GLASS: RAISING THE ENERGY BAR
Even the most advanced (and expensive) windows can’t compete with walls when it comes to restricting the flow of heat between interior and exterior. That’s why window efficiency is typically measured by how fast it loses heat. It’s like saying “Here’s the bad news, but it could be even worse, if you install a lousy window.” This figure, known as U-value, is expressed as “the escape of BTUs per square foot per hour, per degree Fahrenheit.” All you really need to know is that lower values are better, and you can achieve those values by spending more for windows with more layers of glass, special coatings and inert gas such as Argon to slow down the transfer of heat.—Editor
Cold glass can also create uncomfortable drafts as air next to the window is cooled and drops to the floor. This sets up an air movement pattern that feels drafty and accelerates heat loss.
High-performance windows with lower U-values will result in a higher interior window temperature in winter and thus greater comfort. Proper installation along with weatherstripping designed to seal tightly will also improve comfort by reducing cold air leakage.
During the summer, direct sunlight strikes people and surfaces, creating overheating and discomfort. Windows with low solar heat gain coefficients will reduce the solar radiation coming through the glass. Low solar heat gain low-E glass reduces heat gain while still providing sufficient light and view.
LESS FADING AND CONDENSATION BLOCK THE RAYS
High-performance windows with new glazing technologies make homes more comfortable. They create warmer interior glass surfaces, reducing frost and condensation. High-performance windows with warm edge technology and insulating frames have such a warm interior surface that condensation on interior surfaces is significantly reduced under all conditions.
Another concern for homeowners is the effect of sunlight on the home and its contents. Many organic materials, such as carpet, fabrics, paper, artwork, paints and wood may fade upon exposure to sunlight. Window selection can influence the type and intensity of transmitted radiation.
GLOSSARY OF TERMS KNOW THE LINGO
> Air Infiltration
The amount of air that passes between a window sash and frame, measured in cubic feet per minute per square foot of frame area.
> Argon Gas
A colorless, odorless, inert gas that fills the otherwise empty space within an insulating glass unit.
> Efficient Windows Collaborative
Organization that provides detailed reports and studies on the value of using high-performance windows. Visit the website at
www.efficientwindows.org for more information on the studies cited in this article.
> Insulating Glass (IG)
Glass units constructed of two or more glass panes separated by a hermetically sealed space.
> Low Emissivity Coating (low-E)
Low-E coatings reduce energy transfer through insulating glass units, and thereby achieve one of the highest levels of energy performance possible for glass.
> Solar Heat Gain Coefficient (SHGC)
Measures the fraction of solar energy admitted and indicates how well the product blocks heat caused by sunlight.
> U-value
The escape of BTUs per square foot per hour, per degree Fahrenheit.
> Warm Edge Technology
The use of low-conductance spacers to reduce heat transfer near the edge of insulated glazing.
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