Condensation Resistance

Condensation has been a long-standing problem in association with windows. Excessive condensation has been known to contribute to the growth of mold or mildew and it can damage painted surfaces and eventually rot wood trim. Reducing interior humidity is an important component of controlling condensation. Metal window frames can also cause condensation issues due to the fact that they conduct heat very quickly and will "sweat" or frost up in cold weather. The development of thermal breaks for metal windows was a major breakthrough for solving this condensation problem.

The National Fenestration Rating Council (NFRC) has developed a system for rating the condensation resistance (CR) of fenestration products. The Condensation Resistance figure shows the CR for a range of double-glazed windows. The CR is a function of the frame, spacer, and glazing type—a high CR is better. The worst performance occurs with non-thermally broken metal frames where the CR falls in a range of 10–23 regardless of glazing type. The CR for metal frames with thermal breaks is higher—in the range of 30–42.

The greater insulating value of non-metal frames results in better condensation performance resistance. Because the non-metal frame is no longer the dominant factor, the glazing type affects the CR to a greater degree. With clear glass, the CR range is 35–48. With low-E glazings, the range is 40–60. The wide range in CR reflects differences in types of low-E coatings and spacers. Low-conductance spacers are often used in combination with low-E glazings, increasing the condensation resistance.

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Interior Condensation

Maximum Recommended Humidity Levels

Outside Temperature

20° F to 40° F
10° F to 20° F
0° F to 10° F
-10° F to 0° F
-20° F to -10° F
-20° F or below

Inside Humidity

Not over 40%
Not over 35%
Not over 30%
Not over 25%
Not over 20%
Not over 15%

Based on engineering studies at 70° F conducted at the University of Minnesota Laboratories.

Condensation is the result of high humidity levels in your home. Air with high humidity holds water vapor until it comes into contact with a surface temperature less than or equal to the dew point (the temperature at which air becomes saturated and produces dew). Because glass surfaces are usually the coldest part of the home, condensation appears on windows first, generally in the form of water droplets or frost on the interior of your window. As interior air becomes drier or as the glass surface becomes warmer, condensation begins to dissipate.

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Exterior Condensation

Exterior condensation typically occurs in the summer months. It is caused by three main conditions: high humidity in the exterior air, very little or no wind and a clear night sky. When the temperature of the exterior surface of the glass is cooled below the dew point of the outside air, moisture forms on the exterior side of the glass. Then, as the glass temperature rises above the dew point, the moisture evaporates back into the air.

To help eliminate exterior condensation, open window coverings at night to warm up exterior glass and remove or trim shrubbery near windows or doors to enhance air circulation. Increasing the air conditioner setting by a couple degrees warmer might also help.

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Between-the-Glass Condensation

Condensation between two pieces of insulated glass is an indication of glass seal failure.

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Effects of Condensation

High interior humidity can potentially cause structural damage to your home and create health hazards for your family. The sign of condensation on glass is a good indicator that humidity levels are too high. Major structural and health-related issues often occur in the unseen areas of wall cavities, attics, and crawl spaces. The important thing to remember is that your windows are trying to tell you to reduce indoor humidity before it causes hidden, costly problems elsewhere in your home.

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Condensation Prevention

  • Raise the average temperature of the house one or two degrees.
  • Vent stove range hoods to the outside and reduce steam by covering pots.
  • Vent bathroom and kitchen exhaust fans to the outside.
  • Vent the attic and crawl spaces so moisture has a chance to dry out.
  • Do not block heating vents.
  • Properly vent appliances to the outside: use clothes dryer instead of hanging wet clothes indoors.
  • Open window coverings and blinds and make sure interior doors are left open during the day to allow air circulation.
  • Relocate heat vents below windows and patio doors.
  • Be sure all vent ducts are clear of dirt and debris.
  • Make sure furnace is working properly and serviced regularly. Look into dryer heat sources such as gas or electric furnaces.
  • Install an Air-to-Air exchanger to vent moist air outside.
  • Channel water away from the home's foundation.
  • Circulate air with small fans and use a ceiling fan to circulate warm room air toward windows.
  • Run a dehumidifier to reduce excess moisture.
  • Dry firewood outside.

