Educate yourself with the terms commonly used in the window and door industry.


American Architectural Manufacturers Association. A national trade association of voluntary standards for the window, door storefront, curtain wall and skylight industry.
The ratio of radiant energy absorbed to total incident radiant energy in a glazing system
A thermoplastic with good weather resistance, shatter resistance and optical clarity, used for glazing
A transparent silicate foam used as a glazing cavity fill material, offering possible U values below 0.10 BTU.
Air Infiltration:
The amount of air leaking in and out of a building through cracks in walls, windows, and doors.
Air Leaking Rating:
A Measure of the rate of air leakage around a window, door or skylight in the presence of a specific pressure difference. It is expressed in units of cubic feet per minute per square foot of frame area. Formerly expressed as cubic feet per minute per foot of window perimeter length but not now in use. The lower a window’s air leaking rating the better the performance of the window against air infiltration.
Annealed Glass:
Standard sheet of float glass which has not been heat treated.
Heating above the critical or recrystallation temperature, then controlled cooling of metal glass, or other materials to eliminate the effects of cold-working, relieve internal stresses, or improve strength, ductility, or other properties.
American National Standards Institute. Clearing house for all types of standards and specifications.
Argon Gas:
An inert, nontoxic gas used in insulating glass units to reduce heat transfer.
Awning Window:
Window that is hinged at the top and projects out.


A mechanical device (usually spring loaded) used in single and double hung windows as a means of counterbalancing the weight of the sash during operation.
Bay Window:
An arrangement of three or more individual window units, attached so as to project from the building at various angles.
Bead Stop:
A strip against which a swinging sash closes, as in a casement window. Also a finishing trim at the sides and top of the frame to hold the sash, as in a fixed sash or double hung window.
The ideal, perfect emitter and absorber of thermal radiation. It emits radiant energy at each wavelength at the maximum rate possible as a consequence of its temperature, and absorbs all incident radiance.
Building Official and Code Administrators.
Bottom Rail:
The bottom horizontal member of a window sash.
Bow Window:
A rounded bay window that projects from the wall in an arc shape.
Brick Molding:
A standard milled wood trim piece that covers the gap between the window frame and masonry.


A window sash that projects out and swings on the side
Exposed molding or framing around a window or door, on either the interior or exterior, to cover the space between the window frame, jamb and the wall.
A mastic compound for filling joints and sealing cracks to prevent air and water leaking. Compound is primarily made of silicone, bituminous, acrylic, or rubber based material.
Cubic feet per minute.
Check Rail:
the bottom horizontal member of the upper sash and the top of the horizontal member of the lower sash which meet at the middle of the hung window.
A window in the upper part of a lofty room that admits light to the center of the room.
Composite Frame:
A frame consisting of two or more materials. For example, an interior wood element with an exterior aluminum element.
The deposit of water vapor from the air on any cold surfaces whose temperature is below the dew point, such as cold window glass or frame that is exposed to humid indoor air.
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 surface.
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.
Condensation Resistance Factor an indication of a window and door’s ability to resist condensation. The higher the CRF, the less likely it is for condensation to occur.


Degree Day:
A unit that represents a one degree Fahrenheit deviation from some fixed reference point.
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.
Divided Light:
A window within a number of smaller panes of glass separated and held in place by muntins.
A building simulation computer program used to calculate total annual energy use.
Double Glazing:
In general, two thickness of glass separated by an air space within an opening to improve insulation against heat transfer and sound transmission.
Double Hung Window:
A window consisting of two operating sashes in a rectangular frame. Both sashes can be moved up and down and in some cases may tilt in for easy cleaning.
Double Strength Glass:
Sheet glass between 0.115” and 0.133” thick


Edge Effects:
Two dimensional heat transfer at the edge of a glazing unit due to thermal properties of spacers and sealants.
Egress Window:
Fire escape window large enough for a person to climb out. Building Code requires each bedroom to have one egress window.
The ratio of the radiant flux emitted by a specimen to that emitted by a blackbody at the same temperature and under the same conditions.
Energy Star:
Program initiated by the government to help individuals and businesses protect the environment through the purchase of energy efficient products.
Evacuated Glazing:
Insulating glazing composed of two glass layers, hermetically sealed at the edges, with a vacuum between to eliminate convection and conduction. A spacer system is needed to keep the panes from touching.
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.
The process of producing vinyl or aluminum shapes by forcing heated material through an orifice in a die.
Eyebrow Window:
Low inward opening windows with a bottom hinged sash.


