Tips for Choosing Energy-Efficient Windows
Until recently, windows presented a "can't live with them, can't live without them" scenario. We need them in our homes for light, ventilation and as escape routes in emergencies. But even in highly insulated walls, old windows were energy sinks that let out the heated and cooled air inside a house. In very warm climates, all that bright sunshine with no insulation turned homes into greenhouse-like environments.
All that has changed, and a building component that looks rather simple is capable of using a number of sophisticated energy-saving technologies. Most importantly, energy efficiency is not one size fits all. Homeowners can match the windows they place in their home to their local climate.
Parts of a Window
When many people think of windows, they think of the material of the frame. The most common types are solid wood, vinyl, fiberglass, composite materials and, in very warm climates, aluminum. Any one of these materials can be part of an energy-efficient window if it is combined with the right type of glazing.
The old-fashioned single pane of glass has given way to energy-saving glazing. Here are some of the components of energy-efficient glazing. Windows may have one or a number of these elements.
- Two or Three Panes of Glass. Rather than consisting of a single pane, today's windows have two and sometimes three panes. This was the first energy-saving advance for windows, and it has been around for some time. An air space separates the panes, and there are spacers at the top and the bottom of the air spaces. The glazing unit is sealed.
- Heat-Stopping Gases. In some windows, an inert gas, such as argon, fills the space between panes. The gas provides more insulating ability than air.
- Low-Emissivity Coatings. Known as Low-E coatings, these are microscopically thin layers of material applied to one or more surfaces of the panes of glass. The coatings resist heat flow out of the house in cold weather or into the house during hot weather.
- Suspended Films. Nearly invisible films can be suspended in the air space between panes of glass in a double pane window. This turns one air space into two, boosting the insulating value of the space. Low-E coatings can be applied to the film.
- Tints. These are applied to windows to block unwanted heat gain, but they can also block light.
Combing Frames and Glass
Depending on where you live, you will want your windows to help reduce heating costs, cooling costs or, for some places, both. To get the windows that best suit your climate, you will want to consider the following:
- U-Factor. This measures the rate the window, which includes both the frame and the glass, conducts heat. It is the reverse of the more familiar R-value, which measures resistance to heat flow. With U-factors, the lower the number, the better. U-factors for windows in northern climates should be on the low side, which means they prevent heat loss. Some companies provide the U-factor for the center of the glass, which will be lower than one that considers the entire window. For a true energy picture, consider the U-factor for the entire window.
- Solar Heat Gain Coefficient. This measures a window's ability to block unwanted heat from outside, both from direct sunlight and reflected sunlight. It is a number between 0 and 1. The lower the number, the better the window is at blocking heat. Windows in hot climates should have low ratings for heat gain.
- Visible Transmittance (VT) Rate. This is a number between 0 and 1 that indicates the percentage of visible light transmitted through the glazing. The higher the number, the more light gets through. It is important when considering daytime lighting needs. A low VT may require more artificial light for a space.
- Air Leakage. This measures the tightness of window construction and signified by a number between 0.1 and 0.3. Energy Star windows must have a measurement equal to or less than 0.3. You can find these ratings in the product literature and sometimes on a label affixed to the windows. The National Fenestration Ratings Council is an independent organization that tests and certifies the energy ratings on the windows.
What's a Good Rating?
A good place to start your search is to look for products with the Energy Star logo. Energy Star, which is a voluntary program of the Environmental Protection Agency, rates the energy efficiency of many household items, including windows. If the windows meet certain requirements based on the location of the house, the windows can carry the Energy Star logo. Replacing non-Energy Star windows with approved units saves an average of 12 percent in energy bills nationwide. See the map and table "Energy Star Requirements for Windows" below. It's important to note that requirements below are minimum standards for the Energy Star program. You can find windows that are more energy efficient, but this is a good place to start.
Energy Star Requirements for Windows
Source: Energy Star
Solar Heat Gain Coefficient
Equivalent Energy Performance*
Equivalent Energy Performance
Equivalent Energy Performance
*The program allows higher U-factors if they are balanced by higher solar heat gain coefficients.
A window may look energy efficient on paper, but unless it is installed properly, it will not do the job of cutting your heating and cooling bills. As a homeowner, your first step to ensuring a good installation is to hire an experienced installer who has a good reputation and can provide references from previous customers who you call. Here are some things to check while the installer is still at your house.
- If you feel air around the window frame, it may mean that the installer did not fill the openings between the window frame and the house framing with insulation or expanding foam. This is a relatively easy fix. The installer should caulk around the inside trim as well.
- Make sure the window is square and plumb by opening the bottom sash a few inches. The reveal, or open area, should be the same all the way across the window. Repeat for the top sash.
- Make sure the window opens and closes completely. If there is any binding, or a gap when the window should be closed, point it out to the installer.
- Test the window's features. Make sure locks work easily. If the window tilts out for cleaning, give it a test run.
New windows are a big investment, but by selecting energy-efficient models that are installed correctly, you can reduce your energy bills and have windows that will provide years of service.
Fran Donegan is a home improvement author who writes on appliances, lighting and other home related topics online for The Home Depot. Fran is the author of the books Pools and Spas and Paint Your Home. To research window installation options available through Home Depot, you can visit the website here.