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Winter  2001


                            A New Resistance Rating
by Jim Benney

In a nationwide survey with builders earlier this year, NFRC found that respondents feel that the condensation resistance ability of the windows they buy is more important than U-factor (thermal transmission) or solar heat gain ratings. In a 1998 nationwide survey of consumers, NFRC found that 85 percent believe it’s important that new windows “reduce interior condensation” in their homes. 

Condensation Formation
In much of the world, but especially in cold climates, condensation formation on the inside of window surfaces is an issue. Condensation on glass surfaces simply may be an aesthetic concern, reducing visibility or view through the window. But in other cases excess moisture can have a negative impact on curtains, walls, carpets—and possibly even the window itself. 

Condensation may appear in the form of a light coating of water, water droplets, frost, ice or a combination thereof. It will form on a window surface when the temperature of the surface falls below the dew point temperature, which is determined by both ambient air temperature and relative humidity. Heat from inside the house will conduct its way through the parts of the window that are the least energy efficient, causing those parts to have lower indoor surface temperatures thus increasing the potential for condensation. Therefore, to increase the resistance of windows to condensation, it is important to maintain the surface temperature of the window above the dewpoint. To accomplish this, window manufacturers must lower the thermal transmittance (U-factor) of the overall window product, as well as reduce the conductivity of the various window components. 

Manufacturers can take many different steps to reduce thermal transmittance and conductivity. Each affects the overall energy performance—and therefore condensation resistance ability—of a product differently. In order to provide manufacturers, builders, architects, consumers and others with fair, accurate and reliable information on window condensation, NFRC has developed a standardized methodology (NFRC 500) for determining the potential formation of condensation on a window.

Condensation Resistance 
Condensation resistance, as the rating is called, is reported on a scale of 1 to 100, with a higher number being more resistant to condensation than a lower number. The results are based on a set of standardized conditions (0 F outside temperature, 70 F inside temperature) and three levels of relative humidity—30 percent, 50 percent and 70 percent. Surface temperatures for the window are normalized and recorded for the three window areas (center-of-glass, edge-of-glass and frame). The condensation resistance of the window is then determined by the lowest rating obtained from the three component areas. 

The publication of NFRC 500 is significant to the window industry for three reasons:
    • NFRC determines condensation resistance by utilizing computer-modeling tools that determine surface temperatures based on scientifically validated heat flow characteristics. These tools assure that all products are evaluated on a fair and equal basis and provide consistent performance ratings;
    • Computer models are valuable design tools for the fenestration industry, saving time and money in bringing new products to the building community;
    • Finally, the publication of NFRC 500 frees the industry from reliance on physical thermal testing for reporting condensation resistance. 

It should be noted that NFRC 500 only reports condensation formation on the inside surfaces of windows and that, in some cases, condensation can form on the window exterior. Also, remember that in the real world, environmental conditions (temperature and relative humidity) vary from the standardized environmental conditions used to determine condensation resistance. NFRC 500 is not meant to predict condensation; rather, it is meant to be a tool for rating and comparing window products for their potential to form 

Jim Benney is director of education for the National Fenestration Rating Council in Silver Spring, Md.



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