A half circle window over a door or window.
The placement of window openings in a building wall, one of the important elements in controlling the exterior appearance of a building.
A composite material made by embedding glass fibers in a polymer matrix. May be used as a diffusing material in sheet form, or as a standard sash in frame element.
Fixed Light:
A pane of glass installed directly into a non operating frame.
Fixed Panel:
An inoperable panel of a sliding glass door or sliding window.
Fixed Window:
A window with no operating sashes.
Sheet metal or other material applied to seal and protect the joints formed by different materials or surfaces.
Float Glass:
Glass formed by a process of floating the material on a bed of molten metal. It produces a high optical quality glass with parallel surfaces, without polishing and grinding.
A deposit of contamination left on the inside surface of a sealed insulated glass unit due to extremes of temperature or failed seals.
The fixed frame of a window which holds the sash or casement as well as hardware.


Gas Fill:
A gas other then air, usually argon or krypton, placed between window glazing panes to reduce the U-factor by suppressing conduction and convection.
An inorganic transparent material composed of silica (sand) soda (sodium carbonate), and lime (calcium carbonate) with small quantities of alumina, boric or magnesia oxides.
The glass or plastic panes in a window, door or skylight.
Glazing Bead:
A molding or strip around inside of a window frame to secure the glass in place.
Greenhouse Window:
A three dimensional window that projects from the exterior wall and usually has glazing on all sides except the bottom.


Head Track:
The track provided at the head of a sliding glass door. Also, the head member incorporating the track
The upper horizontal member of the window frame.
Heat Absorbing Glass:
Window glass containing chemicals (with gray, bronze, or blue-green tint) which absorb light and heat radiation, and reduce glare and brightness.
Heat Gain:
The transfer of heat from outside to inside by means of conduction, convection and radiation through all surfaces of a building structure.
Heat Loss:
The transfer of heat from inside to outside by means of conduction, convection and radiation.
Heat Strengthened Glass:
Glass that is reheated, after forming, to just below melting point and then cooled, forming a compressed surface that increases its strength beyond that of typical annealed glass.
Hinged Windows:
Windows that include casements, and awning windows with an operating sash that has hinges on one side.
Horizontal Slider:
A window with a moveable panel that slides horizontally.


ICC (International Code Council):
A national organization that publishes model codes for adoption by states and other agencies. Codes include the International Building Code (IBC) and the International Energy Conservation Code (IECC)
IECC (International Energy Conservation Code):
Published by the ICC. The successor to the Model Energy Code, which is cited in the 1992 U.S. Energy Policy Act (EPAct) as the baseline for residential Energy Codes in the United States.
See air leakage.
Infrared Radiation:
Invisible, electromagnetic radiation beyond red light on the spectrum, with wavelengths greater than 0.7 microns.
Insulated Shutters:
Insulated panels that cover a window opening to reduce heat loss.
Insulating Glass:
Two or more pieces of glass spaced apart and hermetically sealed to form a single glazed unit with one or more air spaces in between. Also called double glazing.
Insulated Value:
See U-Factor

Construction materials used for protection from noise, heat cold or fire.
An upright frame member of a panel in a sliding glass door which engages with a corresponding member in an adjacent panel when the door is closed. Also called interlocking site.


Window made of horizontally-mounted louvered glass slats that about each other tightly when closed and rotates outward when cranked open.
A vertical member at the side of a window or door frame.


An inert, non toxic gas used in insulated windows to reduce heat transfer.
Kilo Watt Hour. Unit of energy or work equal to one thousand watt hours.


