Energy-conscious consumers have plenty of options when it comes to new windows. Regardless of what is motivating you–building a new home, energy costs or drafts and frosty frames–understanding how windows work will help you ask the right questions and choose the right products.
Practical benefits of efficient window selection include lower heating and cooling costs; less condensation and fungal growth; reduced fading of carpets, upholstery, window coverings, and artwork; and energy savings. All these benefits are factors of glazing, spacer material, frame composition, and window operation.
Household energy costs in most U.S. homes could be reduced by up to 15 percent by installing more energy efficient windows.
The Frame Game
Frames are major players in window efficiency and cost. Because there are so many materials to choose from–each with different aesthetic qualities and varying levels of durability and maintenance requirements–frame selection can be challenging. Conscientious consumers often consider the origin of a frame’s material; however, Steve Loken of Loken Builders in Missoula, Montana, founder of the Center for Resourceful Building Technology, suggests putting the emphasis on efficiency.
“In the long run, the lifetime energy consumption of the house far outweighs the environmental alternatives of wood versus steel or vinyl,” he says. A good frame does not easily conduct energy. Consider the following metal, wood, vinyl, or fiberglass options.
Aluminum and steel frames are typically the least expensive, at least initially. These are the least thermally efficient frames, so what you save up front you will pay over time to heat and cool your home. They are durable and often recyclable, but they require a lot of energy to produce. Aluminum with a thermal break is better than all-aluminum frames because a less- conductive material separates the interior and exterior components. Some energy codes prohibit the use of solid aluminum windows.
Wood frames are a good thermal choice, but they do require more maintenance than most other materials. A wood frame clad with vinyl or aluminum can significantly ease some maintenance requirements by providing a weather-resistant exterior surface. “Wood is still the best environmental choice because it is renewable,” Loken says. Many wood windows are built with a finger-jointed or laminated wood structural core wrapped with finished lumber. Using smaller pieces of wood in this way reduces waste while retaining the desirable qualities of wood windows.
The wood used for windows is typically chemically treated to resist rot. The preservatives in these products can aggravate health conditions in chemically sensitive individuals, who may need to look for untreated products.
Vinyl is an affordable material for window frames, and it is less conductive than metal. Beware of bargain vinyl frames because they may sag, become brittle, or pull away from seals. Insulated vinyl frames–those with insulation in their hollow cavities–are thermally superior to standard vinyl and wood, according to the Alliance to Save Energy’s Efficient Windows program. Vinyl’s integral color means low maintenance, but a long-term commitment to hue. The vinyl extrusion process uses less energy than some other frame production methods, but there is legitimate concern about vinyl’s chlorine content and the pollution generated during manufacture.
Fiberglass is energy-intensive to produce, but is weather resistant, long lasting, and durable. Fiberglass is stronger than vinyl, so frames can take up less overall window area. As with vinyl, fiberglass frames can have a hollow cavity. When this space is filled with insulation, fiberglass frames are the most thermally efficient of all types. These frames typically can be painted.
A window’s transparent panel, typically made of glass, is called glazing. The glazing is important because it makes up the majority of the window’s surface area. The window and door (or fenestration) industry uses several ways to improve the energy efficiency of glazing products.
Changing the environment between the panes of glass is another way to boost efficiency. The sealed space between the panes contains either air or a gas that is even less conductive than air. Argon and krypton gases, both inert and non-toxic, reduce heat conduction because they are thicker and heavier than air. By putting one or a mixture of both in the space, manufacturers can minimize conduction and convection of heat through the window. “Argon and krypton increase cooling performance by about 10 percent,” says Jim Benney, National Fenestration Rating Council education director.
Coating the glass or suspending a coated film between panes of glass is one method of enhancing glazing. Either method reduces heat gain and glare and improves heating and cooling performance. “There are sophisticated [film] technologies now, and I think a film suspended between two panes of glass would be better [than a triple pane] because it lightens up the window, uses less material, and still gives a good resistance to heat loss,” says Loken.
Low-emittance (low-e) coatings are microscopically thin, transparent layers of metallic oxide. You’ll find low-e coatings for high, moderate, and low solar gain for use in specific situations. For example, a high-solar, low-e glazing is typically designed to reduce heat loss but permit solar gain. This type of window is a good choice in a climate where heating is the primary concern, and also for passive-solar projects. Moderate-solar, low-e glazing reduces heat loss while allowing reasonable solar gain. Such windows perform well in climates where there are both heating and cooling concerns. Low-solar, low-e glazing is sometimes called spectrally selective because it reduces heat loss in the winter and heat gain in the summer. It is an ideal choice for hot climates where cooling is the primary concern.
Low-e coated glass can reduce harmful UV rays by up to 75 percent, a feature that provides protection for interior surfaces, according to the government’s Energy Star program.
Creating more than one air space is another way that glazing can be optimized for energy efficiency. Air space provides an insulative layer that reduces thermal transmission and noise. Adding a third pane of glass or suspending a coated film between two panes creates an additional air space.
Another factor for overall window efficiency is the composition of the spacers separating the panes of glass. “What we found when we started using energy efficient glass [low-e glass] was that cold and heat would ‘short-circuit’ through the edges of the glass via the aluminum spacer,” Benney says. “Warm-edge spacers are typically made of different materials and different designs that reduce the conductivity through the spacer.”
Look for materials with low conductivity. Your choices will range from highly conductive aluminum to increasingly low conductive stainless steel, vinyl, foam, and fiberglass. A thermally improved edge spacer will reduce heat loss and prevent condensation. “The main benefit of warm-edge technology is the reduction of condensation formation in cold climates,” Benney says. “Warm edges do help reduce the overall thermal transmission of the window, but not significantly.”
The type of window operation you select will also have some effect on energy efficiency. For example, horizontal sliders and single- or double-hung windows generally have high air leakage rates because the sash and fixed pane don’t have a tight seal between them. Hinged windows, including casement, awning, and hopper styles, are generally more efficient because the sash locks snugly against the frame when closed. “The thermal transmission of a sliding window may be worse on average than a hinged one because of the higher glass-to-frame ratio of the hinged windows,” Benney says. “This only applies to glazing systems that perform better than framing systems.” Hinged windows also catch more breezes than sliding styles.