January 2004
In this study, the NAHB Research Center investigated building issues related to elevated moisture at 18 sites representing more than 20 residences. The case study reports list symptoms, possible moisture sources, diagnoses and prescriptive remedies for the problems, and analysis of remedial measures if the builder or homeowner adopted the measures. Cases included several areas of the country representing the northern heating climate, the southern cooling climate, and the mixed climate zone situated between the two.
Based on the case study report, moisture problems were not unique to a particular climate or structure. Generally, problems with moisture came from one of three sources:
- construction moisture present in either the building material or as a result of exposure to the weather prior to close-in,
- elevated relative humidity levels causing condensation on building surfaces from both interior and exterior air,
- rain and groundwater entry.
Elevated humidity, while problematic in all climates, operates in different directions based upon climate zone. In cooling climates, the humid air is exterior air that condenses upon surfaces at or below dewpoint within the wall cavity. In heating climates, the humid air is in the house and condenses on wall surfaces or within the wall cavity once dewpoint is attained.
An itemized list of common moisture problems and solutions is presented within the paper. Common moisture problems include wetting of the foundation by watering a garden adjacent to a basement foundation, wetting of exterior cladding by lawn sprinklers, downspouts terminating near the foundation, soil moisture, water vapor, humidity, air leakage into or out of homes, humid summer air entering crawlspaces and condensing on cooler surfaces, rainwater penetrating flashing at window, interior rooms excluded from air circulation and heat and air gaps between exterior cladding and sheathing are all discussed in this report.
Based on the case study report alone, no definitive conclusion can be reached about the nation’s housing stock concerning moisture problems and mold. However, insight from the report reveals several actions that can reduce mold infestations in housing:
- Protect building materials and assemblies from weather.
- Direct precipitation, irrigation, and ground water away from the structure.
- Monitor interior and exterior relative humidity levels and design and build accordingly:
- Heating climates should prevent interior humidity levels from forming condensation on or in exterior walls.
- Cooling climates should prevent exterior humidity levels from creating condensation within exterior walls.
- Mixed climates need to prevent both of the above conditions.
- Conforming to model building code is not a definitive solution for the prevention of moisture problems. For example, ventilating crawlspaces may create a mold problem in some structures instead of preventing problems.
Prepared for:
National Center for Housing and the Environment
Prepared by:
NAHB Research Center, Inc
76 pages