Background Image




AAMA. American Architectural Manufacturers Association. A national trade association that establishes voluntary standards for the window, door, storefront, curtain wall, and skylight industries.

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

Acrylic. A thermoplastic with good weather resistance, shatter resistance, and optical clarity, used for glazing.

Aerogel. A microporous, transparent silicate foam used as a glazing cavity fill material, offering possible U-values below 0.10 BTU/(h-sq ft-°F) or 0.56 W/(sq m-°C).

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

Air-leakage (air infiltration).The amount of air leaking in and out of a building through cracks in walls, windows, and doors.

Air-leakage 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 (cfm/sq ft). Formerly expressed as cubic feet per minute per foot of window perimeter length (cfm/ft) but not now in use. The lower a window's air-leakage rating, the better its airtightness.

Annealed glass. Standard sheet of float glass which has not been heat-treated.

Annealing. Heating above the critical or recrystallization 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.

ANSI. American National Standards Institute. Clearing house for all types of standards and specifications.

Argon. An inert, nontoxic gas used in insulating glass units to reduce heat transfer.

ASHRAE. American Society of Heating, Refrigerating and Air Conditioning Engineers.

ASTM. American Society for Testing and Materials. Organization that sets standards for testing of materials.

Awning. Window similar to a casement except the sash is hinged at the top and always swings out.

Back to top


Balance. A mechanical device (normally spring-loaded) used in single- and double-hung windows as a means of counterbalancing the weight of the sash during opening and closing.

Bay window. An arrangement of three or more individual window units, attached so as to project from the building at various angles. In a three-unit bay, the center section is normally fixed, with the end panels operable as single-hung or casement windows.

Bead. A wood 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 a double-hung window. Also referred to as bead stop.

Blackbody. 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.

BOCA. Building Officials 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, commonly consisting of five sashes.

Brick molding. A standard milled wood trim piece that covers the gap between the window frame and masonry.

Btu (B.T.U.). An abbreviation for British Thermal Unit, the heat required to increase the temperature of one pound of water one degree Fahrenheit.

Back to top


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


Caulking. A mastic compound for filling joints and sealing cracks to prevent leakage of water and air, commonly made of silicone, bituminous, acrylic, or rubber-based material.

CFM. Cubic Feet per Minute.

Check rail. The bottom horizontal member of the upper sash and the top horizontal member of the lower sash which meet at the middle of a double-hung window.

Clerestory. 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 fiberglass element.




CRF. Condensation Resistance Factor. An indication of a window's ability to resist condensation. The higher the CRF, the less likely condensation is to occur. Based on AAMA standard.

Back to top


Degree day. A unit that represents a one-degree Fahrenheit deviation from some fixed reference point (usually 65° F) in the mean, daily outdoor temperature. See also heating degree day.


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

Divided light. A window with a number of smaller panes of glass separated and held in place by muntins.

DOE-2.1E. A building-simulation computer program used to calculate total annual energy use.


Double-hung window. A window consisting of two sashes 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.

Double-strength glass. Sheet glass between 0.115" and 0.133" (2.92–3.39 mm) thick.

Drip. A projecting fin or a groove at the outer edge of a sill, soffit, or other projecting member in a wall designed to interrupt the flow of water downward over the wall or inward across the soffit.

Back to top


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

Electrochromics. Glazing with optical properties that can be varied continuously from clear to dark with a low-voltage signal. Ions are reversibly injected or removed from an electrochromic material, causing the optical density to change.

Electromagnetic spectrum. Radiant energy over a broad range of wavelengths.

Emergency exit window. Fire escape window (egress window) large enough for a person to climb out. In U.S. building codes, each bedroom must be provided with an exit window. The exact width, area, and height from the floor are specified in the building codes.

Emittance. 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.

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.


Extrusion. The process of producing vinyl or aluminum shapes by forcing heated material through an orifice in a die. Also, any item made by this process.

Eyebrow windows. Low, inward-opening windows with a bottom-hinged sash. These attic windows built into the top molding of the house are sometimes called "lie-on-your-stomach" or "slave" windows. Often found on Greek Revival and Italianate houses.

Back to top


Fanlight. A half-circle window over a door or window, with radiating bars. Also called circle top transom.


Fiberglass. 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 and frame element.

Fixed light. A pane of glass installed directly into non-operating framing members; also, the opening or space for a pane of glass in a non-operating frame.

Fixed panel. An inoperable panel of a sliding glass door or slider window.

