Volume 9, Issue 10 - November 2008

AAMA A N A LY S I S

Winter Weather Conditions Call
For Condensation Resistance
B Y J O H N L E W I S

As temperatures lower across our country, excessive wintertime condensation can be a sign of unacceptable thermal performance and resultant energy waste. The collection of moisture can damage interior surfaces and the indoor environment. The latter can result in moisture-induced mold growth and lead to lawsuits over alleged toxic reactions. Green building practices clearly tie into condensation prevention, which is best addressed during the design of windows.

Why Condensation Forms
When water vapor in indoor air comes into contact with glass or framing whose surface temperature falls below the dew point— the temperature at which airborne moisture turns into a liquid—condensation will form. The warmer the air, the more moisture it can hold. So, as the humidity increases, the dew point rises, and condensation will occur at warmer temperatures. For example, bathroom and kitchen areas are well known as high-humidity environments, so condensation on mirrors and windows in these rooms is rather common. If the dew point is below 32 degrees farenheit, the condensation will likely be in the form of frost or even ice.

Controlling Condensation
The first step in controlling condensation is to control the indoor relative humidity through a welldesigned HVAC system (exhaust fans, ceiling fans, dehumidifiers, etc.) or by opening doors and windows when outdoor conditions allow. But, even when the humidity is controlled, interior surfaces of exterior walls and fenestration can, in very cold weather, fall below the dew point and encourage condensation. The problem then becomes one of keeping the inside surface temperature elevated when outdoor temperatures are frigid, which is the same challenge posed by energy-efficient design (i.e., reducing the amount of heat transfer through the window). Therefore, the same design measures work to improve both.

Heat transfer seeks the path of least resistance. In winter, heat from inside the building will conduct its way through the parts of the window that are the least energy efficient (i.e., most conductive), causing those parts to have lower indoor surface temperatures and thus more likely to generate condensation. To minimize the potential for condensation, each component of the window must be thermally efficient.

In most applications it is reasonable and cost-effective to allow for a small amount of condensation on the coldest days of the year, and the following options will improve condensation resistance:

• Alternatives to drive down window thermal conductivity (Ufactor) have evolved well beyond simple two-lite glass with a “dead air” space between. Inert gas such as argon infills and low-emissivity (low-E) glass (featuring a thin coating of metal or metallic-oxide) have driven U-factors down significantly. In cold climates where condensation is a greater risk, a low-E coating on the internal surface of the interior pane of an insulating glass unit reflects heat back into the room, reducing heat loss through the window; and

• Thermally improved framing is a key factor for condensation resistance (thermal barriers and some materials’ native thermal insulating ability). As always, the key should be the demonstrated overall performance of the product and the parameters of the specific project, not any prescriptive bias for or against a specific material.

The industry can ill afford to look at condensation as a mere nuisance. The obvious reason is customer dissatisfaction capable of escalating to lawsuits. Less obvious are the positive improvements that can be obtained in the indoor environment by lessening the likelihood of condensation. These include less maintenance, higher allowable relative humidity indoors (for greater comfort at a lower heat setting and less annoying static electricity in winter) and better clarity of view through the window glass. For these reasons, as well as in the interest of energy conservation, the industry continues to tighten performance and quality standards for all types of applications— commercial and residential alike.


DWM

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