Preventing Condensation and Mold Growth on Window Surfaces

When weather is cool but humid for extended periods, the AC does not run for long (if at all), and there is no way to remove internally-generated humidity. Humidity can build up to the point where it begins to cause not only condensation, but extensive mold growth on window surfaces and elsewhere.

In extreme cases, this can result in widespread heavy mold growth, creating a “fully-involved” heavily contaminated environment requiring extensive remediation. Extensive condensation on or around windows should be considered a warning sign that such conditions may be developing.  Inside relative humidity should not exceed 60% for more than brief periods.  Please contact IET if you have questions.

There are three main approaches to preventing condensation and mold growth on window surfaces like glass, metal and adjacent surfaces.  Each of these approaches can be accomplished in several ways.  Approaches 1 and 2 prevent condensation on window surfaces, while approach 3 allows condensation to occur, but removes it before it has been present long enough to cause mold growth or other problems. Used singly or, more effectively, in combination, these approaches will help to reduce condensation during colder weather as a cause of mold growth.  Discussed in detail below, these approaches are:

  1. Increase surface temperature of window glass and frame
  2. Reduce dew point temperature (humidity) of the air in the room
  3. Absorbing water and cleaning window surfaces regularly
1. Increase surface temperature of window glass and framecondensation on window surface

When temperatures outside are below those inside, wall and window surfaces are warmed by natural air convection currents which cause room air to drop past the surface of the window as the air in contact with the window cools, which causes it to become denser and drop, pulling in less dense, warmer air from further out in the room to replace it.  Window blinds and draperies, when closed, interfere with this convection current and allow the air between the blind and the window to become significantly cooler than it otherwise would. This means more condensation.

In less extreme weather such as we have in Florida, condensation on window surfaces can be partially or fully prevented by allowing the convection currents to operate unimpeded.  Open blinds fully or rotate the slats so that they don’t form as much of a barrier to airflow.  Adjust draperies so that warm room air can circulate past the windows. The less interference with airflow there is, the warmer the surfaces will remain and the less condensation will occur.

If natural convection is inadequate to fully prevent condensation on window surfaces, a fan can be used to blow warmer room air past the surface.  Even a very small, quiet, low-volume fan may be surprisingly effective used this way.

Raising the room temperature also helps minimize condensation by raising surface temperatures in contact with the air, but this is not a very energy-efficient or perhaps comfortable approach. A radiant heater aimed at the problem surface would be more effective.

2. Reduce humidity of the air in the room

The other main way to prevent condensation on window surfaces is reducing or controlling the amount of moisture in the air, also called humidity.

The most convenient way to control humidity in a Florida home is with dehumidification equipment. We like units that bring in outside air, deeply dehumidify it, then inject it into the home.  This creates positive pressure in the home, with multiple advantages for controlling moisture in the building envelope, ensuring good ventilation, etc.  IET has had great success specifying the highly efficient Ultra-Aire ventilating dehumidifiers.  (www.ultra-aire.com) These units control humidity effectively year-round.

However, less elaborate dehumidifiers can also be effective.  40 or 60 pint dehumidifiers from Walmart or Home Depot cost only a few hundred dollars.  One or more of these can easily remove excess humidity and prevent problems.  For convenience, we recommend the auto pump out models or setting the unit up so it can drain rather than having to empty a bucket regularly.

For additional information on controlling humidity in a hot/humid climate, see http://www.nrel.gov/docs/fy05osti/36643.pdf.

To learn more about controlling humidity throughout the year, see article: Controlling Humidity in Florida Homes Year Round.

3. Absorbing water and cleaning

A final approach to preventing or at least minimizing mold growth on window surfaces is to just wipe it off each morning when the night temperature was cool enough to cause condensation.  This removes the water necessary for growth, and as a convenient side benefit also removes the dust particles or other food source that they could feed on.  A variant of this approach might be to place several paper towels on each windowsill to absorb the moisture and discard them in the morning.  If window blinds or drapes were closed overnight for privacy, open them so that convective airflow past the still-damp surfaces accelerates the evaporation of residual moisture into the room air.

Existing mold growth on surfaces should be carefully cleaned using a detergent and water solution.  The use of a biocide, such as a solution of one part standard household chlorine bleach to ten parts water, is not necessary, but may be used if desired.

Indoor Environmental Technologies (IET) provides mold and moisture investigations and other services to improve the indoor air quality of your home or office.  Contact us today!

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