P.O. Box 717, Loveland, CO.

The authors wish to acknowledge the contributions of Pat Huelman, College of Natural Resources, University of Minnesota.

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