Key Terms

AIR INFILTRATION: The amount of air that passes between a window sash and frame. In windows it is measured in terms of cubic feet or air per minute, per square foot of area. The lower the number, the less air the window lets pass through.

ARGON GAS: An odorless, colorless, tasteless, non-toxic gas which is six times more dense than air. It is used to replace air between the glass panes to reduce temperature transfer.

AWNING: A top-hinged window that swings outward for ventilation.

BALANCE: A mechanical device spring loaded used in double hung windows as a means of balancing the weight of the sash during opening and closing.

BAY WINDOW: An angled combination of three windows that project out from the wall of the home. The windows are commonly joined at 30- or 45-degree angles.

BOW WINDOW: An angled combination of windows in 3-, 4- or 5-lite configurations. The windows are attached at 10-degree angles to project a more circular, arced appearance.

CASEMENT: A window with a side-hinged sash that opens outward for ventilation.

CAULKING: A mastic compound for filling joints and sealing cracks to prevent leakage of air and water. Commonly made of silicone or a rubber based material.

CONDUCTION: Energy transfer from one material to another by direct contact.

CONVECTION: Heat transfer by currents that flow from a warm surface to a colder one.

DAYLIGHT TRANSMITTANCE: The percentage of visible light that a glazing transmits through glass.

DOUBLE-HUNG WINDOW: A window that has two operable sash that can be raised from the bottom or lowered from the top for two levels of ventilation.

EFFICACY FACTOR: The ratio of Visible Transmission (TV) to Shading Coefficient (SC). K = TV/SC. Efficacy Factor is simple a ratio of Light to Heat. The higher the number, the more light per amount of heat. A low number means that either it is a dark glass or a lot of heat gets through. A superior, spectrally-selective glass will have a K>1.0. This may also be called Light to Heat Ratio (LHR) or Coolness Factor.

EGRESS: The act of leaving an enclosed space. In the window industry the term refers to the dimensions of the opening of a window or door (the horizontal and vertical clear distance). Established by building codes. The purpose for establishing minimum egress dimensions is to insure that in an emergency situation a person attempting to leave a building has adequate area to escape.

FENESTRATION: Openings in a building wall, such as windows and doors, designed to permit the passage of air, light, and people.

FUSION-WELDED: The process of joining materials by melting them together with extreme heat (over 500ºF), resulting in the materials uniting into a one-piece unit.

GEOMETRIC: Specially designed windows classified as either Straight line Geometrics such as rectangles, triangles, trapezoid, octagons, pentagons, etc., or Radius Geometrics which include Half-rounds, Quarter-rounds, Circles, Ellipses, Eyebrows, etc.

GLASS: An inorganic transparent material composed of sand (silica), soda (sodium bicarbonate), and lime (calcium carbonate) with small quantities of alumina, boric or magnesia oxides.

GLAZING: The process of sealing the glass to the sash.

GLIDER WINDOW: A window unit operating horizontally. Typically consisting of two sash, with one sash operable. Unit may include one or more locking devices to secure the sash in the closed position. One sash must remain stationary.

GRILLES/GRIDS: Decorative horizontal or vertical bars installed between the glass panes to create the appearance of the sash being dividing into smaller lites of glass.

SUSPENDED FILM: A high performance soft-coat low-e sputtered directly on an optically clear substrate then suspended within the airspace of an insulated glass unit.

I.G. UNIT (INSULATING GLASS UNIT): Two or more lites of glass separated by spacers with or without suspended films that are hermetically sealed at the glass edges.

JAMBS: Vertical members of a window or doors outside frame.

KEEPER RAIL: The horizontal section of the sash where the keeper is attached.

KRYPTON GAS: An inert, odorless, colorless, tasteless, non-toxic gas which is about 12 times denser than air. It is used to replace air between the glass panes to reduce temperature transfer and deter convection.

LIFT RAIL: A handhold for raising and lowering the sash. Rail implies that the handhold is continuous across the sash.

LOW EMISSIVITY COATINGS: Coatings applied which allow shortwave energy to be transmitted through glass, but reflect long wave infrared radiation. The lower the emissivity, the lower the resultant U-value.

MEETING RAIL: The horizontal sections of a pair of sash that meet when the sash are closed.

