Installing Energy Efficient Windows, by Stu Silverman
There's more to replacing windows than updating the look of your home. If you live in a cold climate, you can cut your heating costs by as much as 40 percent by switching from low-efficiency to high-efficiency windows. And if you live in a hot climate, you can save up to 30 percent on your cooling costs with energy-efficient windows.
The five main factors that affect the energy efficiency of windows are:
- The type of glazing material (e.g., glass, plastic, treated glass)
- The number of layers of glass
- The size of the air space between the layers of glass
- The thermal resistance or conductance of the frame and spacer materials
- How well the window is put together to prevent air leaks
Windows are given ratings for their energy efficiency, so you can compare various types of windows. The National Fenestration Rating Council (NFRC), which is a non-profit organization created by the Window, Door and Skylight Industry, has developed a Window Energy Ratings System based on product performance.
Labels help you compare windows
You will see NFRC labels on windows which give ratings for U-Factor, Solar Heat Gain, and Visible Light Transmittance. Ratings for Air Leakage and Condensation Resistance are optional, and may or may not be included by window manufacturers.
The U-Factor is a measure of how well a product prevents heat from escaping. It is most important in colder climates, where loss of indoor heat means more work for your furnace. A window with a low U-Factor loses less heat, and is a better insulator than a window with a higher U-Factor, which transfers more heat. U-values usually range from 0.20 (most energy efficient) to 1.20 (least energy efficient).
U-Factor is more comprehensive than R-Value
The NFRC warns that the U-Factor is a better indicator of energy efficiency for windows than the popular R-Value. While the R-Value- which measures resistance to heat loss- is a good measure for insulation in walls and ceilings, windows are different. Windows react to outside air temperatures, sunlight, wind, indoor air temperatures and occupant use, and are strongly affected by solar radiation and the airflow around them, and the R-value does not accurately reflect these interactions.
The U-Factor also takes into account the emissivity of the glass, which is its ability to absorb certain types of energy (specifically infrared) and radiate it to the other side, such as the passage of warm air in a room to the outside. This is especially important due to the developments over the last 10 years involving low emissivity (low-E) glass. (More about the increasingly popular Low-E glass below).
Rating shows proportion of heat passing through window
The Solar Heat Gain Coefficient (SHGC) indicates how well the window blocks the sun's heat from entering the building. It is the proportion of incident solar radiation admitted through a window, and is expressed as a number between 0 and 1. The lower a window's SHGC, the less solar heat that will come in from the outside.
So a window with a SHGC rating of .40 admits 40 percent of the heat that hits it, and will let in twice the amount of heat as a window with a .20 SHGC. The SHGC is most important in warmer climates, where heat entering from the outside increases discomfort or makes air-conditioners work harder, using more energy.
How much light can pass through?
Visible Transmittance (VT) is the proportion of light that comes through the window, and is expressed as a number between 0 and 1. So a window that allows 65 percent of the light to pass through will have a VT of .65. The more light that passes through, the higher the VT. But remember that VT is measured for the entire window, so windows with wide frames that block more light will have a lower VT. Similarly, windows with special coatings which increase energy efficiency or security, such as Low-E or laminated glass, respectively, will sacrifice some visible transmittance.
Air leaks mean thermal inefficiency
The Air Leakage (AL) rating measures amount of air that can pass through cracks in the window assembly, leading to heat loss or gain. It is expressed as the equivalent number of cubic feet of air passing through a square foot of window area. The lower the AL, the less air will pass through cracks, and the more energy efficient it will be. This rating is optional and may or may not be included by manufacturers.
Condensation Resistance (CR) measures the window's ability to resist the formation of condensation on its interior surface. This rating cannot predict the appearance of condensation, which results from indoor humidity. However, it can provide a way to compare the relative likelihood of condensation forming on various windows. CR is expressed as a number between 0 and 100. The higher the CR rating, the better that product is at resisting condensation formation. This rating is also optional and may or may not be included by manufacturers.
The Energy Star Windows Program
 Another way to ensure you're buying energy-efficient windows is to look for the Energy Star logo. An Energy Star sticker means the window has met the minimum standard for insulating ability, as set by the Energy Star Windows Program- a program developed by the U.S. Department of Energy and Environmental Protection Agency to promote sales of energy-efficient windows, doors and skylights. The program is intended to help homeowners cut their heating and cooling costs, reduce air pollution, and make homes and buildings more comfortable. Energy Star windows generally exceed building code requirements and cost about 15 percent more than windows without the rating.
Different climate zones have different standards
Since the energy efficiency performance of windows can vary due to climate, different climate zones have been designated within the U.S., and color-coding is used to indicate whether a window meets minimum U-Factor and SHGC requirements for each zone.
To see a map of the different zones and the associated minimum standards, click here.
More panes give more energy efficiency
Single-pane windows have very little insulating value and are therefore very energy-inefficient. With one pane of glass, there is only a thin barrier to the outside, which can lead to considerable heat transfer- loss in the winter and gain in the summer. Double-paned windows improve energy efficiency, as multiple layers of glass increase the window's ability to resist heat flow. You can even choose triple-paned windows for even better energy efficiency, or get windows which have argon or krypton gas between the layers to reduce the transfer of heat.
The energy efficiency and quality of a window is a partly determined by the type of glass, or the glazing, used. Clear glass has been the most common material used for window panes in homes. But in the past few years, many types of special glazing have been more widely used- such as glass that helps control heat loss and condensation, and improves security.
Low-E glass a popular choice
The type of glass that has gotten the most attention in the past few years is Low-E glass (which stands for “low emissivity”). Low-E glass has a microscopically thin metal oxide coating that reduces heat transfer through the glass by about 40% -70% compared to clear glass. It is therefore more energy-efficient by keeping the heat in in the winter (reducing heating costs), and keeping the heat out in the summer (reducing cooling costs). And with Low-E glass, there is no reduction in the amount of light that can pass through the window, yet it blocks or reflects ultra-violet and infrared rays, which can be harmful to your skin and damage or fade wood, drapes, carpets and furniture.
Choose your Low-E according to climate
There are three common types of Low-E glass that you can choose according to your climate; High, Moderate and Low Solar Gain. High Solar Gain Low-E windows are used in colder climates, with cooler summers and colder than normal winters, because they allow as much solar heat to enter the home as clear glass, while restricting the heat loss. For climates with cold winters and mild summers, choose Low-E windows rated as Moderate Solar Gain, which will allow a high amount of solar heat to enter in the winter, and only a moderate amount of heat gain in the summer. And for climates with hot summers and mild winters, choose Low Solar Gain windows, as they let the least amount of solar heat enter the home.
Stu Silverman is a home improvement advice columnist and freelance writer.
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