Installing Energy Efficient Windows

Windows may provide light and great views, but compared to the walls around them, older windows do little to block heat and cold. An average home may lose up to 30% of its heating and cooling energy through the windows, which makes installing energy efficient windows a priority on the home improvement list.

Energy efficient windows, especially those that have earned an Energy Star, help keep heating and cooling in your home, which lowers your overall energy use. As a bonus, they also protect carpets and furniture from the damaging rays of the sun.

The Benefits Are Transparent
Installing energy efficient windows will reduce your heating and cooling requirements. According to the US Department of Energy, Energy Star windows save $60 to $120 annually over older double-pane windows and up to $465 a year over single-pane windows. If you're building a new home, using energy efficient windows may allow you to install a smaller HVAC system.

Tight seams and weather stripping are the obvious advantage of energy efficient windows, but the real improvement lies in the glass itself. Low-emissions glass, also known as low-e glass, is specially coated and manufactured to act as an insulating barrier. Using double- or triple-paned glass offers further advantages.

The higher insulating properties of energy efficient windows helps to keep the overall temperature of a room more constant. Cold or hot spots will be a distant memory and you won't mind standing next to your window on a cold winter morning.

The special coatings used in energy efficient windows keep ultraviolet (UV) rays from getting through. This will help to control the fading of carpets and furniture.

Energy Efficient Window Technology

• Multiple windowpanes. Two sheets of glass with an air or gas-filled space between them have better insulating properties than a single pane of glass. Some manufacturers even use more than two panes of glass for increased efficiency. The theory is simple: glass transmits a temperature from one side to the other with a certain level of resistance. Adding panes increases the resistance, making it harder for heat or cold to get through.
• Gas fills. Some multi-pane windows use argon, krypton or other gasses to fill the space between panes. These odorless, colorless and non-toxic gases resist changes in temperature, giving them a better insulating value than air.
• Low-E glass. Microscopic layers of metal or metal oxides are used to coat the surface of windowpanes. These coatings reflect infrared radiation to prevent heat buildup. They also reflect ultraviolet radiation, protecting furniture and carpeting from fading.
• Insulating edge spacers. A spacer maintains the correct distance between multi-pane windows. Energy efficient spacers reduce heat transfer and the formation of condensation.
• Improved frame materials. Frames made of vinyl, fiberglass or wood composites reduce heat transfer and improve insulation. Energy efficient windows avoid frames made of aluminum or other metals that easily transmit heat and cold.

How to Gauge Efficiency
When you're selecting windows, looking for the Energy Star is a step toward higher efficiency. These windows surpass government standards for energy efficiency, but it's a little trickier to know the exact savings with windows and doors, because efficiency varies by region.

Some windows and doors have earned the Energy Star in all 50 states, while others only qualify in certain regions. To get the best balance of cost and efficiency, check the Energy Star Climate Zone map. Check with your local utility company, too, as Energy Star windows may qualify you for energy discounts.

To get the most efficient windows, you'll want to compare several standard measurements of performance. These figures should appear in store displays, on Web sites and in catalogs. You'll also find them on a window's shipping carton.

• U-Value. A unit of measure that records the overall heat transfer of a window. Lower values provide better insulation. Look for a value of 0.33 or less.
• Visual Transmission (VT). Windows with a high VT are easy to see through and admit lots of natural light, reducing your need for artificial light. Some tints and coatings can reduce visible light and will have lower VT ratings. For most people, VT ratings above 60% look clear. VT ratings at or below 50% begin to look dark or reflective.
• Solar Heat Gain Coefficient (SHGC). SHGC calculates the available amount of solar heat that passes through a window. It uses a scale of 0 (none) to 1 (100% of available sunlight). In cold climates, consider windows with a SHGC of 0.40 to 0.55. In warm climates, windows should have a SHGC rating of less than 0.40.
• Air Leakage. Properly constructed and installed windows should allow very little air to pass through them when they're closed. Air leakage is measured in cubic feet per minute per square foot of surface area. Only consider windows with a value of less than 0.30 cfm/ft2.

Green in So Many Ways
Energy efficient windows are not only good for the environment, they're good for your wallet as well. The average payback time for a window with a 20-year warranty is 4 to 6 years. This means that for at least a decade after installation, your windows will continue to pay you in energy savings. With the spiraling cost of energy, the return on your investment will only increase over time.