Window

A window is an opening in a wall or vehicle that allows light and air to enter, typically fitted with glass and surrounded by a frame. It may also provide a view, contribute to the building's aesthetics, and help regulate temperature and moisture when paired with insulation and weatherstripping.

Windows evolved from simple openings sheltered by shutters or coverings to glass panes in later centuries. The use of transparent glass expanded in medieval and early modern Europe, and by the 19th century manufacturing processes enabled larger and more panes, improving insulation, daylighting and weather protection.

Common configurations include fixed panes and operable types such as casement windows, sash windows (with movable or hung panels), double-hung or sliding sash windows, bay and bow windows, and skylights. Special forms address safety, egress, or architectural needs, and some employ multi-layer glazing for energy efficiency.

Frames are made from wood, vinyl or uPVC, aluminum or steel, and may incorporate composite materials. Glass may be plain, tempered or laminated; multiple panes separated by spacers form insulated glazing units. Additional components include sashes or panels, muntins and mullions for division, weatherstripping and seals, glazing beads, and hardware for operation.

Window performance is described by U-value, solar heat gain coefficient (SHGC), visible transmittance, and air leakage. Building codes and energy standards drive required performance, with many jurisdictions encouraging or mandating double or triple glazing, low-emissivity coatings, and airtight frames. Safety glass and compliant hardware are used in appropriate locations.

In architecture, windows influence daylight, views and ventilation, while selection affects energy efficiency and acoustics. Privacy and glare can be managed with coatings, films, blinds or shades. Regular maintenance includes cleaning, seal replacement and weatherstripping checks to prevent leaks and drafts.