Windows are one or more panes of glass held in place by a frame and built into a wall or roof as a means to admit light and usually ventilate an interior space. A single unit of framed glass is called a sash, and there is often multiple sashes in each window. While the materials used to make windows are relatively limited and their application fairly consistent, the diversity of buildings and climates has created a market for windows of nearly limitless shapes, sizes, and features.
- Casement: a rectangular sash with a hinge that swings it open outward from the side
- Awning: a rectangular sash with a hinge that swings it open from the top to the outside
- Picture: a window that is fixed in place and does not open, usually rectangular, so that it frames the outside similar to a picture
- Double-hung: two rectangular sashes hung one over the other, but overlapping, so that each can be slid open or shut
- Single-hung: identical visually to a double-hung window, but only the bottom sash slides vertically to open and close
- Hopper: a rectangular sash with a hinge that swings it open from the lower section to the outside
- Horizontal sliding: two rectangular sashes mounted side-by-side, but overlapping, where one sash is fixed and the other slides horizontally
- Double horizontal sliding: three rectangular sashes mounted beside each other where the outer two can slide horizontally
- Bay and bow: typically a combination of three or more windows mounted in an architectural bay, with the center window unable to open
- Garden: a mix of windows that extend out from the exterior wall, typically in a kitchen, to create a small greenhouse-like space
- Dadius: an arched window that is fixed in place like a picture window
- Skylight: a fixed window in the roof set to allow in light from above
The transparent materials of a window allow natural light in while preventing the escape of air from the building, thereby improving ambience, slightly raising internal temperatures (via the greenhouse effect), and ventilating interiors, while also allowing operators to restrict rain, wind, and convection. Frames support glass frames to provide a functional surface to open and shut windows.
An important secondary function is the window's use as access to a fire escape or other emergency exit.
Windows have highly variable energy efficiency attributed to their ability to regulate convection, conduction, and radiance. Material insulation and transmittance is measured by R-value and U-Value, respectively. The National Fenestration Rating Council (NFRC) provides energy performance ratings for windows, and highly efficient windows carry the ENERGY STAR service mark.
Windows can be single- or double-paned, with double-paned windows having better energy performance. Double-paned windows often include a gas fill of argon or krypton, which resist heat flow better than air, between the two panes to improve thermal performance. Many also have heat-absorbing window glazing with special tints that change the color of the glass to reduce glare, and light and solace heat transmittance. Coatings, including low emissivity (low-e), reflective, and spectrally selective can be added to the surface of glass to alter these parameters. Highly advanced windows with transparent solar coatings can even produce their own electricity.
Windows are found in almost every inhabited structure; in fact building codes typically require windows based on building usage and size. Even in buildings that have no intention of ever opening a window, such as a hospital, windows are provided to improve ambience and morale.
The most relevant specifications for windows are their energy efficiency ratings, which are broken down into three categories by the NFRC:
- U-factor: the lower the U-factor, the better a window is at insulating the interior.
- Solar heat gain coefficient (SHGC): the lower the SHGC, the less solar heat transmitted into the interior.
- Visible transmittance (VT): the higher the VT, the more light that is allowed through the window.
The material used to create the overwhelming majority of window panes is glass. Frame materials are more diverse and include wood, aluminum and other metals, vinyl, fiberglass, composites, or other materials. Many wood frames get covered in cladding made of aluminum or vinyl. Recycled conent is often used for windows.
Style and energy attributes tend to be the two primary considerations when selecting a window. While the style and design of a window is an extremely personal choice generally driven by the aesthetic of a building, it is important to also consider the type of window based on the climate the building is in. For example, in warm climates, windows with a low SHGC that block out unwanted heat from the sun are preferable, but the opposite is true in cold climates. If the sun causes a high level of glare in a building, it might be important to select a window with a particular tint or coating to reduce VT or UV light, which discolors paints, furniture, etc.
Calibration and Standards
Installation of windows largely falls under building codes. In the U.S., this would be the International Building Code.