Air & Water Sciences - Environmental Consultants
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Indoor Air Sciences Masthead

Air& Water Sciences

WHAT CAUSES MOLD?

Microbial growth can be found almost anywhere and will grow on virtually any organic substance as long as moisture is present. Molds can grow on wood, paper, carpet, food and insulation. Leaking appliances, wet drywall and wet crawlspaces contribute to the ideal conditions for growth. When excessive moisture accumulates in buildings or on building materials, mold growth will often occur, particularly if the moisture problem remains undiscovered or unaddressed. It is impossible to eliminate all mold and mold spores from the indoor environment however.

Mold growth is a naturally occurring process that provides for the breakdown of organic matter and returns it as nutrients to the earth. Mold growth in your home or building is just that, deterioration of the building. MOLD DOES NOT BELONG IN BUILDINGS!


mold patch on ceiling


Mold spores are essentially everywhere. Mold can appear in humid locations and in certain parts of your home where moisture exists such as bathrooms, laundry rooms, basements and crawlspaces. You may also find mold growth on moist surfaces or where there has been flooding, leaking pipes or roofs, standing water, poor drainage or places where water is collected (i.e. dehumidifiers, air condition drip trays and refrigerator collection pans).

In the summer when air conditioners are in use, mold growth can occur in and around vents and duct work. In the winter when buildings are heated, mold often grows in wall spaces between the indoor air and the cold outdoor air. Ideal conditions for mold growth are inside exterior windows and walls where one side of the wall is colder than the other, especially uninsulated closet walls along exterior walls. Water vapor can condense on building surfaces just as it does on the outside of a glass of ice water on a warm, humid day.

Water Intrusion

Rainwater can enter a building through leaks in siding, walls, windows or the roof. Surface or ground water may enter when there is poor foundation drainage. 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.

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.

Damp Concrete

Wet or damp concrete slabs often contribute to indoor mold problems. Concrete absorbs water like a sponge. Wet concrete in and of itself, is not a problem. However, construction materials that are attached to wet concrete, such as wood framing, wood flooring, carpet, pad, tack strip, cabinets, etc., can all absorb the moisture from a concrete slab, causing those materials to deteriorate over time and establishing the ideal conditions and food sources for accelerated mold growth.

Other sources of Mold:

Cladosporium On Wall
  • Sprinkler spray hitting the house
  • Damp basement or crawl space
  • Swamp coolers/sump pumps
  • Building Construction defects
  • Overflow from sinks and sewers
  • Unventilated bathrooms and kitchens
  • Leaking shower valves and appliances

Links for more information:

EPA - Molds & Moisture
CDC - Mold
Calif Dept of Public Health - Indoor Mold

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