Windows consist of one or more panes of glass and a framework made of wood, metal, plastic or composite materials. They are built into a wall or roof to admit light or air, and are designed for architectural, commercial or professional, and consumer or residential applications. Although some windows are designed for new construction, replacement widows for home improvement projects are also commonly available. When selecting windows, product specifications include height, width, and energy performance ratings such as U-Factor and SHGC. U-Factor is a measure of the rate of heat loss through a window. The lower the U-factor, the greater the window's insulating value. Solar heat gain coefficient (SHGC) measures how well windows block heat from sunlight. As a rule, windows with a lower SHGC transmit less heat.
There are many different types of windows. Awning windows are hinged at the top and open at the bottom, usually with a hand lever. Bay windows consist of three windows arranged at 30 or 45-degree angles. Bow windows consist of multiple windowpanes that form a gentle curve. Casement windows open and close from the side, usually with a crank handle. Double-hung windows have both a top and a bottom window. Typically, the lower window slides up and down. Gliding windows or sliding windows open by moving horizontally past another window. Hopper windows tilt in from the top and are often used in basement applications. Jalousie windows have overlapping slits that permit light and ventilation; however, they may not be weather-tight. Picture windows are square or rectangular in shape, and large enough to fill a room with light. Skylights or roof windows are angled to provide daytime lighting. Some skylights open to provide ventilation. Tilt-turn windows have a handle and can function as fixed window, open or close from the side, or tilt-in from the top.
Windows may incorporate features such as low-E glass, C5 wind resistance, safety or security glass, and stained glass. Low-E glass is designed to reflect heat before it passes through a window. Some Low-E glass can also reflect ultraviolet (UV) light. Windows with C5 wind resistance can withstand extremely high wind-pressures without blowout or permanent deformation. C5 windows are suitable for some high-rise buildings, and for low-rise buildings in locations with hurricanes or severe weather conditions. Safety glass or security glass is designed for protective applications and includes both bulletproof glass and shatter-proof glass. Typically, safety glass is used in cars because it shatters in large pieces that are less likely to cause injury. Stained glass is colored by adding metallic salts during fabrication. It is associated with buildings such as churches, but can be also be used in decorative applications.
Frame material, glazing, and grille type are important considerations when selecting windows. Choices for frame materials include aluminum, fiberglass, vinyl, and wood. Windows with aluminum-clad wood, bronze-clad wood, copper-clad wood, and vinyl-clad wood are also available. Choices for glazing include single, dual, or tripling glazing; as well as laminated, reflective, spandrel, and tinted glazing. There are five main grilled patterns for windows: colonial, custom, fractional, prairie, and specified equal-light patterns. Specialized windows that use other frame materials, glazing styles, or grille types may also be available.