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Windows

Since windows need to provide quality lighting, views, and ventilation, they are important structural elements. Efficient windows are crucial, because they are often the weakest link for heat transmission through the building shell — they are a significant source of heat loss in cold climates, and heat gain in hot climates. Benefits of energy-efficient windows include energy savings; lower heating, ventilation, and cooling costs; improved comfort; increased light and view; reduced fading of inside furnishings; and less condensation.

Energy-efficient windows have caught the attention of local and state policy makers and consumers. The U.S. Department of Energy (DOE) states that more energy is lost through leaks from poor windows each year in the United States than there is energy supplied by the entire Alaskan pipeline. DOE also reports that in 1990 alone, the energy used to offset unwanted heat losses and gains through windows in residential and commercial buildings cost the United States $20 billion, one-fourth of all the energy used for space heating and cooling. Billions of dollars are wasted annually through inefficient windows. Some of the largest energy savings are possible in buildings that have poorly performing windows, such as single-paned, clear windows.

According to The Efficient Windows Collaborative, windows are a major source of unwanted heat loss in climates that mainly require heating and heat gain in climates that mainly require cooling. Energy-efficient windows can reduce energy use by 20%-30% in some cases, and therefore allow a designer to reduce the size and cost of the air-conditioning system. Windows are an important element in whole-building design. When architects use this concept to incorporate energy-efficient windows, it reduces energy use and saves building owners energy and money.

A window is actually a system with a few key components: glazing, sash, and frame. Glazing is the clear glass or plastic used in the window. The sash is where the glass or plastic panes of the window are set, and the frame is the complete structure that holds the sash and glazing.

The thermal performance of a window varies significantly, based on the number of panes, the space between the panes, the type of material between the panes, the emissivity (the ability of the surface to emit thermal radiation) of the glass, the frame in which the glass is installed, and the type of spacers that separate the panes of glass.

Because the sash and frame represent 10%-30% of the total area of the window unit, the frame influences total window performance. Frames can be made of a variety of materials, each of which have different thermal properties. Vinyl and wood generally have comparable thermal characteristics. Aluminum conducts heat readily, which makes it prone to condensation and rapid heat loss in cold climates. When cavities are filled with insulation, fiberglass frames have thermal performance superior to wood or vinyl.

Energy performance also varies by operating type. Horizontally sliding windows have higher air-leak rates than projecting or hinged windows. Hinged windows such as casements have lower air-leak rates than sliding windows from the same manufacturer because the sash closes by pressing against the frame.

There are several types of special glazings are available that can help control heat loss and condensation.

• Low-emissivity (low-e) glass has a special surface coating to reduce heat transfer back through the window. These coatings reflect 40%-70% of the heat from sunlight that is normally transmitted through clear glass, while allowing most of the visible light to pass through.

• Heat-absorbing glass contains special tints that allow it to absorb as much as 45% of the incoming solar energy, reducing the amount of heat that passes through the window into the room. Some of the absorbed heat, however, still passes through the window by conduction and reradiation.

• Reflective glass has been coated with a reflective film and is useful in controlling heat during the summer. It also reduces the passage of light all year long.

• Plastic glazing materials—acrylic, polycarbonate, polyester, polyvinyl fluoride, and polyethylene—are also widely available. Plastics can be stronger, lighter, cheaper, and easier to cut than glass. Some plastics also allow more light through than glass. But plastics tend to be less durable and more susceptible to the effects of weather than glass.

In addition to these common types of windows, two new technologies are under development and hold great promise for future energy savings from windows.

• Electrochromatic glazings promise to be the next major advance in energy efficient window technology. These smart windows can be reversibly switched from a clear to a tinted state with a control. Incorporating electrochromatic glazings could reduce peak electric loads by 20%-30% in many commercial buildings and increase daylighting benefits throughout the United States. Full-scale tests on these products are underway, and the product is well on the way to commercialization. Please see How Stuff Works for an excellent interactive look at how electrochromatic windows work.

• Suspended Particle Devices are being investigated as a way to create a window that can be changed from clear to opaque with the flip of a switch. This type of window will use small light-absorbing microscopic particles known as suspended particle devices (SPDs). In the window, millions of these SPDs are placed between two panes of glass or plastic, which is coated with a transparent conductive material. When electricity comes into contact with the SPDs via the coating, they line up in a straight line and allow light to flow through. Once the electricity is taken away, they move back into a random pattern and block light. SPDs are an example of the cutting edge research underway in the windows field.

Manufacturers usually represent the energy efficiency of windows in terms of their U-values or R-values. Most window manufacturers use the window's R-value, which is a measure of how good the window is at resisting heat flow. The higher the R-value, the less heat it will lose. The U-value is the reverse, so lower is more efficient. The R-value of the window as a whole should be used because efficient glazings can be compromised by poor frame designs. Usually, window R-values range from 0.9 to 3.0, but there are energy-efficient exceptions.

Another measure of window efficiency is the solar heat gain coefficient (SHGC), which measures how well a product blocks heat caused by sunlight. The SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed, then subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window's SHGC, the less solar heat it transmits.

The Efficient Windows Collaborative provides objective information about the benefits of energy-efficient windows, descriptions of how they work, and recommendations for their selection and use. The Efficient Windows Collaborative is a coalition of window, door, skylight, and component manufacturers, research organizations, federal, state and local government agencies, and others interested in expanding the market for energy-efficient windows. Its goals are to double the current market penetration of efficient window technologies, and to make the NFRC (energy rating) labeling of new windows a near-universal practice in U.S. markets. Their individual State Fact Sheets are perhaps the single-best window purchasing tool available for commercial and residential consumers.

New window technologies are proven, so purchases carry very little risk. More states are expected to adopt stronger residential and commercial building energy codes in the near future, further improving the market for energy-efficient windows. As more large cities and counties adopt "green building" guidelines for residential and commercial buildings, window technologies are likely to benefit. If enforcement efforts were ramped up in the states that already mandate the commercial building energy code required by the Energy Policy Act of 1992, the markets for these products would improve even more.

Retrofit or replacement window costs commonly range from $5 to $50 per square foot of window area. Spectrally selective glass (windows that filter out as much as 70% of the heat transmitted through clear, single-paned windows) only adds about an extra $1.00 per ft² to the cost of a new home. Double-pane glazing with a spectrally selective coating costs 10%-20% more than ordinary double-pane glazing. Using spectrally selective windows in retrofit applications, where labor accounts for a significant proportion of the cost, adds only about 5% to the total price of the job.

According to DOE, retrofitting with selective glazings in most parts of the United States can pay back in 4-10 years for commercial buildings. The payback is even faster in new buildings where the incremental cost is lower and the air-conditioning system can be downsized.

Today spectrally selective products are manufactured by the major glass manufacturers and some films manufacturers, and are used in about 15% of new low-e windows.