mold spores

Where does Mold Grow

Posted on November 19, 2012 · Posted in Uncategorized

Where Does Mold Grow?

Mold spores may be found lying dormant on almost every surface in a building. Unless large numbers of spores become airborne, there is usually little problem. However, when mold spores are on a surface with an appropriate moisture content, nutrients, and temperature, the spores will germinate and mold will grow. The key to identifying locations where mold is likely to grow is finding where these conditions exist, have occurred, or are likely to develop.

Mold should not grow indoors unless there are moisture problems in the building. Obvious causes of moisture problems include occupant-generated sources, floods, roof leaks, and problems with drainage or plumbing. A less obvious source of moisture is the effect of temperature gradients (temperature differences), especially in locations where relatively warm and moist air comes in contact with relatively cool surfaces. These conditions can cause water vapor to condense on building surfaces, just as it does on a glass of ice water on a warm, humid day.

Most molds must get their food from the environment, living and feeding on dead organic matter. Outdoors, molds are very important in decomposing organic materials and recycling nutrients. Indoors, many building components and contents contain materials that are excellent food sources for mold, such as wallpaper glue, some paints, greases, paper, textiles, and wood products. Indoor dusts may contain fibers, dead skin cells, and other organic matter that can serve as a food source for mold when adequate moisture is available.

Temperature also affects mold growth. Different types of mold have minimum, optimum and maximum temperature ranges for growth. Many fungi grow well at temperatures between 60 and 80 degrees Fahrenheit, which are also ideal temperatures for human comfort. In addition, as mentioned above, temperature gradients often produce the moisture needed for mold growth.

In the summer, when air-conditioning is in use, mold growth can occur in buildings where the cooling systems are oversized, undersized or poorly maintained. Unplanned air flow in buildings can also create conditions favorable to mold growth. A competent heating and air conditioning contractor should be able to address these issues.

In the winter, when buildings are heated, mold often grows in cold, uninsulated exterior windows and walls, including uninsulated closets along exterior walls where building surfaces are generally cold relative to the indoor air temperature.

Conditions That Promote Mold Growth

All of the conditions needed for mold growth (food sources and appropriate temperatures) are present in the indoor environment with the exception of adequate moisture. Prevention of mold growth indoors can not be achieved without proper moisture control. The following are some of the moisture problems that cause indoor mold growth.

Water Intrusion

Rainwater can enter a building through leaks in walls, windows or the roof. Surface or ground water may enter when there is poor foundation drainage. Flooding can, of course, cause catastrophic intrusion. In buildings that have slab construction, water can seep or wick up through the cement floor causing mold to grow on carpet pads or carpet backing. The building envelope (walls, windows, floors , roof , etc.) must be well maintained to prevent water from coming in, both to prevent mold growth and to maintain the structural integrity of the building.

Water Vapor

When relative humidity (a temperature-dependent measure of water vapor in air) becomes elevated indoors, building materials and furnishings absorb the moisture. Those damp materials can then provide a good place for mold to grow. If there are no cold condensing surfaces and the relative humidity (RH) is maintained below 60 percent indoors, there will not be enough water in those materials for mold to grow. However, if the RH stays above 70 percent indoors for extended periods of time, mold will almost certainly grow.

In the summer, air conditioning can de-humidify indoor space. But if the system is too large or too small for the space it serves, the cooling system can create high humidity by cooling without removing water vapor. A properly sized and maintained system will dehumidify and cool a building.

When there are cold surfaces in a building, water vapor can condense on those surfaces, just as water condenses on the outside of a glass of ice water. Insulation of exterior walls can prevent condensation and mold growth during the winter.

You should always be mindful of indoor sources of water vapor that can be problematic. Clothes dryers must be vented to the outdoors. Unvented gas or kerosene space heaters can generate enormous amounts of water vapor (as well as other air contaminants), and should be used sparingly and never as a primary heat source. Always run the bathroom exhaust fan when showering or bathing, and make sure the vent is exhausted to outdoors. A properly vented kitchen exhaust fan can remove steam created during cooking.