Laminated Glass:
Two sheets or more of glass with an inner layer of transparent plastic to which the glass adheres if broken. Used for safety glazing and sound reduction.
Handle for raising the lower sash in a hung window.
A window; pane of glass within a window.
Light to Solar Gain Ratio:
A measure of the ability of a glazing to provide light without excessive solar heat gain. It is the ratio between the visible transmittance of a glazing and its solar heat gain coefficient. Abbreviated LSG.
A horizontal member above a window or door opening that supports the structure above.
Liquid Crystal Glazing:
Glass in which the optical properties of a thin layer of liquid crystals are controlled by an electrical current, changing from a clear to a diffusing state.
Low Conductance Spacers:
An assembly of materials designed to reduce heat transfer at the edge of an insulated window. Spacers are placed between the panes of glass in a double or triple glazed window.
Low Remittance Coating:
More commonly know as Low E. Microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or glazing surface primarily to reduce the U-factor by suppressing radiative heat flow. A typical type of Low-E coating is transparent to the solar spectrum (visible light and short waved infrared radiation) and reflective of long-waved infrared radiation.


Meeting Rail:
The part of a sliding glass door a sliding window, hung window where two panels meet and create a weather barrier.
Metal Clad Windows:
Exterior wood parts covered with extruded aluminum or other metal, with a factory applied finish to deter the harsh elements.
Model Energy Code:
The Model Energy Code (MEC) is cited in the 1992 U.S. Energy Policy Act (EPAct) as the baseline for residential energy codes in the United States. It has been succeeded by the International Energy Conservation Code (IECC) published by the International Code Council (ICC).
A major structural vertical or horizontal member between window and door units.
A secondary framing member (horizontal, vertical or diagonal) to hold the window panes in the sash.
Muntin Grilles:
Wood, plastic, vinyl or aluminum grids designed for a single-light sash to give the appearance of muntins in a multilight sash.


Nailing Fin:
An integral extension of a window or patio door frame which generally laps over the conventional stud construction and through which nails are driven to secure the frame in place.
National Fenestration Rating Council. Established to help consumers compare various window and door products.


Obscure Glass:
Any textured glass (frosted, etched, fluted, ground, etc.) used for privacy, light diffusion or decorative effects.
Operable Window:
Window that can be opened for ventilation.
Crank-operated device for opening and closing casement or awning windows.


One of the components of a door or window consisting of a single sheet of glass in a frame; also a sheet of glass.
A major component of a sliding glass door, consisting of a light of glass in a frame installed within the frame of the door. A panel may be operable or fixed.
In replacement window work, the outside aluminum trim that can extend around the perimeter of the window opening; used to cover up the old window material. Panning can be installed in the opening before the window or can be attached directly to the window prior to window installation.
Particle Dispersed Glazing:
Glazing in which the orientation of small particles between two sheets of glass is controlled electrically, this changing the optical properties.
Parting Stop:
A narrow strip, either integral or applied, that holds a sash or panel position in a frame.
Peak Load:
The maximum thermal load to be provided by a heating or cooling system in a house. Glazing with the optical properties that change in response to the amount of incident light.
Picture Window:
A large, fixed window framed so that it is usually, but not always longer horizontally than vertically to provide a panoramic view.
Pivot Window:
A window with a sash that swings open or shut by revolving on pivots at either side of the sash or at top and bottom.
Plastic Film:
A thin plastic substrate, sometimes used as the inner layers in a triple glazed window.
Artificial substances made of organic polymers that can be extruded or molded into various shapes including window frames and sashes.
Plate Glass:
A rolled, ground, and polished product with truly flat parallel plane surfaces affording excellent vision. It has been replaced by float glass.
Otherwise know as PVC. An extruded or molded plastic material used for window framing and as a thermal barrier for aluminum products.
Projected Window:
A window fitted with one or more sashes opening on pivoted arms or hinges. Refers to casements, and awning windows.


