How to prevent mold and mildew

How to prevent Mold and Mildew

Mold and Mildew

How to reduce or eliminate the risk of mold in the Southeast USA and other areas of the world with the same type of hot humid climate.

Mold prevention measures are not difficult or costly; however, the reality is that once you have a mold problem, it then becomes costly and difficult to remedy.

You cannot reliably control mold and mildew growth using only the air conditioning (cooling) system. To prevent mold and mildew growth the indoor RH (relative humidity) should not exceed 55%. In hot weather the air conditioning system does this fairly well. However the highest relative humidity in the home is likely to occur during mild weather when your air conditioner is not needed much during the day. In mild, cool but damp weather the cooling system is on very little or not at all; this is the most likely weather to promote mold growth. In Florida, the outdoor RH reaches nearly 100% at night, regardless of daytime temperatures. This moisture needs to be removed from the home during the day (while the temperature is higher) by the air conditioner or at night with a dehumidifier. By far, the best method is to use a dehumidifier. For best results use a dehumidifier with a humidistat control, a timer and a permanent drain.

Preventing mold by lowering Relative Humidity while maintaining a comfortable environment and reducing the cost of cooling.

Thermostat Settings for hot humid weather:

  • Every Fahrenheit degree increase in your thermostat setting decreases air conditioning costs by about 10%.
  • Set the thermostat to the highest temperature that is comfortable for you.
  • A temperature of 78 degrees Fahrenheit or greater is recommended.
  • Set the thermostat to the “cool” mode and not the “auto” mode (auto / heat / cool).
  • Set the fan mode of operation to the “auto” mode not the “on” (continuous) mode.
  • Do not try to control mold by lowering the thermostat setting as this will not work.

Never leave outside doors or windows open in hot, humid weather while running the air conditioner as this not only fosters mold growth, but will also waste electricity and possibly damage the AC.

Ceiling fans can keep you comfortable while saving money.

Change air filters regularly and get professional service once a year to clean the coils.

Make sure sprinkler systems do not hit exterior walls of the building.

Route bathroom exhaust fans and dryer vents outside of your home, not into some other part of the home (garage, attic, utility room, etc).

Minimize live indoor house plants.

Eliminate any and all water leaks and standing water in or under your home. For example: water heater overflow pans, air conditioner overflow pans, drips under sinks or other hidden places.

The Vacant Home

If a home is to be vacant for an extended period of time then install a dehumidifier controlled by a humidistat. This is the best and most cost-effective solution to the high relative humidity and mold problem.

If you are going to leave the heating or cooling system on in a vacant home set the thermostat fan selection switch to “auto” in order to save energy and avoid clogging the air filter. Open all interior doors to allow unrestricted air circulation.           Keep all exterior doors and windows closed.

Mold can be eliminated effectively in most cases.

There are four elements needed to create an optimal environment 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. Mold is basically everywhere so negating the availability of mold spores is impossible. Mold spores eat anything but especially love paper, wood and other organic fibers (the main items found in a living environment), so reducing their food source is highly unlikely. Regarding temperatures, mold grows in our refrigerators as well as hot tropical climates. So the comfortable living temperature that humans enjoy is also an excellent environment for mold growth. Moisture level (relative humidity) is the component that can most easily be controlled and is the factor we will focus on in our effort to control the growth of mold.

Air Conditioner Fan Mode of Operation.

Keep the air conditioner fan switch set to “auto” mode instead of the “on” mode. When set to the “on” position the blower fan runs continuously and the moisture which has condensed on the evaporator coil during the cooling cycle is re-evaporated and blown back into the home before it can drain off the coil and out of the home. This will cause the RH in your home to be significantly greater than using the “auto” mode.

Match the AC capacity to the house size.

Air conditioner units are commonly oversized. The larger and more oversized the unit is, the poorer it will be at removing humidity. This is because, during each air conditioning cooling cycle, moisture removal does not reach full effective capacity during the first three minutes of operation. The larger oversized system has a shorter on cooling time during which interior humidity is removed.

Do not try to control mold by lowering thermostat setting as this will not work.

Two things result from lowering the temperature in the home, first relative humidity increases when the temperature decreases. Second, lowering the temperature decreases the temperature of the materials in the walls, floors and ceilings of the home, thereby significantly increasing the potential for actual moisture condensation on these items. A side benefit of setting your thermostat at higher temperatures is that it significantly decreases cooling energy costs. This is exactly the news you were hoping to hear.
This Chart Shows the Impact of the Thermostat Set-Point Temperature and The Fan Mode of Operation on Interior Relative Humidity.

Indoor Temp degrees F Avg RH % Fan= Auto Compressor Run Time Fraction Avg RH% Fan=On
80 55% 0.27 65%
77 60% 0.35 70%
76 67% 0.60 70%
71 74% 0.80 75%

The AC was operated alternatively for one week in each mode. Based on experiments conducted by The Florida Solar Energy Center.

If you would like to learn more about Mold and Mildew prevention and control, click here to download the full PDF version.

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Filed under: FAQ's,Going Green