What Causes Mold to Grow in Homes?

The single most important thing to know about mold growth, whether in a house or commercial building, is that excessive moisture is primarily the culprit of what causes mold to grow.  However, below are other factors along with moisture that can contribute to a mold problem.

For mold to grow in a building, it requires:

  1. A mold spore that can germinate and grow. Since microbial spores are everywhere, inside and outside, in the air and on surfaces, it can be assumed they are always present.
  2. Food source. This can be a material itself, such as wood, paper or a paint film, or it can be a dust film on a non-food material such as metal or glass.  Since even a very slight dust film can support mold growth, a food source is also pretty much always present.
  3. Moisture. In sufficient quantity, this can be either liquid water, or humidity at a high enough level. This article discusses the different patterns of mold growth created by liquid water as compared to humidity.
  4. Time. In other words, with all these factors in place, if the moisture isn’t present long enough, germination fails or significant growth doesn’t occur.

what causes mold to growMold grows in different patterns and has different effects on the environment. This depends on whether the necessary moisture was provided by the following:  liquid water or humidity.  To be sure, the two sources often overlap.  For instance, when relative humidity (RH) reaches 100%, it begins to condense onto surfaces, forming liquid water.  Similarly, liquid water evaporates into the air and then raises the humidity. In a confined space, the percentage can be high enough for the humidity as well as the liquid water to contribute to microbial growth.

Liquid water and mold growth

Liquid water usually enters a building by a building envelope or plumbing leak, condensation associated with HVAC systems, or from a variety of less common causes.  It then tends to soak into porous materials such as drywall and, much more slowly, into semi-porous materials such as wood framing.

mold growth building envelopeIf discovered and dried quickly enough by a restoration professional, almost all materials can be dried without mold growth and many without physical damage that would require replacement.  If not dried quickly enough, microbial growth will likely begin in as little as 48 hours and may be well established by a couple of days after that.

Gravity directly affects liquid water, so mold growth from this cause tends to be concentrated around the base of walls, especially behind baseboards, inside wall cavities and under and behind cabinetry and other built-ins.  In these locations, it cannot evaporate as easily as it can from the surface of a wall exposed to room air.  Thereby, what causes mold to grow here is the material in the confined space that remains wet/damp longer, and also the growth is likely to be more extensive.

Mold growth, because of contact with liquid water, tends to grow in a dense pattern.  It also is more likely to be of varieties, such as Stachybotrys and Chaetomium, that require very wet conditions for an extended period.  Many such species are particularly problematic for the health of the occupants of the structure.

High humidity and mold growth

The humidity in question is the relative humidity (RH) on the surface, which may be quite different from the measured RH in the room air.  For instance, on an exterior wall during cold weather, a cool, high RH microclimate often forms behind a dresser or behind window treatments.

mold growth activityMold species vary greatly in the moisture they require to grow.  (See Fact Sheet: Moisture Requirements for Mold Growth.) Each species has a “band” of moisture availability (water activity) within which it can grow.  The very lowest is 0.61 water activity, which equates to 61% RH on the surface.  Hence, maintaining RH below 60% is an excellent way to prevent mold growth.  However, it is probable that all mold species require water activity well up into their “band” of growth to germinate.  Once established or colonized, they may be able to continue growth at water activity levels towards the bottom of the band.  The chart shows water activity requirements for various mold genera.  Individual mold species within each genus will normally be able to germinate only within a narrower band.  What causes mold to grow and which species in a location depends on water activity, food source, temperature and other factors.

moldy shoesHigh humidity, unless localized to a microclimate, such as on an exterior wall during cold weather, around an AC supply vent, or inside a wall, affects the entire structure and everything inside it.  This includes inside cabinetry, furniture and even inside upholstered items such as sofas and mattresses.  Anywhere there is a spore, the humidity is high enough for long enough, and a food source is present (which is basically everywhere, since growth can occur even in a very thin dust film), mold growth can occur.

What this means is that in a building, if there is high humidity for an extended period, mold growth can be present on almost any surface.  Since mold organisms are microscopic in size and only become visible to the naked eye when a great many of them are clumped together, such mold growth is often sub-visible, and can only be detected by microscopic analysis of samples taken from the surface.

See Micrographs of Common Species in our Mold Glossary

The species of mold that grow under these conditions are more likely to be those associated with a tolerance for somewhat lower water activities.  They often colonize more rapidly than molds requiring high water activity.  Unfortunately, many of them also have negative health effects for people.

The two patterns of mold growth

Microbial growth from liquid water is usually denser and more concentrated but is often limited to relatively small surface areas where the water originated or accumulated.

Mold growth from high humidity, on the other hand, may be more dispersed and seldom as heavy at any given point.  However, because almost any surface may be supporting the growth, the surface area affected may be hundreds of time greater, and thus the total amount of mold particles produced may be greater than in a building with a small area of heavy growth caused by contact with liquid water.  This can lead to a potentially higher concentration in the air and on surfaces to which occupants can be exposed.

As a result of liquid water, microbial growth is generally obvious to the naked eye, assuming it’s not hidden behind materials or inside wall cavities.  Determining the extent and severity of contamination with mold growth caused by high humidity, however, will generally require sampling and microscopic analysis by a mold expert.

 

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