- Transom: A smaller window above a door or another window. A transom joint is also the horizontal joining area between two window units which are stacked one on top of the other.
- Triple glazing: A sash glazed with three lights of glass, enclosing two separate air spaces.
- True divided light: A term which refers to windows in which multiple individual panes of glass or lights are assembled in the sash using muntins.
- U-Factor: A measure of heat transmission through a wall or window. The lower the U-factor, the better the insulating value.
- Unison lock: A casement locking system which secures the window at two locking points by operation of one handle.
- Vapor barrier: A watertight material used to prevent the passage of moisture into or through floors, walls and ceilings.
- Venting unit: A window or door unit that opens or operates.
- Windload: Force exerted on a surface by moving air.
- Windowpane divider: A short bar used to separate glass in a sash into multiple lights. Also called a muntin or a grille.
