Purchasing Energy Efficient WindowsApr 23, 2009
Windows is one of the most common culprits in loss of home energy efficiency. If the windows in your home are not efficient, 70 percent of your home’s heat and 46 percent of the air-conditioning you pay for is lost through the windows and doors. New windows can pay for themselves through energy savings alone in a short period of time.
There are several factors to look at when shopping for new windows. There are several valuable resources out on the web or your local bookstore or library that can assist you in making the best, most informed decision there is.
The U. S. Department of Energy provides some excellent guidelines and explains some of the technical information that you will need to compare when shopping.
Energy Performance Ratings
You can use the energy performance ratings of windows, doors and skylights to tell you their potential for gaining and losing heat, as well as transmitting sunlight into your home. Look for the ENERGY STAR certification label. The manufacturers use a wide array of technologies to make ENERGY STAR qualified windows.
Heat Gain and Loss
Windows, doors and skylights can gain and lose heat in the following ways: direct conduction through the glass or glazing, frame and door; the radiation of heat into a house and out of a house; and air leakage through and around them.
- U-Factor – The rate at which a window, door or skylight conducts non-solar heat-flow. The lower the U-factor, the more energy-efficient the product.
- Solar heat gain coefficient (SHGC) – The fraction of solar radiation transferred through a window, door or skylight. The lower the SHGC, the less solar heat transmits and the better its shading availability. What SHGC you need is determined by the climate in which you live. A product with a high SHGC the better for cooler climates, the lower the SHGC the more effective it is at reducing cooling loads so should be used in warmer climates.
- Air leakage – The rate of how much air is allowed to escape around a window, door or skylight. A product with a low air leakage rating is tight than one with a high air leakage rating.
This is the products ability to transmit sunlight into a home can be measured and rated by these performance characteristics:
- Visible transmittance (VT) – This is the measurement of how much sunlight is allowed to lighten a room weighted by the sensitivity of the human eye. A product with a higher VT transmits more visible light. The amount of VT that is needed should be measured by your homes daylighting requirements.
- Light-to-solar gain (LSG) – This is the ratio between the SHGC and VT. It examines the relative efficiency of different glass types while it transmits daylight while blocking heat gains.
Nothing changes the look of your home, inside and out, like new windows. They can open up a room, improve ventilation, and let in more natural light. And, the maintenance-freedom, durability and ease of operation from today’s energy-efficient windows also give you more time and money for the things that matter most.