|Poor building design leads to fungal growth Moisture problems due to construction, and how to control moisture by Philip R. Morey, Ph.D., Director of Consulting Services, Air Quality Sciences, Inc.|
|Modern buildings are expensive investments. Consequently, owners and investors expect the structure to remain physically sound for a lifetime of 50 to 100 years. Building occupants and tenants also expect their occupied spaces to be both a comfortable and a healthy place to work.|
Several litigations in the United States provide examples of poorly designed new buildings that were both poor investments and unhealthy workplaces. Moisture damage and consequential fungal growth on interior surfaces in two USA courthouses necessitated occupant evacuation because of allergic respiratory disease. The restoration in each building (cost about 20 to 40 million USA dollars) exceeded the original capital cost of each building. In a recent 11 million dollar settlement in a California condominium case, moisture incursion and extensive fungal growth occurred on exterior sheathing (paper fiber gypsum board) in the building envelope. Owners of condominium units were compensated because of construction defects and fungal growth in the envelope walls.
Reasons for moisture and fungal growth problems in modern buildings are complex and involve considerations such as the integrity of the building envelope and the susceptibility of construction and finishing materials to biodeterioration.
As we enter the 21st century it should be remembered that many of the materials used in modern construction such as paper fiber gypsum board, porous insulation, vinyl wall covering, pressed wood products, porous ceiling tiles, and textile wall and floor coverings were not used or seldom used prior to the 1930s. Some of these construction and finishing materials are highly susceptible to fungal biodeterioration.
With the advent of air-conditioning in the mid-20th century, temperature gradients on various surfaces in buildings have become non-uniform. Elevated relative humidity (dampness) and even condensation can occur on surfaces that are relatively cool compared to the temperature of the surrounding air. Fungal colonization of interior surfaces occurs when biodegradable materials are chronically damp or wet. If growth is extensive, the consequence can be a structural defect (e.g., fungi degrade the paper fiber surface of wallboard) or a health problem (e.g., allergic respiratory disease).
Several reviews have been published on prevention and control of moisture/fungal problems in buildings (Morey 1996; ACGIH 1999). Fungi will grow on damp/moist biodegradable construction and finishing materials. It is immaterial to the fungus if the building is located in Miami, Atlanta, Hong Kong, Singapore, or Kuala Lumpur.
The primary environmental factor controlling the growth of fungi in buildings is moisture availability. Moisture can enter buildings from sources such as rainwater or pipe/sprinkler leaks. Moisture can also occur in building materials from less obvious sources involving water vapor migration and infiltration of humid outdoor air into the building envelope.
Wind driven rain can enter the building envelope and saturate construction materials especially when roof and window flashing fails. Water that enters the building envelope should be removed by drainage to the outside or by collection of water vapor by the air-conditioning dehumidification cooling coil. Construction defects in building envelopes where water drainage to the outside is blocked by mortar and other construction debris is common. Because of this defect, rainwater that enters the envelope chronically drains into the occupied space. This results in wetting of the paper fiber gypsum board and flooring materials with subsequent fungal growth.
In warm humid climates condensation occurs on walls, ceilings, and floors when their surface temperatures are cooled (by air-conditioning) below the dew point temperature of the surrounding air. If warm moist outdoor air infiltrates through the envelope wall (in a negatively pressurized building) condensation or dampness occurs on cool surfaces (e.g., wall paper, paper fiber gypsum board). Fungi then colonize these surfaces.
The following principles are useful in preventing moisture and fungal growth problems in modern air-conditioned buildings in warm humid climates:
If vapor diffusion and air retarders are used, install them near the exterior surface of the envelope.
Operate the building so that the indoor air is slightly positive in pressure relative to ambient (outdoor) air.
Dry construction materials that are wet or moist before sealing them into building structural components.
Use permeable wall coverings (permeance greater than 5 perms) on interior surfaces of envelope and interior walls that may be subject to water vapor or moisture incursion. A permeable wall covering allows water molecules in wetted structural components to diffuse through the wall and be removed by the air-conditioning dehumidification cooling coils.
Avoid cooling the interior space below the mean monthly outdoor dew point temperature. This reduces the likelihood of condensation on interior surfaces.
Substitute biodeterioration-resistant materials for those susceptible to fungal growth. For example, substitute concrete board for paper fiber gypsum board in walls that are likely to be chronically wet or damp.
Finally, if you invest in buildings (for example, your residence) inspect all structural components for water damage and visible fungal growth (see references) prior to making that investment. Purchasing a chronically wet or damp building is likely to be a poor investment.
Copyright ? 2001 Aerias, LLC All rights reserved. Reprinted with permission.
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