The Mold Equation: What Causes Mold Growth?

Molds reproduce by producing and emitting mold spores into the air. These mold spores can travel anywhere air goes. So that means that mold spores are everywhere doesn’t it?

It sure does.

So what determines whether mold will actually start to grow once a mold spores touches down? It all depends on a few simple factors that I like to call “the mold equation”.

 

The mold equation

Mold only requires a few things to grow. We’ve entered them into this very simple equation:

Mold spores + Temperature + Food source + Moisture = Mold growth

 

Seems simple enough, right? But how does this help us?

If all four of these factors are present, it’s very likely that you’re going to see mold growth. But maybe we can figure out a way to change the equation that will eliminate the risk for mold growth. Let’s take a look.

Mold spores

As we just mentioned before, once mold spores become airborne they can go just about anywhere. They can travel indoors on people’s clothes, or through an open window and even through HVAC air intakes. Mold spores can even make their way into otherwise sterile environments by traveling through air ducts or non-pressurized spaces. In other words, while we can try to control the presence of mold spores, any measures we take will not be 100 percent effective. So we have to assume that mold spores are a constant in the equation.

Temperature

Molds need an environment between 40 and 100 degrees Fahrenheit to grow, so human-occupied buildings provide a very comfortable environment for mold growth. Unfortunately, using temperature to control mold growth in buildings is impractical because we, as humans, need to keep indoor temperatures between about 65 and 75 degrees Fahrenheit to be comfortable. So temperature acts as a second constant in our mold equation.

Food source

Molds use organic materials as a food source. Unfortunately, this includes just about every building material you can imagine. If so mold spores are present indoors, they’re going to be able to find a never-ending supply of nutrients. Once again, we have a constant in the mold equation.

Moisture

Mold needs moisture to survive. That could mean liquid water from faucets, water-damaged building materials from a leak, or high-humidity indoor humidity levels. Moisture sources and humidity levels in a building can vary greatly depending on the temperature, weather, ventilation, and location and use of the area in question. But generally, indoor moisture is something that we have control over. That means that moisture is the one factor in the mold equation that will determine whether or not there is mold growth.

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In sum, the solution to controlling mold growth is moisture control.

 

What causes mold growth?

We already know that mold needs a moisture and a food source to survive. But what other factors encourage mold growth? Here a just a few of the ideal conditions for mold growth:

Warm temperatures – Molds grow enthusiastically in warm environments, with temperatures between 70 and 90 degrees Fahrenheit as the sweet spot.

High humidity – Even if liquid water is not available, mold can get the moisture it needs from water vapor in the air (also known as humidity). Indoor relative humidity levels 70 percent and above are a major stimulant for mold growth.

Condensation – Condensation forms because of differences between air temperature and surface temperatures in a space. This can become a problem when condensation forms regularly on moisture-sensitive materials that can absorb the moisture and create an ideal environment for mold growth. Condensation inside wall cavities is especially common in very warm or cold climates, where extremely hot or cold air regularly meets cooled or heated surfaces.

Inadequate ventilation – Less air movement means less drying, inadequate humidity control and fewer disturbances to mold growth.

Damp building materials – Building materials can become damp for a variety of reasons. In some cases, building materials are not properly dried after a water event. Sometimes, regular moisture infiltration or water leaks prevent materials from drying completely.  In other cases, high humidity and condensation can cause building materials to absorb and retain moisture from the air. And in the most unthinkable of circumstances, wet building materials are sometimes enclosed before they are completely dry, leading to trapped moisture and prolonged systematic dampness. In any case, damp building materials beg for mold growth because they provide both food and a moisture source in one package.

 

Where does mold grow?

We finally know what circumstances cultivate mold growth. But where is mold most likely to grow?

Mold can grow in just about any indoor space, but areas that feature any of the ideal conditions mentioned above will have greater risk for mold growth. For example, some building materials tend to trap or absorb moisture more than others. Other building areas have higher humidity, less ventilation or are more prone of condensation. Here are a few places where all the stars align to create ideal environments for mold growth:

• Areas of water leaks or moisture damage
• On or around window frames
• On walls
• Behind wallpaper
• Behind bubbling or blistering paint
•  On paper or wood products
• On carpet or the back of carpet
• Underneath floor tiles or other laminate flooring
• On ceiling tiles (water stained)
• In the space above drop ceilings
• In attics or basements
• Inside exterior walls

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Additionally, it’s important to note that exterior corners are especially prone to mold growth due to the fact that these areas tend to have poor air circulation and limited insulation, provide a greater surface area of heat loss, and suffer from exterior wind-washing.

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