The first step in addressing any mold growth problem in a building is identifying and correcting moisture source(s) (see Where Does Mold Grow?). If moisture problems are not corrected, then any mold cleanup or removal that takes place will most likely be only a short-term solution; at some point the mold growth will recur. It is critical to control moisture at the beginning, during, and at the end of a mold growth removal project.

One of the most common misconceptions about mold is that it can be removed by spraying the surfaces with products such as disinfectants, biocides or cleaners. That will not take care of the problem because the allergenic and toxic properties of mold are not removed by using such products. Whether viable (living) or nonviable (dead), mold spores and other parts of the mold, when they get into the air, still present a health risk to exposed individuals.

While disinfectants and biocides may kill mold spores and take away their ability to reproduce, these products should not be used alone in addressing a mold growth problem. Either the mold must be completely removed from the affected material, or the mold-contaminated material must be completely removed from the building.

In determining which materials can be cleaned and what should be removed, the two important factors are how porous (absorbent) the material is and how extensive the mold growth is. Generally, non-porous materials (such as metals, glass and hard plastics) and semi-porous materials (wood, plaster and concrete) that are visibly moldy but structurally sound can usually be cleaned and reused. Moldy porous materials (carpeting, wallboard, ceiling tile, wallpaper, fabric, upholstered furniture, mattresses) should usually be discarded, since they absorb and hold moisture, may be internally moldy, and cannot be completely cleaned and thoroughly dried.

Cleanup and mold removal activities can expose people to mold particles and other hazards, so it is important to wear protective equipment and follow procedures safely. For complete instructions, see:

Air duct systems in buildings can also become contaminated with mold. Air duct systems can be constructed of bare sheet metal, sheet metal with exterior or interior fibrous glass insulation, or made of entirely out of fibrous glass (ductboard). If mold growth has occurred on fibrous glass or other porous surfaces, then effective cleaning will not be possible and the ductwork and/or insulation will need to be discarded. Mold growth on metal ductwork may be cleaned and disinfected following the instructions for non-porous materials. For additional details on addressing air duct cleaning see Should You Have the Air Ducts in Your Home Cleaned?, a publication of the Environmental Protection Agency (EPA).

Cleaning Moldy Non-porous
and Semi-porous Materials


When cleaning non-porous (metals, glass, hard plastic) and semi-porous (wood, plaster, concrete) materials, follow these steps:

  • Wear protective equipment and follow safety procedures (see Health Precautions During Mold Cleanup and Removal).
  • First, wet the moldy area down by spraying lightly with a water-and-detergent solution to help keep mold dust and spores from getting into the air. A high-efficiency particulate air-filtered (HEPA) vacuum cleaner can also be used to help in removing surface mold growth and removing the spores from nearby materials.
  • Using a non-ammonia soap or detergent and hot water or commercial cleaner, thoroughly scrub all moldy surfaces.
  • Rinse the scrubbed surfaces with clean water. A wet-dry vacuum may be used to collect excessive water.
  • After cleaning, a disinfectant solution such as household bleach and water (¼ to 1½ cup liquid chlorine bleach to one gallon of water), can be applied to the affected surface. Never mix bleach with ammonia – toxic gases can be created. Follow all label directions on all products used in this step. Make sure the area is well-ventilated when using disinfectant solutions. For this step to be most effective, the disinfectant solution should be allowed to stay on the surface for 6 to 8 hours and the solution should be allowed to dry naturally.
  • After cleaning and disinfecting, the affected surfaces should be dried out as quickly as possible. The use of fans and dehumidifiers may speed up this process. If materials are not dried properly, the mold is very likely to re-grow. Moisture levels in wood should be less than 12 to15 percent prior to rebuilding, painting, etc.



Nestor Quaas

President & Ceo.

QC Cleaning LLC

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