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 opposite or inverse of R-value.) Therefore, the lower the U-value the better. In the past, many homes had single-paned windows. These windows provide little insulating value or resistance to 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.

Adding an additional pane of glass to a window creates a trapped air space, usually from 1/4" to 1/2", 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. There are also triple-paned windows with U-values approaching 0.333 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 higher insulating value and its relatively low cost. Gas filled double-paned windows can achieve U-values of 0.4 or lower.

One of the recent advances in window technology is to add a type of clear coating to the glass. This type of coating, called Low-E (for low-emissivity) is used in double-paned windows to keep heat outside during summer and inside during winter. A virtually invisible coating made from a semi-conductor or metal is applied to one of the inside panes, or on a transparent sheet suspended between the two panes. The Low-E coating is usually placed on the surface of one of the panes adjacent to the trapped air space. This Low-E coating serves to absorb and reflect radiant heat and ultraviolet rays. In cold climates, the Low-E coating should be on surface 3 as shown in the diagram. In warm climates, the coating should be on surface 2.

With typical Low-E windows, most savings will occur in winter months. Low-E glass reflects between 70%-75% of the heat that would normally be lost to the outdoors back into the home. During the cooling season, Low-E glass reflects about 25% more heat back outside than a single-paned window and 11% more than a standard double-paned window. The typical U-value for a double-paned window with Low-E glass is about 0.4. A newer type of Low-E glass, often referred to as "southern" Low-E glass, is designed to be most effective during the summer, by reflecting much of the sun's heat away from the inside of the home. Very often Low-E glass is combined with gas filling for U-values as low as 0.3.

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

To best evaluate the performance of a window, check the National Fenestration Rating Council (NFRC) sticker. Many window manufactures now provide this rating sticker, which lists the U-value for that window. For windows that are not rated, look for the lowest overall "U-value" which includes both the glass and the framing material.