NAILING FIN: An extrusion attached to the main frame of a window used to secure the unit to the rough opening. Used in new construction.

OBSCURE GLASS: Glass that has been made translucent instead of transparent.

OPERATING FORCE: The forces required to maintain sash or panel motion in either the initial opening or closing direction.

PANEL: A part of a fenestration product composed of a light of glass surrounded by a door frame. Similar to a sash.

PATIO DOOR: A glass door that slides open and close on adjustable tandem rollers. Available in 2- or 3-lite configurations with the operable panel available in any position.

PICTURE WINDOW: A non-operating window unit. A window consisting of frame, sash, and glass, with no hardware.

PYROLYTIC LOW-E: Glass with a coating, either visually reflective or low emissivity, applied at high temperatures and fired into the glass surface during float glass production.

R-VALUE: Measures the insulation effectiveness of the window. The R-value equals one divided by the U-value (R=1/U). The higher the R-value, the better the insulating performance.

REINFORCEMENT: Material added to individual sash or frame members to increase strength or stiffness.

RELATIVE HEAT GAIN (RHG): A number which represents the amount of heat entering a building due to both solar radiation and conductive/convective heat gain. It is the total heat flow through the glass for a specific set of conditions. This value considers indoor/outdoor temperature differences and the effect of solar radiation (Btu's/hr/ft2).

ROUGH OPENING: The opening in a wall into which a door or window is to be installed. Also known as shim space.

SASH: The portion of a window which includes the glass and framing sections which are directly attached to the glass. Typically the operating section of the window unit, however sash may also be stationary. See also – Picture Window.

SCREEN: A product used with a window or door consisting of a four--sided frame surrounding a fiberglass mesh designed to keep insects out.

SEALANT: A compound used to fill and seal a joint or opening. Also the material used to seal edges of insulated glass

SHADING COEFFICIENT (SC): Measures the total solar heat gain through the glazing compared to 1/8" clear glass under the same conditions. It includes both the solar energy transmitted directly plus any absorbed solar radiation that is re-radiated as heat into the interior. The number ranges from 0.0 to 1.0. Standard insulating glass has a SC of .81. The lower the shading coefficient, the lower the solar heat gain.

SIDELIGHT: Fixed (non-venting) frame of glass beside a window or door.

SINGLE HUNG: A window in which one sash slides vertically and the other sash is fixed.

SLIDER WINDOW: A window in which the sash move horizontally. Sliders are available in a 2- or 3-lite configuration, with the 3-lite having operable end vents.

SLOPED SILL: The sill of the window that has a downward slope to the outside. This sill has sufficient degree of slope to aid in water runoff.

SOFT COAT LOW-E: Glass with a coating applied in a secondary process at ambient temperature within a vacuum chamber.

SOLAR HEAT GAIN COEFFICIENT (SHGC): The percent of solar radiation that actually enters a building through a window as heat gain. A high (up to 1.00) coefficient means high solar heat gain. The smaller the number, the better the product is at preventing solar heat gain. If shading coefficient values are required, take SHGC value and divide by 0.87.

SPACER: Structural member used to separate glass panels in an insulating glass unit.

TEMPERED GLASS: Glass manufactured to withstand greater-than-normal forces on its surface. It is approximately five times stronger than annealed glass. When it breaks, it shatters into small pieces to reduce hazard. Tempered glass cannot be cut after tempering.

TILT-IN/LIFT-OUT SASH: A sash that can be tilted to the interior and removed for cleaning and is manufactured by welding.

U-VALUE: Measures the heat loss or gain due to differences between indoor and outdoor air temperatures (Btu's/hr/ft2). The U-value equals one divided by the R-value

(U=1/R). The lower the U-value, the better the insulating performance. U-value is particularly important during the heating season because the temperature difference is greater between inside and outside.

UV BLOCK: The percent of ultraviolet rays blocked from being transmitted through the glass. The higher the number the lower the percentage of ultraviolet rays transmitted through the window.

VISIBLE LIGHT TRANSMITTANCE (VT): The percentage of light that is transmitted through glass in the visible light spectrum (380 to 720 nanometers). The higher the number the higher the percentage of visible light transmitted through the window.

WDMA: Window and Door Manufacturers Association.

WEATHERSTRIPPING: Material used to form a weather-resistant seal around operable sash.