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  • Writer's pictureCorey Provencal

Mold Humidity Bloom

Florida Humidity Bloom Florida Indoor Air Quality Solutions IAQS

Florida Humidity Bloom Florida Indoor Air Quality Solutions IAQS

A humidity bloom is the growth of mold within a living space due to the elevation of indoor relative humidity at or above 60 percent relative humidity for a period of time greater than 72 hours and must be maintained for growth to continue.  When the humidity is above 60 percent relative humidity, molds will germinate causing what is referred to as a mold bloom.   These molds can bloom in many colors and are often confused with dust, dirt, foxing, or cobwebs.

Humidity for an extended period of time, typically in excess of 72 hours to initiate the bloom and must be maintained for growth to continue.  When the humidity is above 60 percent relative humidity, molds will germinate causing what is referred to as a mold bloom.   These molds can bloom in many colors and are often confused with dust, dirt, foxing, ghosting, or cobwebs.

Both active and inactive mold can have a distinctive smell, which most people describe as musty.

  1. Active mold in the early stages of a bloom has hair-like filaments in webs, which develop a more bushy appearance as the bloom matures. This is more easily seen under magnification. Active mold is soft and may smear when touched with a fine brush. It may also be slimy and damp.

  2. Inactive mold is dry and powdery and will seem to brush off materials readily.

Mold and mildew are words that refer to more than 100,000 species of fungi. Mold spores are present everywhere in our environment, generally in a dormant state where they do little damage. Spores require moisture to become active. They do not require light.

When water or high relative humidity provides the necessary moisture, dormant spores will germinate, grow fine web-like structures, and eventually produce fruiting bodies that release more spores. Most molds will germinate at 60 percent relative humidity. Increases in temperature can speed the growth rate of active mold.

There are 4 critical requirements for mold growth – available mold spores, available mold food, appropriate temperatures and considerable moisture. The removal of any one of these items will prohibit mold growth.

The only way to reduce the threat of mold in a home is to maintain an environment that is not hospitable for the germination of mold spores. The temperature should be 74-78 degrees F, and the relative humidity of 60% or less. It is important that the air conditioning system (HVAC) be kept on 24 hours a day, 7 days a week. Inconsistent operation or fluctuations in the temperature and humidity are the cause of many serious mold outbreaks.


Humidity is simply vaporized water in the air. Your breath contains hundreds of droplets of invisible water vapor. You can see them when you breathe on a pair of cold glasses.

The term most often used to define the amount of water vapor in the air is “relative humidity.” Relative humidity is the percentage of water vapor in the air at a specific temperature, compared to the amount of water vapor the air is capable of holding at that temperature. Warm air holds more water vapor than cold air. When air at a certain temperature contains all the water vapor it can hold at that temperature, its relative humidity is 100 percent. If it contains only half the water vapor it is capable of holding at that temperature, the relative humidity is 50 percent.

If the outside air temperature in winter is 0°F and the relative humidity is 75 percent, that same air inside your 70°F home will have a four percent relative humidity. The Sahara Desert has an average relative humidity of 25 percent.

When air is saturated with water vapor, it has reached the dew point; at this point, water vapor condenses and produces visible water or “condensation.” In winter it usually occurs first on windows. When warm, moist air comes in contact with a cold window, air temperature drops and it can no longer hold the water vapor; condensation results.

Desirable Humidity Levels

The human body is comfortable when relative humidity ranges between 20 and 60 percent. In your home, an average relative humidity of 35 to 40 percent is appropriate when the outside temperature is 20°F or above. However, during cold weather, higher humidity ranges may cause structural damage because of condensation on windows and on the inside of exterior walls. As outdoor temperatures fall, condensation problems inside may develop.

The construction of a home also influences how much humidity is desirable. Tightly constructed buildings with properly installed vapor barriers and tight fitting doors and windows retain more heat and moisture. This is where mechanical ventilation becomes important. If a home does not have the proper mechanical ventilation, excess water vapor can move through walls and ceilings, causing wet insulation, peeling paint, and mold on walls and woodwork.

The following table shows recommended indoor humidity levels in relation to outdoor temperatures.

Outside temperature (0°F)

Recommended relative humidity

+20° and above

35% to 40%








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 (VOC’s). 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 is, 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.

Mold spores are everywhere all the time, entering from outdoor air as well as on pets and clothing. A mold spore landing on an indoor surface is likely to be insignificant and amount to little more than a common component of indoor dust, until such a mold spore lands on a moist organic surface (such as drywall.) High indoor humidity causes the surface moisture level to be sufficient for mold sporulation.  Since a mold spore requires moisture to propagate and grow, the indoor humidity level is a key gating factor in the control of indoor mold (and dust mites) in buildings.

Certain common mold genera and species, such as some members of the Aspergillus sp. and others grow readily on common building materials if they also have enough moisture. While there are fungal species that are able to grow under a remarkably wide range of environmental conditions, keeping indoor humidity at the appropriate level will reduce the chances of growth of the most common indoor problem molds.

High indoor humidity can encourage more issues than indoor mold. The same moisture conditions that support growth of problematic indoor molds also encourage the development of bacterial hazards, dust mite populations, mite fecal allergen problem, and possibly other insect problems in buildings.

The same measures of humidity control to prevent mold growth are needed to discourage the dust mite population that exists in all living areas. Measures including choosing and maintaining the proper humidity level to avoid indoor mold will also work to minimize the level of dust mites and dust mite allergens.

Keep the indoor humidity level in the mid-comfort range. A maximum indoor relative humidity of 50% RH may be acceptable, 45% RH better. At 60% indoor RH, we’re entering the indoor mold-formation risk zone of high interior moisture in building wall or ceiling cavities or on wall and floor surfaces, possibly conducive to mold growth.

John Lapotaire, CIEC

Indoor Air Quality Solutions, IAQS

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