A measure of the resistance of a glazing material or fenestration assembly to heat flow. It is the inverse of the U-factor (R=1/U) and is expressed in units of hr-sq ft-F/Btu. A high R-value window has a greater resistance to heat flow and a higher insulating value then one with a lower R-value.
The transfer of heat in the form of electromagnetic waves from one separate surface to another. Energy from the sun reaches earth by radiation, and a person’s body can lose heat to a cold window or surface in a similar way.
Horizontal member of a window sash.
The ratio of reflected radiant energy to incident radiant energy.
Reflective Glass:
Window glass coated to reflect radiation striking the surface of the glass.
The deflection of a light ray from a straight path when it passes the oblique angle from one medium (such as air) to another (such as glass).
Relative Humidity:
The percentage of moisture in the air in relationship to the amount of moisture the air could hold at the given temperature. At 100 percent humidity, moisture condenses and falls as rain.
Adding or replacing items on existing buildings. Typical retrofit products include replacement doors and windows, insulation, and accordion shutters.
A computer program used to calculate energy use based on window selection in residential buildings.
Roof Window:
A fixed or operable window similar to a skylight.
Rough Opening:
The opening in a wall into which a door or window is to be installed.


Safety Glass:
A strengthened or reinforced glass that is less subject to breakage or splintering.
The portion of a window that includes the glass and the framing sections directly attached to the glass, not to be confused with the complete window frame into which the sash sections are fitted.
Woven mesh of metal, plastic, or fiberglass stretched over a window opening to permit air to pass through, but not insects.
A compressible plastic material used to seal any opening or junction of two parts, such as between the glass and a metal sash, commonly made of silicone, butyl tape, or polysulfide.
Shade Screen:
A specially fabricated screen of sheet material with small narrow louvers formed in place to intercept solar radiation striking a window; the louvers are so small insects cannot pass through. Also referred to as a sun screen.
Shading Coefficient (SC):
A measure of the ability of a window or skylight to transmit solar heat, relative to that ability for 1/8” clear, double strength, single glass. It is being phased out in favor of the solar heat gain coefficient, and is approximately equal to the SHGC multiplied by 1.15. It is expressed as a number without units between 0 and 1. The lower a window’s solar heat gain coefficient or shading coefficient, the less solar heat it transmits, and the greater is its shading ability.
Sheet Glass:
A transparent, flat glass found in older windows, now largely replaced float glass.
Short-wave Infrared Radiation:
Invisible radiation, just beyond red light on the electromagnetic spectrum (between 0.7 and 2.5 microns), emitted by hot surfaces and included in solar radiation.
The lowest horizontal member in a door, window or sash frame.
Sill Track:
The track provided at the sill of a sliding glass door. Also, the sill member incorporating such a track.
Simulated Divided Lights:
A window that has the appearance of a number of smaller panes of glass separated by muntins, but actually is a larger glazing unit with the muntins placed between or on the surfaces of the glass layers.
Single Glazing:
Single thickness of glass in a window or door.
Single-Hung Window:
A window consisting of two sashes of glass, the top one stationary and the moveable.
Single Strength Glass:
Glass with thickness between 0.085” and 0.100” (2.162.57 mm).
A roof window that gives light and ventilation.
Sliding Glass Door:
A door fitted with one or more panels that move horizontally on a track. Moving action is usually of rolling type. Also called gliding door, rolling glass door, and patio sliding door.
Sliding Window:
A window fitted with one or more sashes opening by sliding horizontally or vertically in grooves provided by frame members. Vertical sliders may be single or double hung.
Smart Window:
Generic term for windows with switchable coatings to control solar gain.
Solar Control Coatings:
Thin film coatings on glass or plastic that absorbs or reflect solar energy, thereby reducing solar gain.
Solar Heat Gain Coefficient:
(SHGC) The fraction of solar radiation admitted through a window both directly transmitted, and absorbed and subsequently released inward. The solar heat gain coefficient has replaced the shading coefficient as the standard indicator of a window’s shading ability. It is expressed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the less solar heat it transmits, and the greater its shading ability. SHGC can be expressed in terms of the glass alone or can refer to the entire frame.
Solar Radiation:
The total radiant energy from the sun, including ultraviolet and infrared wave lengths as well as visible light.
Solar Screen:
A sun shading device, such as screens, panels, louvers, or blinds installed to intercept solar radiation.
Solar Spectrum:
The intensity variation of sunlight across its spectral range.
Sound Transmission Class:
(STC) The sound transmission loss rating of a material over a selected range of sound frequencies. The higher the number, the less sound transmitted.
Spectrally Selective Coatings:
A coated or tinted glazing with optical properties that is transparent to some wavelengths of energy and reflective to others. Typical spectrally selective coatings are transparent to visible light and reflect short-wave and long-wave infrared radiation.
The upright or vertical edges of a door, window or transom.
The shelf-like board of the interior part of the window sill, against which the bottom rail of the sash closes.
Also called bead, return, side stop, window stop and painting stop.
Storm Windows:
A second set of windows installed on the outside or inside of the primary windows to provide additional insulation and wind protection.
Sun Control Film:
A tinted or reflective film applied to the glazing surface to reduce visible, ultra violet, or total transmission of solar radiation. Reduces solar heat gain in summer and glare. Some can be removed and reapplied with changing seasons.
A window with a very low U-Factor, typically less then .15, achieved through the use of multiple glazings, Low E coatings, and gas fills.


