Ever want to learn more about how to choose energy efficient windows?
Important things to know about windows…
Gas Fill
The space between the two panes can also be filled with argon, or less often krypton, gas. Both of these gases insulate better than air. Krypton is somewhat more effective in windows with less space between the panes (1/4 -3/8 inch), so it is often used in triple-pane windows to keep the thickness down. Windows filled with air or argon work best when the space is about 1/2 inch. Windows with krypton are usually more expensive, both because krypton itself is expensive and because the designs tend to be upper scale. Argon is nearly as effective and does not add much to the cost of a double-pane window.
Low-Emittance Coatings
Wouldn’t it be nice in winter if your windows would let in heat from the sun, and keep the home’s warmth inside as well? This is essentially what a high-transmission, low-emittance (low-e) coating does. A clear microscopic layer of metal oxide installed on one surface of one pane of glass allows short-wavelength radiation from the sun to pass through it, but reflects long-wavelength radiation. The heat energy from inside the home (including from people) that radiates toward windows is long-wavelength radiation, which is reflected back into the home.
High-transmission, low-e windows are excellent for south-facing windows in passive solar house designs.
Manufacturers are now also designing low-e windows specifically to help reflect solar heat back outside in hot climates. These are considered low-transmission, low-e windows. They usually have tinted glass in addition to the low-e coating, and they provide glare control as well as high insulation values and good solar heat rejection.
If you live in an area with cold winters and hot summers, selective-transmission (or spectrally selective) low-e windows are the best. They reflect solar heat, which helps in summer, and they trap winter heat inside.
Improved Frames and Spacers
Window frames are made from aluminum, wood, vinyl (polyvinyl chloride), or fiberglass. There are also composites of two materials (for instance, vinyl and wood) mixed together and formed or extruded like plastic. To achieve a certain look, manufacturers also offer vinyl or fiberglass frames with a thin veneer of wood on the inside (wood-clad vinyl). Others offer wood frames with vinyl or aluminum cladding on the outside for increased durability.
The frame can account for about 15% of the energy loss through a window. Aluminum frames have high U-factors, unless they include a thermal break–a strip of urethane that interrupts the transfer of heat through the metal. Wood, vinyl, and fiberglass are much better insulators than standard aluminum frames (without the thermal break). Of these, fiberglass performs slightly better than the rest and is also the most durable. You’ll find that vinyl and wood frames generally have similar U-factors. Some very expensive vinyl frames are filled with urethane foam insulation.
In double- and triple-pane windows, the panes of glass are separated by spacers. The spacers are traditionally made of aluminum–even in wood, vinyl, or fiberglass frames–creating greater conductivity around the window edges. This makes the windows colder at the edges in winter, and water vapor may condense there as it hits the cold surface. New warm-edge spacers are made from better-insulating materials, and are recommended for cold climates. The biggest advantage to warm-edge spacers is that they reduce condensation around the edge of the window.
Keeping Air Out, or In
Windows should seal tightly when closed. Otherwise, drafts can enter the house, and heated or cooled air can leak out. How well a window will prevent air from moving in and out depends both on how it was constructed and on how it is installed.
Windows with compressing seals are generally more airtight than those that open along sliding tracks. Casement, awning, and hopper windows tend to seal best. Jalousie windows are extremely leaky; it’s nearly impossible to get the overlapping panes of glass to seal tightly. However, jalousies may be desirable in hot, muggy climates (in Hawaii, for instance), because they can be left open for nighttime ventilation, but still keep rain out and offer some deterrent to intruders.
Windows have an enormous effect on summer comfort. In the winter, we want to get as much light and heat from the sun as we can, but the more intense summer sun often adds unwanted heat. Solar transmission through windows can account for up to 40% of a home’s cooling requirements in some climates. Even in parts of the country that are cold in winter, summers can be uncomfortably hot. If you have air conditioning, keeping the house cool can be a major expense.
Shading Windows
The best way to keep the sun from overheating your home is to block it before it strikes your windows. With exterior overhangs, awnings, or shade trees, you stop the heat before it hits the window, and you still get indirect light into the house. Long roof overhangs and awnings can shade the window in summer when the sun is high in the sky, and still let in the lower winter sun. Similarly, trees that lose their leaves in the fall provide more shade in summer than in winter. However, these are best placed on the east and west sides of the house, because the leafless trees can still block a good portion of the winter sun through south windows.
Other options include exterior sunscreens, shutters, and window films. Sunscreens can be put up in summer and removed in winter. They generally block a lot of light but provide very effective shading. Shutters require ongoing attention, as they usually need to be opened and closed by hand.
Spectrally Selective Windows
The battle against unwanted heat gain through windows is usually tempered by the desire for light, a view, and the sense of openness that windows bring to a room. The best new windows are spectrally selective, meaning that they let in the wavelengths of visible light, but block out the wavelengths of the sun’s energy that carry only heat. You still get the view and the light, but much less heat comes along for the ride.
The amount of solar heat hitting the window that passes through it is called the solar heat gain coefficient (SHGC). A typical single-pane window with a SHGC of 0.79 allows 79% of the solar heat to pass through the window (including the frame). But some windows have SHGCs as low as 0.3, allowing only 30% of the heat to come in. Windows with lower SHGCs include double-pane windows with tints or low-e coatings.
Dark tints (including many of the older bronze and gray tints) can drastically reduce light transmission or visible transmittance (VT). Spectrally selective windows include those with multiple low-e coatings or light blue or light blue-green tinted glass (see Table 3).
Keep the Drapes from Fading
Windows with multiple panes and low-e coatings reduce the transmission of ultraviolet light (UV), which can fade and damage drapes, carpets, furniture, and paintings. Don’t count on them to prevent fading entirely, however. Double-pane windows block about 50% of the UV; low-e windows and window films block between 65% and 85%.
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