Windows can be responsible for anywhere from 10 to 25 percent of your heating and cooling costs! All of the components of the building envelope (such as building materials, windows, doors, insulation, drywall and concrete) determine how well a home performs. Windows bring light, warmth and beauty into homes and give a feeling of openness and space to living areas. They are also potential sources of heat loss in the winter and heat gain in the summer. That is why windows are one of the most high-tech products in residential construction. Since the early 1980s, the energy performance of typical windows has increased by more than 50 percent, the result of both improvements in glazing and in frame construction.
The National Fenestration Rating Council (NFRC) publishes the energy performance of certified window products, and in some states manufacturers are required to label their windows with the NFRC’s rating. These ratings are like the EPA mileage rating for cars—they may not provide actual energy consumption in a particular application, but are useful for comparison. The windows you select for your house- whether retrofit or new, must meet your local energy code requirements at a minimum, and ideally should be Energy Star qualified for you home’s climate.
Maximizing Thermal Performance
If you want to reduce your utility bills, you need to consider the impact of windows. In the Gulf Coast, we mainly require cooling; windows typically represent a major source of unwanted heat gain low-e (a finish that reduces energy transfer through insulating glass units roughly as much as adding another pane of glass) glass coatings, which increase the R-value (a measure of how effectively a material resists heat flow-the higher the better) of standard double-glazing from 2 to about 3, have significantly reduced solar heat gain and improve comfort. The cost for low-e easily pays for itself in a few years in most applications. The added benefit is a warmer window surface that is more comfortable to be near both in cold weather and in very hot weather. Double low-e and HeatMirror™ coatings on suspended films are available in premium windows, and can increase the center-of-glass insulating value up to R-9.
By careful selection of low-e coatings, windows can be “tuned” to optimize the performance of a structure—balancing heat loss, solar gain, and visible light transmission through the glass. In hot climates, coatings that transmit less solar gain should generally be preferred. On the east and west, less solar gain is preferable even in cold climates, because solar gain is greatest on these orientations during the summer, when air conditioning is likely to be used. Use of an inert, low-conductivity gas in the space between layers of glazing is another way to improve thermal performance. Most low-e windows have argon gas fill. Some super-energy-efficient windows have krypton or a mix of argon and krypton between the glazing layers.
Window Frame Materials
Frame materials for windows provide varying degrees of energy efficiency depending on their inherent conductivity properties. Although standard for many years, aluminum windows are disappearing from most cold-climate markets. If aluminum frames are used, they should be constructed with a thermal break between the inner and outer surfaces to improve energy performance. Aluminum windows are rapidly being replaced by vinyl frames.
Vinyl frames are much better than aluminum in terms of thermal performance, but there are some environmental concerns associated with the production and eventual disposal of PVC (polyvinyl chloride). Vinyl windows vary greatly in quality. A particular weakness of vinyl windows is that vinyl expands considerably (more than wood or fiberglass) when heated by sunlight, and many consumers complain about weather-sealing problems over the life of the window due in part to this issue.
Wood windows are still the standard for energy efficiency. Vinyl or aluminum cladding adds value because of its low-maintenance qualities. Wood-window manufacturers are facing increasing difficulty in finding affordable, knot-free material from which to manufacture their product, and some are using finger-jointed material with an interior coating and exterior cladding.
Other energy-efficient frame materials include fiberglass, with or without foam insulation in the hollow channels, and composites such as a combination of recycled vinyl and wood fibers. With any window materials, durability of the edge seals and spacers that separate the layers of glass is extremely important, as failure of this seal will cause condensation inside the window (fogging), and the loss of any low-conductivity gas fill.
Old windows lose large amounts of heat to the outside during cold days and gain unwanted heat on hot days. Replacing your old windows with good quality, energy efficient windows can save a significant amount of energy and provide increased comfort in your home.
If you are replacing your windows, don’t let the attraction of low prices sway you to buy inferior windows. Quality windows will last a long time and you will realize great savings over time through increased energy savings. Also, qualifying energy efficient windows are eligible for government tax rebates and incentives.
See www.resnet.us and www.energy.gov for more information.
Once you’ve got that taken care of, it’s time to look at your window treatments. This is an opportunity to not only enhance the look of your home, but also create energy efficient windows that will help reduce your monthly utility costs. Here are a few tips that will help reduce heat loss or gain around your windows.
- Awnings: Adding an awning over a window or even the entire side of a house will reduce the amount of direct sunlight entering a home and can decrease solar heat gain in the summer by up to 77%.
- Draperies: They’ve always been available as window coverings, however, the way you use them is important to control heat. On summer days, keep the drapes closed on windows that are receiving direct sunlight. This will help prevent unwanted heat gain.
- Shades: When properly installed, interior shades are one of the least expensive and simplest methods for saving energy. They are highly effective at reducing the amount of solar heat gain and are an attractive window treatment.
- Storm windows: A storm window added to a single pane window can reduce winter heat loss by as much as 50 per cent. They cost less than a replacement window and are easier to install while being very effective.
- Shutters: These can be either interior or exterior additions. They require a little elbow grease to install but in addition to providing excellent coverage, they also provide added security to your home. Reducing the amount of heat loss or gain around your windows is one of the easiest ways to help reduce your energy costs. By taking a few simple steps and ensuring you have energy efficient windows, you can already begin to reduce your monthly energy bills and at the same time enhance the look of your home.