Tempered Glass:
Treated glass that is strengthened by reheating it to just below the melting point and then suddenly cooling it. When shattered, it breaks into small pieces. Approximately five times stronger than standard annealed glass; it is required as safety glazing in patio doors, entrance doors, side lights, and other hazardous locations.
Thermal Break:
An element of low conductance placed between elements of higher conductance to reduce the flow of heat. Often used in aluminum windows.
Thermal Expansion:
Change in dimension of a material as a result of temperature change.
Thermal Mass:
Mass in a building (furnishings or structure) that is used to absorb solar gain during the day and release the heat as the space cools in the evening.
Glazing with optical properties that can change in response to temperature changes.
An image of an object taken with an infrared camera that shows surface temperature variations.
The member that lies on the bottom of a door.
Tilt Window:
A single or double hung window whose operable sash can be tilted into the room for interior cleaning and wash ability.
Tinted Glass:
Glass colored by incorporation of a mineral admixture. Any tinting reduces both visual and radiant transmittance.
The percentage of radiation that can pass through glazing. Transmittance can be defined for different types of light or energy. Examples include visible light transmittance, UV transmittance or total energy transmittance.
A horizontal transverse beam or bar in a frame; a crosspiece separating a door from a window.
Transom Window:
The window sash located above another window or door.
Triple Glazing:
Three panes of glass or suspended film with two air spaces between.


(U-Value) A measure of the rate of non-solar heat loss or gain through a material or assembly. It is expressed in units Btu/hr-sq-ft-F (W/sq M-C). Values are normally given for NFRC/ASHRAE winter conditions of 0 degrees outdoor temperature 70 degrees indoor temperature, 15MPH wind, and no solar load. The U-factor may be expressed for the glass alone or the entire window. The lower the U-factor, the greater a window’s resistance to heat flow and the better its insulating value.
Uniform Building Code
Ultraviolet Light:
(UV) The invisible rays of the spectrum that are outside of the visible spectrum at its short-wavelength violet end. Ultraviolet rays are found in everyday sunlight and can cause fading of paint finishes, carpets and fabrics.


Vapor retarder:
A material that reduces the diffusion of water vapor across a building assembly.
The moveable framework or sash in a glazed window that is hinged or pivoted to swing open.
Polyvinyl Chloride material, which can be both rigid and flexible, used for window and door frames.
Vinyl Clad Windows:
A window with exterior wood parts covered with extruded vinyl.
Visible Light:
The portion of the electromagnetic spectrum that produces light that can be seen. Wavelengths range from 380 to 720 nanometers.
Visible Transmittance:
(VT) The percentage or fraction of the visible spectrum (380 to 720 nanometers) weighted by the sensitivity of the eye, that is transmitted through glazing.


Warm-Edge Technology:
The use of a low conductive spacer system to reduce heat transfer near the edge of the insulated glazing unit.
A strip of resilient material for covering the joints between the window sash and frame in order to reduce air leaks and prevent water intrusion.
Weep Hole:
A small opening in a window sill through which water may drain away from the building.
A glazed opening in an external wall of a building; an entire unit consisting of a frame, sash and glazing.
Window Hardware:
Various devices and mechanisms used for the operation of a window or door.

Industry definitions by The Efficient Window Collaborative.