The glass used in sashes may also be complexly produced in double or triple-glazed panes. In the double-glazed window, two sheets of glass are joined by a spacer at the edges, the spacer sealed in with plastic backed by a silicone seal to prevent loss of heat. A layer of air between the panes provides insulation as well. More sophisticated versions of double- and triple-glazing substitute argon gas for the air between two or more layers. The third layer may be a thin sheet of translucent plastic.
The object of double- or triple-glazing is to reduce heat loss from the interior. The insulating power of a material is typically measured by R-Value. The letter stands for resistance to heat loss. A six-inch layer of fiberglass has an R-Value of 19, a single pane of glass a value of 1.04. Glass is thus a poor insulator. Double-glazed glass panes will have an R-Value of 2 and triple-glazed panes an R-Value of almost 7. With large areas of a room open to the outdoors, increasing the R-Value translates, over time, into real energy savings. In the windows industry customers may encounter references to the U-Factor, another form of the R-Value. It is derived by dividing the R-Value into
The higher the R-Value, the lower the U-Value. The U-Factor is apparently used by the industry for marketing purposes. The better the insulation the lower the U-Factor will be-thus it is easier to explain low heat loss by low numbers rather than by high R-Values. Math can be used to improve the sales message.
Glass used in windows may also be specially coated with metallic oxides to produce so-called low-e glass. The e stands for emittance. The coatings limit radiation gain from the sun or heat loss from within. The invisible portions of sunlight, in the infrared segment of light, penetrate glass but only introduce heat. Ultraviolet light is also invisible but can cause materials to fade in color. Metal oxide coatings, applied in microscopic quantities and thus invisible to the naked eye, act to reflect back at least a part of the invisible spectrum in the summer months while letting visible light in. Low-e windows are coated on both sides. The inner coating, like the outer, reflects back energy reaching the window as invisible rays and keeping the house warm in the winter. Low-e coatings applied to the outer surface must be applied during the glass-making process itself which makes the glass more expensive. The interior coating may be applied later but is less durable so it is never used on outside surfaces. Special coatings are also applied to high-end windows to cause rainwater to run off rather than to dry in place, thus reducing user maintenance activity.
The sash itself fits into the window frame. The frame is a basic rectangle with a head on top, a sill on the bottom, and jambs on either side. Centered around this rectangle but on its outer perimeter is a nailing flange. The installer uses it to affix the frame to structural beams left by the builder to mark out the window hole and to provide support for the window. The top of this rough framing is the header, its bottom is called the rough sill. The interior portions of the frame are equipped with rails to accommodate sliding windows or they hold hinges to allow the sash to move inward or out.
Where mechanisms like springs or weights are needed, the frame will be more complicated in order to provide room for such devices. Most windows, or portions of them, are protected from insects by a screen when the sash is opened. Mounting for screens are affixed to the frame as well, inside or out, depending on the type of window. Screens may be permanently affixed, move in rails of their own, and may be removable. The structural framing, that which does real work, is finished inside and out with a decorative casing. When we look at a window, that is what we see-the casing and the sash. The head-casing at the top may extend on from the house to serve as a drip cap for catching rain drops; attached to the sill at the bottom may be a decorative piece known as the apron.
Wood, Metal, and Plastic
The material forming the frame and sash of windows plays a major part in differentiating products in this market. The oldest, traditional, and still universally considered as the best material for holding glass is wood. As a natural material it has superior appearance, high R-Values, and filters out noise effectively. Wood windows, however, are subject to environmental effects, will rot unless protected by paint, generally require more maintenance, and are the most expensive window material on the market.
Steel-framed windows were commonly installed early in the twentieth century in residential as well as in commercial buildings. Such window had the advantage of great strength but had very unattractive R-Values. For energy conservation, homeowners had to install fixed storm windows that stayed shut all winter-and had to remove them again in spring. At the time these early windows were put in place people did not worry about energy inefficiency. They did concern themselves with costs, but storm windows saved fuel. Aluminum appeared as the next dominant material for metal framing. The metal continues to be important in the twenty-first century as well in commercial applications. Commercial builders value the strength of aluminum as it enables them to offer large glass surfaces with low noise penetration from the outdoors. The cost of aluminum windows is the lowest, also making it an economical choice for commercial builders. Aluminum windows, however, have the worst R-Values despite insulating spacers used in their installation. In the commercial market aluminum continues to be dominant, but in the replacement market, where aluminum once reigned supreme because of its low cost, it is now a very small factor. Aluminum windows are almost never used in new residential construction.
Although polycarbonate, polyethylene, polyurethane, and polystyrene are used by some producers, the two most important plastics in use in the first decade of the twenty-first century are polyvinyl chloride (PVC), the vinyl window in industry parlance, and acrylic, at a distance, the fiberglass window in trade terminology. Vinyl windows are the most purchased category in the residential replacement market. They are inexpensive, have much better insulation characteristics than aluminum, and are maintenance-free. Fiberglass windows are stronger than vinyl or wood, available in dark colors (whereas vinyl is available in lighter colors only), and comparable to vinyl in insulation value. Fiberglass windows, however, are expensive, costing only slightly less than wood, and thus have the lowest share of the market.
