Fanlight: A semicircular or half-circle window above a door, usually inset with radiating glazing bars.

Fixed window: A window that cannot be opened, whose function is limited to allowing light to enter. Also known as picture window.

Fixed panel. An inoperable panel of a sliding glass door, single hung window or slider window.

Frame: The enclosure in which window sash or door panels are mounted.

French sliding door: A sliding door which has wider panel members around the glass.

Fogging: A deposit or film left on the inside surface of a sealed insulating glass unit due to extremes conditions, temperatures or failed seals.

Fusion-welded: Strong, weathertight method of heat-joining materials by melting them together with extreme heat resulting in the materials uniting into a one-piece unit