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Traditionally, windows have been made of a single layer of glass framed in wood placed into an opening in the wall — a low tech method that is one of the biggest energy wasters in your house. Recent technological advances for windows have increased window efficiency by more than 50%.
 Sustainability considerations for new or replacement windows are primarily the energy performance, secondarily the framing materials and third, the style appropriate for the placement. Other considerations are the maintenance you’re willing to perform and the durability of the hardware.
If you have an existing home with single pane, inefficient windows, ask yourself Should You Recaulk, Reglaze, Retrofit, or Replace? Determine your budget and take into consideration the energy cost savings of your choices. Decide how durable your windows need to be based on your climate. This narrows down the choices that you need to consider. Sometimes simply caulking around the edges of your windows increases their efficiency.
Energy Performance: Many window suppliers have been developing windows that meet or exceed criteria for high performance energy efficiency. Although many people automatically assume a higher cost for these higher performance windows, usually they are cost competitive upfront, although sometimes there can be as much as 15% higher up front cost, but, it's important to remember that since you’ll own your windows for many years, they contribute significantly to the value of your home, the increased value and savings on your energy bills will more than offset it. Make use of Energy rating systems to help guide your decisions in energy cost savings. Select windows that have some sort of rating sticker on them and compare the U-value of each window.
The type of glazing (the glass) and frame materials are the greatest determining factors for energy performance of windows. (Other factors include placement, design of the window, and the amount of interior and exterior shading.)
The Glazing System (the glass) can be single pane, double glazed, or triple glazed. (These terms refer to the number of panes of glass incorporated into the window unit: double-glazed—two panes, etc.) The number of panes affect the insulation value of the window. A good glazed window should feel comfortable to sit next to even on cold days. As a guide, double panes are recommended for colder, more northern climates. Storm windows over the single panes are a less expensive alternative option.
High-performance glazing systems further increase energy efficiency by incorporating such technologies as low-E coatings, heat absorbing glass, reflective glass, or transparent plastic films and inert gas filling inserted between glazings —some which can increase the insulating value of a double-glazed window to more than equal that of a standard triple-glazed window.
Low-emissivity (low-e) glass has a virtually invisible coating on the inside surface of a double-paned window, or on a transparent sheet suspended between the two panes. It basically acts as a mirror by reflecting heat that is normally transmitted through the glass back inside. But, unlike a mirror, it allows the full amount of light to pass through. Often Low-E glass is combined with gas filling for even more energy savings.
Heat-absorbing glass contains special coatings and tints that absorb as much as 45% of incoming solar energy. This is useful in reducing heat gain. Some of the absorbed heat passes through the window by conduction and reradiation. Conduction occurs through the edges and frames of the window and radiation through heat which moves from warmer to colder surfaces.
Reflective glass is glass coated with a reflective film. It is useful in controlling heat gain during the summer. Reflective glass reduces overheating. It also reduces the amount of light that can pass through the window.
Plastic glazing materials are generally considered less durable and less weather resistant than glass. Typical plastics which are generally available for glazing include acrylic, polycarbonate, polyester, polyvinyl fluoride, and polyethylene. They are generally stronger, lighter, cheaper, and easier to cut than glass. Some have higher solar transmittance.
Transparent plastic films, or pressure-sensitive films, can be applied directly to existing windows. Some of these solar control films have low-E coatings and can significantly reduce the amount of heat and radiation that comes in through your windows. Use of these films are only recommended for areas which tend to overheat in the summer. Use of these films could void the window warranty.
Gas-filled pane windows are generally filled with an inert gas of argon or krypton between the panes instead of air. These gases allow less heat transfer than air, increasing the window’s insulating capabilities. Although air is cheaper as a filler, it is much less efficient then an inert gas fill, and air will likely cost more for energy over time. Argon and krypton are the usual gas choices.
Framing materials: The window sash and frame could occupy 10 to 30 percent of the total window area and can be a major source of heat loss resulting from conduction through the material. The insulating value, or R-Value of the material, helps determine its efficiency.
Framing materials used for windows include aluminum, wood, vinyl, fiberglass or a combination of different materials such as wood clad with vinyl or aluminum-clad wood or fiberglass filled with foam insulation. Good quality windows are available using any of these materials. Each material has advantages and disadvantages in terms of insulating value, strength, durability, cost, aesthetics and maintenance requirements. Low-conductivity materials include wood and vinyl. Wood and fiberglass are the most efficient, but they also require more maintenance and are more expensive. Aluminum conducts heat and loses heat faster than other options.
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