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Air-to-Air Exchangers

With the emphasis on energy conservation and efficiency, new home construction can create a problem of indoor air pollution. Vapor barriers, super insulation packages, thermal windows and doors, weatherstripping, and caulk have reduced or stopped fresh air from infiltrating and replacing stale air. Entering and exiting the house through doors doesn't provide enough air exchange. Cooking, aerosol sprays, cleaning agents, paints, and in some cases, excess humidity if the house is sealed too tightly can create an undesirable environment. A device known as an air-to-air exchanger is used to recover heating or cooling and improve air quality.

There are many different designs, depending on the manufacturer, but the principle is the same. Fresh air is drawn in from a port open to the outside of the building and passed through a chamber, also known as the exchanger, which is surrounded by indoor air. Highly conductive metal or other materials remove the energy (heat) from the warmer air and give it to the cooler air. The fresh air is then ducted into the house and the indoor air is ducted to a port and expelled outside. Up to 80% of the energy can be exchanged. During the energy exchange, moisture (humidity) can condense into water. A drain pan inside the cabinet will allow the water to be collected for removal. If the unit is installed in the basement, a condensate pump might be used to eject the water outside.

Most air exchangers are controlled simply by on and off switches, but in applications for removal of humidity, a humidistat can be used to turn the machine on and off to achieve the desired level.
Routine maintenance involves replacing or cleaning the filters if so equipped and keeping the outside air intake free of debris. After years of service the exchanger will become crusted with grease and dirt that collects during condensation. Removing this accumulation will probably take solvents or degreasers and require opening up the cabinet and compartments.

One method of adding fresh air to the space is to duct outside air directly into the return air plenum of the heating or air conditioning system. Usually required by code for commercial applications, it is an inexpensive way of improving air quality, but not the most efficient, since it means heating or cooling outside air, which could be very cold in winter or extremely humid on a midsummer day.

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Frequently Asked Questions

What is condensation?
Condensation is a process in which water vapor (gaseous state) turns into water droplets (liquid state), when it comes in contact with a cooler surface such as window glass.

Is interior condensation caused by windows and doors?
Because the most often visible condensation is seen on windows and doors, it is easy to blame condensation as being the fault of a window or door. This is not true in most cases. Windows and doors do not manufacture water.

What is dew point?
The temperature of air at which it can no longer hold all of its water vapor and some of the water must condense into liquid.

Why do I have interior condensation?
Interior condensation is when water appears on the interior of windows and doors because conditions are just right. The cause of condensation is air saturated with too much humidity or water. When this happens, air cannot hold the excess humidity. It gets rid of it by condensing it on the most convenient cool surface.

Is interior condensation more likely to occur in certain climates or times of year?
Yes, in areas where January temperatures average 35°F or less.

When is window condensation only temporary?
Building materials used in new construction or remodeling contain moisture, which is gradually released into the air of the home. This excess moisture may cause condensation but will usually disappear after the first few heating seasons.

Why do my new windows have condensation and my old ones didn't?
The older less efficient windows allowed air to move across the glass by letting air inside or allowing inside air to escape outdoors, preventing the air temperature of the glass from reaching dew point. The tighter the window the less air infiltration occurs.

Why do only one of my windows in a room have condensation and the others do not?
Potential reasons for this occurrence are: Air circulation patterns within the room or home, fluctuating room temperatures, types of windows installed, window size, air register location, glass type, window coverings, window screens, the direction the windows are facing, elevation of the windows, wind direction, or direction of the sun or partial blocking of the sun due to trees, buildings, etc.

Is interior condensation on windows or doors caused by window coverings or drapes?
No, but they can enhance the problem by restricting the flow of air over the glass surface making condensation more likely to occur.

Is interior condensation impacted by air circulation?
Air circulation affects the supply of fresh air to all areas of your home and supplying fresh air to the glass area slows down the cooling process and reduces condensation.

Can humidity cause problems?
High interior humidity can potentially cause structural damage to your home and create health hazards for your family through mold and mildew growth.

Why do I still have interior condensation even though I am running a dehumidifier?
The humidity is probably still too high. Check for varying air temperature in the home as well as proper air circulation.

What can I do to control interior condensation?
Reduce humidity levels.

Is exterior condensation caused by windows and doors?
No, exterior condensation is a sign of energy efficiency, indicating the outside pane is thoroughly insulated from the heat of the home's interior.

Why causes exterior condensation?
Exterior condensation happens when the exterior surface temperature of the glass falls below the dew point of the outside air.

What does condensation between the glass mean?
Condensation between the two sealed panes of insulating glass is an indication of seal failure.


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