Fixed window. A window with no operating sashes.

Flashing. 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.

Fogging. A deposit of contamination left on the inside surface of a sealed insulating glass unit due to extremes of temperatures or failed seals.

Frame. The fixed frame of a window which holds the sash or casement as well as hardware.

Back to top




Glazing. The glass or plastic panes in a window, door, or skylight.

Glazing bead. A molding or stop around the inside of a window frame to hold the glass in place.


Back to top


Head track. The track provided at the head of a sliding glass door. Also, the head member incorporating the track.

Header. The upper horizontal member of a window frame. Also called head.

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. See also Tinted glass.

Heat gain. The transfer of heat from outside to inside by means of conduction, convection, and radiation through all surfaces of a house.

Heating degree day. Term used by heating and cooling engineers to relate the typical climate conditions of different areas to the amount of energy needed to heat and cool a building. The base temperature is 65 degrees Fahrenheit. A heating degree day is counted for each degree below 65 degrees reached by the average daily outside temperatures in the winter. For example, if on a given winter day, the daily average temperature outdoors is 30 degrees, then there are 35 degrees below the base temperature of 65 degrees. Thus, there are 35 heating degree days for that day.

Heat loss. The transfer of heat from inside to outside by means of conduction, convection, and radiation through all surfaces of a house.

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 (casement, awning, and hopper) with an operating sash that has hinges on one side. See also Projected window.

Hopper. Window with sash hinged at the bottom.

Horizontal slider. A window with a movable panel that slides horizontally.

Hybrid Tubular Daylighting Device (HTDD). A tubular daylighting device (TDD) whose light transmitting tube consists of more than one material and/or has more than one geometry throughout its length. Typically used with suspended ceilings or to illuminate spaces without ceilings.

Back to top


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.

Infiltration. See air leakage.

Infrared radiation. Invisible, electromagnetic radiation beyond red light on the spectrum, with wavelengths greater than 0.7 microns.

Insulated shutters. Insulating 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.

Insulating value. See U-factor.

Insulation. Construction materials used for protection from noise, heat, cold or fire.

Interlocker. 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 stile.

Back to top


Jalousie. Window made up of horizontally-mounted louvered glass slats that abut each other tightly when closed and rotate outward when cranked open.

Jamb. A vertical member at the side of a window frame, or the horizontal member at the top of the window frame, as in head jamb.


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

KWH. KiloWatt Hour. Unit of energy or work equal to one thousand watt-hours.

Back to top


Laminated glass. Two or more sheets of glass with an inner layer of transparent plastic to which the glass adheres if broken. Used for safety glazing and sound reduction.

Lift. Handle for raising the lower sash in a double-hung window. Also called sash lift.

Light. A window; a pane of glass within a window. Double-hung windows are designated by the number of lights in upper and lower sash, as in six-over-six. Also spelled informally lite.

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.

Lintel. 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.

Long-wave infrared radiation. Invisible radiation, beyond red light on the electromagnetic spectrum (above 3.5 micro meters), emitted by warm surfaces such as a body at room temperature radiating to a cold window surface.

Low-conductance spacers. An assembly of materials designed to reduce heat transfer at the edge of an insulating window. Spacers are placed between the panes of glass in a double- or triple-glazed window.

Low-emittance (Low-E) coating. Microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a window or skylight 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-wave infrared radiation) and reflective of long-wave infrared radiation.

Back to top


Meeting rail. The part of a sliding glass door, a sliding window, or a 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 elements.

Micron. One millionth (10-6) of a metric meter.

Mil. One thousandth of an inch, or 0.0254 millimeter.

Model Energy Code (MEC). The Model Energy Code 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).

Mullion. A major structural vertical or horizontal member between window units or sliding glass doors.

Muntin. A secondary framing member (horizontal, vertical, or diagonal) to hold the window panes in the sash. This term is often confused with mullion.

Muntin grilles. Wood, plastic, or metal grids designed for a single-light sash to give the appearance of muntins in a multilight sash, but removable for ease in cleaning the window.

Back to top


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.

NFRC. National Fenestration Rating Council.



Operable window. Window that can be opened for ventilation.

Operator. Crank-operated device for opening and closing casement or jalousie windows.

Back to top


Pane. One of the compartments of a door or window consisting of a single sheet of glass in a frame; also, a sheet of glass.

Panel. A major component of a sliding glass door, consisting of a light of glass in a frame installed within the main (or outer) frame of the door. A panel may be sliding or fixed.

