GLOSSARY OF WINDOW TERMS The Learning Center Air Leakage Rating A measure of the rate of infiltration around a window or a skylight in the presence of a strong wind, expressed in units of cubic feet per minute per foot. Airspace The space in the cavity between two panes of glass in an insulated glass unit. Argon An inert, nontoxic gas placed between glass panes in insulated windows to improve the insulating value of sealed glass units. Awning Window This style of window is hinged at the top, swinging out a few inches from the bottom. Closely related to a Hopper Window and a Casement Window. Air Leakage Rating - BTU Balance System This is the mechanism in a window that provides force to the bottom half of the window so it is easy to move it up and down. Older windows have a weight on a line that is hidden in the cavity of the adjacent wall. Modern systems use either constant force spiral springs or very small block and tackle arrangements. Bay Window A bay window protrudes outward from the wall in which it's installed. It is usually constructed from one fixed central pane to which two casement panels are attached at an angle. BTU An abbreviation for British thermal unit - a standard measure of the amount of energy required to raise one pound of water by one degree Fahrenheit. main menu       next > 3245 Miracle Drive Murrysville, PA 15668 724-387-2991 PA Contractor ID PA001856