Mold Control On a Budget

Mold grows if spores are present with food (organic matter) at the right temperature and adequate moisture. As pointed out by the EPA, “The key to mold control is moisture control.” Filtration cannot remove all the spores. Mold does well at classroom temperatures. Since mold eats organic matter, there’s always plenty of food available at a school.

Controlling moisture falls into a couple of categories.

1. Water Intrusion: Leaks need to be repaired, and the wet area must be dried quickly.

2. Condensation & High Humidity: Unless a school is located in an arid climate, there are times of the year that there is enough moisture in the air to grow mold.

Controlling moisture in the air means ideally maintaining relative humidity below 50% according to the EPA. Since air conditioning only does dehumidification as a byproduct of cooling, it will not maintain the desired humidity levels on a day-to-day basis without ‘reheat.’ Reheat is simply a method of adding heat to an area to make the air conditioner run more. This is quite energy-intensive. Electric reheat removes only about 6/10 lb of water per kWh. The only other option is some kind of dehumidifier. The most efficient units in the industry — manufactured by Therma-Stor — remove about 5 lbs of water per kWh.

During humid periods, schools can see massive amounts of humidity from ventilation air and internal moisture loads from students and staff. For example, when the outside air has a 70°F dew point (a moderately humid day), a classroom with 30 students (465 cfm) has to remove about two gallons of water per hour to maintain 50% RH. If it’s cloudy/rainy, overcast weather, the air conditioner would run very little, or not at all. A few days of this kind of weather means conditions are perfect for mold growth.

Is it ever 70°F and rainy for 3+ days where you live? If so, then even the most ‘perfect’ building with a ‘perfect’ HVAC system will have ‘perfect’ conditions for mold growth.

If mold growth is unacceptable, then a school district must be prepared to deal with humidity under these conditions, when the A/C unit alone does little or no dehumidification. This would be a major financial commitment for the equipment, and/or the operational and maintenance costs.

There is a cost-effective alternative that effectively keeps mold from growing —mold control on a budget. Since mold growth needs 24–48 hours of high humidity, interrupting the cycle by controlling humidity during unoccupied times is a cost-effective strategy to keep mold dormant.

On a warm, rainy day, the ventilated, occupied classroom has a water load of 16–17 lbs. of water per hour. When the students go home and the ventilation system shuts down, the water load drops by about 90%! A modest dehumidifier can ‘dry down’ the space during the unoccupied time and thus break the mold growth cycle. The cost of purchasing and operating dehumidifiers during unoccupied times is minimal vs. attacking humidity loads at their peak. Shutting off the ventilation and A/C units during these unoccupied times will have substantial energy savings.


As stated by the EPA, “the key to mold control is moisture control.”

Water intrusion from leaks must be dealt with promptly.

Another source of moisture for mold growth is high humidity levels due to high moisture content in the air. Since an A/C unit alone does not always operate enough to remove excess humidity, either ‘reheat’ or a dehumidifier is necessary to assure humidity control. “Reheat” is expensive to operate — electric reheat removes about 6/10 lb. of water per kWh vs. 5 lbs per kWh with the most energy efficient dehumidifiers.

Controlling humidity during the most challenging periods is tremendously expensive.

Since mold needs 24–48 hours of high humidity to reproduce, short periods of high humidity are tolerated. “Drying down” each evening and on the weekends assures no mold growth at a modest cost—mold control on a budget.

Therma-Stor manufactures the most energy-efficient dehumidifiers in the industry and offers free-standing as well as ducted models.

An Ultra Aire 155H whole house ventilating dehumidifier is typically ducted to 4 classrooms. Depending on the design of the A/C, the 155H can be installed with a separate supply and return to each room, or it could be tied into existing ductwork.

As a rule of thumb, the Ultra Aire handles about 3,500 sq. ft.

The maximum energy consumption would be 7 KwH per classroom per day. The net energy costs would be minimal since the A/C units would be shut down during unoccupied times. In fact, there could be significant energy savings if A/C units could be kept off for long periods such as during the summer.

Therma-Stor also offers a powerful, quiet, free-standing unit — the Santa Fe Rx. Although the Santa Fe Rx draws only 5.4 amps, it removes about 74 pints of water per day. Simply plug it in, run the hose from the built-in condensate pump to a drain, and set the dehumidistat. The unit is quiet enough to place in a classroom, media centers, etc. Since it has the options of a HEPA and/or charcoal filtration, it is commonly used to deal with indoor air quality complaints from specific individuals.

Since the Santa Fe Rx typically handles up to 2,200 sq. ft., a “fleet” of units can be located throughout the school for “mold control on a budget.” Units can be either temporarily or permanently installed throughout the structure. Since moisture readily equalizes through the air, locating the Santa Fe units in hallways is acceptable, as long as interior doors are left open.

Therma-Stor also distributes the HOBO® Data Logger, which records temperature, relative humidity and dew points. The latest data loggers can make a school district proactive in dealing with mold growth. A high humidity alarm can trigger an auto dialer. A district has a few hours to act to control humidity to avoid mold growth conditions. This could very likely mean mobilizing a fleet of dehumidifiers. The cost of preventing mold growth is far less than dealing with the aftermath.