Energy Efficiency E-Library > Structural Info > Energy-Efficient Windows

Energy-Efficient Windows

Unlike other insulating features of the home, the efficiency of windows is typically expressed in terms of a U-value. U-value measures the conductivity of the window (this is the inverse of R-value.) Therefore, the lower the U-value the better.

In the past, many homes had single-paned windows, which provide little insulating value or resistance to radiant heat flow. Also, some windows have metal frames which conduct heat (into the home in summer and out of the home in winter). More efficient windows have a thermal break in the frame to prevent this. Many technological advances have been made in an effort to increase the energy efficiency of windows. Vinyl (PVC) is now the material of choice for residential window frame.

Double- and Triple-Paned Windows

Adding an additional pane of glass to a window creates a trapped air space, usually half an inch, which provides a higher insulating R-value (or lower U-value) than a single-paned window. Double-paned windows usually have U-values of 0.55 or lower, whereas single-paned windows have a U-value of about 1.1. Triple-paned windows have U-values approaching 0.33 or lower.

Some manufacturers are using one of several types of inert gas to replace the air between panes of glass. Argon gas is most commonly used because of its high insulating value and its relatively low cost. Gas-filled double-paned windows can achieve U-values of 0.33 or lower.

Low-E Windows

Clear glass has a high emissivity – heat easily passes through it. So while the sun's energy easily passes through the glass during the day, this property also allows radiate heat to easily leave the warmer house to the cooler outdoors. A low-emissivity, or Low-E, coating is a very thin, transparent layer applied to the glass to reduce the amount of energy (radiant heat and ultraviolet rays) that passes through the window. That helps to save energy and reduce fading of room furnishings. The Low-E coating is usually placed on the surface of one of the panes adjacent to the trapped air space – surface 2 or 3 (see diagram).

A double glazed window with a Low-E coating has about the same insulating value as an uncoated triple glazed unit. The Low-E coating improves the insulating value of the double glazed window from R-2 to R-3 or R-4 – depending on the type of coating. There are two main types of Low-E coatings. Often referred to as northern or hard-coat Low-E, this coating type maximize solar heat gains and reduce heat loss at night. Southern or soft-coat Low-E on the other hand reduces solar heat gain and heat loss at night. Ask your window contractor what type is best for your home. Either type will reduce your heating costs giving you a payback on the incremental cost within a reasonably short period of time.

Advantages of Low-E windows

• Reduced fading of carpets, drapes and furniture
• Reduced mildew and deterioration of the window frame
• Increased comfort near windows
• Lower energy bills for years to come

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