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 oftentimes air in the case of opening windows, into an interior space. Each unit of framed glass is often called a sash, and there can be multiple sashes in each window. While the materials used to make windows are relatively limited and their application fairly consistent and straightforward, the great diversity of buildings and climates has created a market for windows of nearly limitless shapes, sizes, and features.
The first glass windows appeared thousands of years ago. Within the last century, window technology has increased so rapidly with the use of ultra-strong, durable glass and frame materials that now entire skyscrapers and other massive buildings are covered with nothing on the exterior but windows. Windows have long been sought after for their qualities in helping save energy, but now technology advances with transparent coatings even allow windows to act as solar panels producing their own electricity.
In the case of custom and specialty windows, any window can be one of a kind, but the most commonly used forms of windows include:
- 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 vertically for efficient ventilation
- 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 open and shut horizontally
- double horizontal sliding - three rectangular sashes mounted beside each other where the outer two can slide open and shut horizontally
- bay and bow - typically, a combination of three rectangular windows with the outer two extending outward and the central window parallel but extended out from the wall
- garden - a mix of windows that extend out from the exterior wall, typically in a kitchen, to create a small greenhouse space
- radius - 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
How Windows Work
The transparent glass panes in a window allow natural light in while preventing the escape of air from the home, thereby helping regulate the indoor climate, and stopping other elements like rain, wind and extreme temperature. Window frames provide structural support for the glass panes and provide a more durable, functional surface to open and shut windows.
Besides the necessary and readily observable size, shape, design and functionality of windows, there are several key features that may be unnoticeable at first sight. Windows have highly variable energy efficiency attributes concerning their ability to transmit solar heat in from outside and to insulate the inside. 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 typically 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 the window's thermal performance. Many also have heat-absorbing window glazing with special tints that change the color of the glass to reduce glare, the amount of light entering and the amount of solar heat entering a room. Coatings including low emissivity (low-e), reflective and spectrally selective can be added to the surface of a pane of glass to alter a window's attributes with regard to light allowed in, keeping heat in or keeping solar radiation out. Highly advanced windows with transparent solar coatings can even produce their own electricity.
Depending on the type of material chosen, window frames also have important features to consider. Each frame material has varying levels of durability, requirements for upkeep and energy efficiency qualities.
A window can be used to let in natural light while protecting the interior space from elements like wind, rain and extreme temperature. Opening and closing windows can also be used for ventilation and climate control, although many windows are fixed and serve the purpose of providing an idyllic view to the outside world as much as anything. Fixed sunlight windows in the roof can do a stronger job of evenly lighting a room, particularly the center and parts further from exterior walls.
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 is allowed through the window
While the raw material used can differ widely, the post-fabrication material used to create the overwhelming majority of window panes is glass. Frame materials are more diverse and include wood, aluminum or other metals, vinyl, fiberglass or composite 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
Calibration and standards for windows depend on which type is being used. Large commercial windows have many different standards compared to standard residential windows, for example. The NFRC sets energy ratings standards for most windows. Consult manufacturer's guidelines to determine other calibration and standards requirements.