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Indoor Mold and Moisture Basics

John Huang, Ph.D., PE and John Poullain, PE

This two-hour online course provides guidance on the basics of mold growth in buildings and the sources and typical conditions that cause mold problems. The course discusses how vapor pressure and surface temperatures interact to cause mold, discusses the relationship between temperature, relative humidity and moisture and shows their combined influences with an ASHRAE psychrometric chart. The course presents methods to identify sources of mold; mold related health effects, ways to avoid mold growth and reduce exposure to mold and how various building wall cavities can be protected. The course has references to informative sources in the text topics.

Mold may be indicated on room surfaces but may also be growing inside wall and ceiling cavities. Any moisture migrating from the inside and into the wall or ceiling during the heating season may condense someplace inside the walls where dewpoint temperatures are reached and cause mold problems. In some cases condensation may be severe enough to also cause certain types of wall insulation to collapse and settle inside wall cavities. This is usually caused from poorly insulated exterior walls and windows.

Moisture is transported in a building in four ways. The first way includes bulk types that include rainwater and ground water and the second is by capillary actions such as concrete floor slabs. Measures to control the first two are relatively easy and straightforward. The latter two, air transported and vapor diffusion, are far more difficult to control.

Air transported moisture is a very destructive mechanism. Air transported moisture may be a bigger problem than vapor diffusion because diffusion is a slow process and does not contribute the vast amount of moisture as air transported. Air often has a considerable amount of moisture in a vapor form and as it moves from inside to outside or outside to inside in a building the vapor moves along with it. Air movement is caused by air pressure differences and depends on whether itís the heating or cooling season time of year. If a surface within the wall or ceiling cavity is below the dew point temperature, condensation will form, mold will grow and insulation and structural damages may occur from repeated cycles.

Vapor diffusion depends on the differences in vapor pressure across materials, on their permeability and usually goes from higher to lower pressures. All materials are vapor permeable to a degree. Diffusion has less of an effect than air transported moisture; failure of a building envelope from vapor diffusion is rare. Paint with a low permeability can help to retard vapor diffusion.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

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