Energy Efficient Windows

Windows are obviously an important design feature of a home. They can enhance the home's appearance and provide ventilation, emergency egress, lighting and views of the outdoors. But they also must function as part of the home's building envelope. In most homes, windows account for a major portion of the total winter heat loss and summer heat gain. Energy efficient windows can reduce heat losses and gains, saving energy and dollars year-round, while improving indoor comfort and adding to the value of the home.

There are two main factors that affect the energy efficiency of a window. The first is the overall conductivity or the rate at which heat passes through the window. Conductivity is the inverse of resistance and is expressed as a number called a U-value. The lower the U-value, the greater the window's resistance to heat transfer. During cold winter weather, a window with a lower U-value will allow less heat to escape the home, saving space heating energy and improving comfort.

The second factor is called a Solar Heat Gain Coefficient (SHGC) and provides a measure of how much of the sun's radiant heat will pass through the glass. The lower the SHGC, the less heat gain through the glass, saving on summer air conditioning costs. Since windows can be one of the greatest sources of summer heat gain (and high air conditioning costs), choosing windows with a low SHGC is important, particularly in climates with high air conditioning requirements.

Double-Paned and Triple-Paned Windows

Years ago, most windows were made with a single pane of glass, which offers little resistance to winter heat loss. Double-paned windows add a second pane of glass with a sealed air space between the panes. This sealed air space works like a thermos bottle, insulating the window and reducing winter heat loss. The result is lower space heating costs and improved comfort, in addition to reduced condensation on the interior of the glass. Windows with triple-paned glass are also available, but the third pane of glass has less of an effect on efficiency, and factors like weight often make them impractical.

In addition to insulating glass, efficient windows also incorporate some type of thermal break to reduce heat loss through the frame, and built-in weather-stripping to eliminate air leakage around the window.

Low-E and Gas-Filled Windows

Low-E ("low emissivity") windows use an almost invisible coating bonded to one of the panes of glass to reflect a portion of the heat that would otherwise pass through the glass. The most common type of Low-E glass has the coating facing inward, to reduce winter heat loss and improve comfort in cold weather. Another version, sometimes called "Southern" Low-E glass, has the coating facing outward to reflect some of the sun's radiant heat in summer. Virtually all Low-E windows are double-paned (or triple-paned) glass, with the Low-E coating facing the trapped air space to protect it from being scratched or otherwise damaged.

Another development in insulated glass substitutes an inert gas (usually argon or krypton) for the trapped air between the panes. These invisible gases have a higher resistance to heat transfer than air, resulting in a lower overall U-value and a more efficient window.

What to Look For

When considering new or replacement windows, look for the best combination of overall U-value and Solar Heat Gain Coefficient, based on your climate. In cold winter climates, the U-value is most important, as it has the greatest effect on space heating energy costs and winter comfort. In climates where air conditioning is the primary concern, the SHGC is most important. Look for windows whose U-value and SHGC have been certified based on testing procedures developed by the National Fenestration Rating Council. The table below shows ENERGY STAR criteria for efficient windows based on climate zone.

Climate Zone U-value SHGC
Northern (mostly heating) < .35 Any
North central (heating and cooling) < .40 < .55
South central (cooling and heating) < .40 < .40
Southern (mostly cooling) < .65 < .40

In addition to the U-value and SHGC, be sure to check the effectiveness of the window's weather-stripping. Some manufacturers provide data on overall air leakage. When comparing different windows, always ask for written documentation of efficiency ratings and any claims of energy savings, and be skeptical of any claims that seem unrealistic, particularly when they have not been verified by a respected third party. When it comes to installation, be sure the installer properly caulks or otherwise seals around the edges of the window and insulates any small gaps between the window and adjacent wall framing.

Installing new energy efficient windows will pay dividends in energy savings for years to come, while potentially improving the comfort, appearance and value of your home.

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