Molds do not require liquid water to grow. They only require relative humidity levels from 65% to 99% at the surface on which they grow. If you keep the humidity low enough, you can prevent mold growth. Maintaining relative humidity below 50% inhibits mold and mildew growth, dust mite infestations, and bacteria. This lower relative humidity also reduces the out-gassing of volatile organic compounds (VOCs). In colder climates, wintertime humidity levels must be even lower — generally, 25% to 40% (to prevent condensation on windows).
To protect your respiratory system indoors, the relative humidity should be above 25%.
- Keep the air pressure in the lowest rooms slightly higher than the soil gas pressure to minimize entry of water vapor through pores and cracks in the concrete.
- Use subslab depressurization to expel water vapor and radon from the soil to the exterior so they don't leak into your home through pores and cracks in the concrete.
Molds are incapable of obtaining the moisture needed for their development directly from the atmosphere, but they can obtain it from a substrate, which has absorbed moisture from moist air (60% to 100% relative humidity). The relative humidity of the air has an indirect effect on fungal growth, and the more hygroscopic a material was, the more susceptible it is to mold growth. The minimum moisture content at which mold growth occurs depends on the material and usually ranges from 10% to 14%. Suitable substrates include carpet fibers, gypsum, concrete, bricks, etc.
Buildings contain a mixed community of yeast, each of which may experience optimal conditions [temperature and relative humidity (Table #1) and for nutrients which extend the viable range. Even strains among species have different requirements.
There are generally surfaces that are at a lower temperature than the bulk of a room. Consequently, although the moisture content will be the same as in air in the center of the room, the relative humidity of the air adjacent to a cooler wall will be higher. In a poorly insulated [or leaky ] building, the temperature differential between the ambient air and an outer wall may be 5°C (9°F); for an ambient air temperature of 20°C (68°F) and a wall temperature of 15°C (59°F) this could mean a difference between 60% and 80%, a difference between a relative humidity that reportedly would not support mold growth and one that would. Where the temperature of a surface is at or below the dew point, water condensing on that surface will allow germination and mold growth on it, irrespective of the ambient humidity. Where moist air permeates a porous material like concrete, brick or gypsum, condensation may be interstitial. This can then act as a reservoir of water which will permit mold growth to continue under ambient conditions that would have dried the surface and prevented growth.
The widespread practice of shutting down ventilation systems during unoccupied hours should be stopped. Instead buildings should continually be slightly pressurized to prevent infiltration of moist air and radon. Some dehumidifiers can help you control humidity and provide necessary ventilation at the same time.
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Large quantities of water vapor and soil gasses can enter through the lower surfaces of crawl spaces. Here are a few suggestions:
May 18, 2005
ATLANTA – Due to the proliferation of mold in buildings, sound moisture management should take precedence over energy cost savings, according to a new position document from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).
Energy conservation goals may conflict with moisture management goals. In fact, traditional methods of dehumidification, such as reheat systems, may increase energy use, Ron Vallort, ASHRAE president, said.
“Considering energy conservation and moisture management goals in the design, construction, operation and maintenance of HVAC systems can minimize energy use and cost,” he said. “However, the impact of mold proliferation suggests that energy cost savings should not be achieved at the expense of sound moisture management.”
Minimizing Indoor Mold Through Management of Moisture in Building Systems outlines ASHRAE’s position on the management of moisture in buildings by describing issues related to the topic and highlighting resources available through the Society regarding the management of moisture and mold in buildings.
The document recommends that for proper moisture management include:
•
Building envelopes, penetrations and building systems be designed and built to protect the indoor environment and the building materials from water infiltration or accumulation.•
Building and system design consider internal or exterior moisture that could cause condensation on surfaces or within materials.•
Building and system design, operation and maintenance provide for drying of surfaces and materials prone to moisture accumulation under normal operating conditions.•
Building and system design, operation and maintenance provide for water management of surfaces and materials that are expected to have moisture present.•
Mechanical system design should properly address ventilation air.•
Building and system design, construction and operation take into account occupant uses.•
An operation and maintenance plan for each building.•
The sequence of operation for the HVAC system contain appropriate provisions to manage humidity, control air pressure and monitor critical conditions.•
Moisture accumulation be investigated in a timely manner and steps be taken to identify and control the course of water.The position document, Minimizing Indoor Mold Through Management of
Moisture in Building Systems, can be downloaded for free at:
http://membership.ashrae.org/template/AssetDetail/assetid/43903
The position document is also available via the “position documents” shortcut on ASHRAE.org
ASHRAE, founded in 1894, is an international organization of 55,000 persons. Its sole objective is to advance through research, standards writing, publishing and continuing education the arts and sciences of heating, ventilation, air conditioning and refrigeration to serve the evolving needs of the public.
Contact: Jodi Dunlop
Public Relations
678-539-1140
jdunlop@ashrae.org
ASHRAE
1791 Tullie Circle NE
Atlanta, GA 30329
Basement wall drainage alternatives employing exterior insulation
The following manufacturers make ERV systems that can help you to prevent excessive indoor humidity during warm weather and assure adequate indoor humidity during cold weather:
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NOTE: Good drainage of rainwater away from foundation walls is important. If water in the soil near a foundation freezes, it can sometimes create enough pressure to crack foundation walls. While tremendously strong, even the best concrete is porous. Water vapor can move through it, causing dank musty smells...rust and condensation...damage to mechanical equipment...cracked plaster...chipped paint, efflorescence, etc. Up to 80% of the moisture entering some structures is from the soil; moving into the structure both as a liquid (capillarity) and as a gas (vapor). One way to eliminate the costly problems of excessive moisture migration is to completely isolate the structure from the soil during original construction by installation of a barrier that is both water-proof and vapor-proof. |
