Absorptance : The ratio of radiant energy absorbed to total incident radiant energy in a glazing system.

Air infiltration : The amount of air that leaks in and out of a building through cracks in walls, windows and doors.

Air leakage rating : A measure of the rate of infiltration around a window or skylight in the presence of a specific pressure difference. It is expressed in units of cubic feet per minute per square foot of window area (cfm/sq ft) or cubic feet per minute per foot of window perimeter length (cfm/ft). The lower a window's air leakage rating, the better its air-tightness.

Argon : An inert, nontoxic gas used in insulating windows to reduce heat transfer.

BTU : An abbreviation for British Thermal Unit - the heat required to increase the temperature of one pound of water one degree Fahrenheit.

Casement : A window sash that swings open on side hinges; in-swinging are French in origin; out-swinging are from England.

Condensation : The deposit of water vapor from the air on any cold surface whose temperature is below the dew point, such as a cold window glass or frame that is exposed to humid indoor air.

Conduction : Heat transfer through a solid material by contact of one molecule to the next. Heat flows from a higher-temperature area to a lower-temperature one.

Convection : A heat transfer process involving motion in a fluid (such as air) caused by the difference in density of the fluid and the action of gravity. Convection affects heat transfer from the glass surface to room air, and between two panes of glass.

Desiccant : An extremely porous crystalline substance used to absorb moisture from within the sealed air space of an insulating glass unit.

Dew Point : The temperature at which water vapor in air will condense at a given state of humidity and pressure.

Double glazing : In general, two thicknesses of glass separated by an air space within an opening to improve insulation against heat transfer and/or sound transmission. In factory-made double glazing units, the air between the glass sheets is thoroughly dried and the space is sealed airtight, eliminating possible condensation and providing superior insulating properties.

Double-hung window : A window consisting of two sashes of glass operating in a rectangular frame, in which both the upper and lower halves can be slid up and down. A counterbalance mechanism usually holds the sash in place.

Edge effects : Two-dimensional heat transfer at the edge of a glazing unit due to the thermal properties of spacers and sealants.

Exterior stop : The removable glazing bead that holds the glass or panel in place when it is on the exterior side of the light or panel, in contrast to an interior stop located on the interior side of the glass.