Panning. 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 before installation.

Particle dispersed glazing. Glazing in which the orientation of small particles between two sheets of glass is controlled electrically, thus changing its optical properties.

Parting stop. A narrow strip, either integral or applied, that holds a sash or panel in position in a frame.

Peak load. The maximum thermal load to be provided by a heating or cooling system in a house.

Photochromics. 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- or quadruple-glazed window.

Plastics. 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 true flat parallel plane surfaces affording excellent vision. It has been replaced by float glass.

Polyvinylchloride (PVC). An extruded or molded plastic material used for window framing and as a thermal barrier for aluminum windows.

Projected window. A window fitted with one or more sashes opening on pivoted arms or hinges. Refers to casements, awnings, and hoppers.

Pyrolytic low-E. A low- E coating which typically uses tin oxide with some additives deposited directly onto a glass surface while it is still hot. The result is a baked-on surface layer that is hard and durable and thus sometimes referred to as a "hard coat." Pyrolytic coatings are typically used in insulated glass units with the low-E surface inside the sealed air space, but can also be applied to single-pane glass and separate storm windows.

Back to top


R-value. 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 than one with a low R-value.

Radiation. The transfer of heat in the form of electromagnetic waves from one separate surface to another. Energy from the sun reaches the earth by radiation, and a person's body can lose heat to a cold window or skylight surface in a similar way.

Rail. Horizontal member of a window sash.

Reflectance. The ratio of reflected radiant energy to incident radiant energy.

Reflective glass. Window glass coated to reflect radiation striking the surface of the glass.

Refraction. The deflection of a light ray from a straight path when it passes at an 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 that given temperature. At 100 percent relative humidity, moisture condenses and falls as rain.

Retrofitting. Adding or replacing items on existing buildings. Typical retrofit products are replacement doors and windows, insulation, storm windows, caulking, weatherstripping, vents, landscaping.

RESFEN. 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 placed in the sloping surface of a roof.

Rough opening. The opening in a wall into which a door or window is to be installed.

Back to top


Safety glass. A strengthened or reinforced glass that is less subject to breakage or splintering.

Sash. 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 frame into which the sash sections are fitted.


Sealant. 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 that only extremely small insects can pass through. Also called sun screen. Also, an awning with fixed louvers of metal or wood.

Shading coefficient (SC). A measure of the ability of a window or skylight to transmit solar heat, relative to that ability for 1/8-inch 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 by 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.

Sill. 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 bottom movable.

Single-strength glass. Glass with thickness between 0.085"and 0.100" (2.16–2.54 mm).

Skylight (operable or pivot). 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 and/or in grooves. Moving action is usually of rolling type (rather than sliding 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 absorb or reflect solar energy, thereby reducing solar gain.

Solar heat gain coefficient (SHGC). The fraction of solar radiation admitted through a window or skylight, 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 window assembly.

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 glazing. Glazing that is transparent to some wavelengths of the solar spectrum and reflective to others. Typical spectrally selective coatings are transparent to visible light and reflect short-wave and long-wave infrared as well as UV radiation. Spectrally selectivity can be achieved with low-E coatings and/or high-performance tints.

Stile. The upright or vertical edges of a door, window, or screen.

Sputtered low-E. A multi-layered low-E coating deposited on glass or plastic film. Typically consists of three primary layers with at least one layer of metal. Sputtered coatings often use one, two or three silver layers and must be protected from humidity and contact. For this reason, they are often referred to as "soft coat." Sputter coatings are typically used in insulated glass units with the low-E surface inside the sealed air space, but can also be applied to plastic and used in suspended films or retrofit window films.

Stool. The shelf-like board of the interior part of the window sill, against which the bottom rail of the sash closes.

Stop. The molding on the inside of a window frame against which the window sash closes; in the case of a double-hung window, the sash slides against the stop. Also called bead, side stop, window stop, and parting 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.

Superwindow. A window with a very low U-factor, typically less than 0.15, achieved through the use of multiple glazings, low-E coatings, and gas fills.

Switchable glazings. Glazings with optical properties that can be reversibly switched from clear to dark or reflective.

Back to top


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; is required as safety glazing in patio doors, entrance doors, side lights, and other hazardous locations. It cannot be recut after tempering.

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.

Thermochromics. Glazing with optical properties that can change in response to temperature changes.

Thermogram. An image of an object taken with an infrared camera that shows surface temperature variations.

Threshold. The member that lies at the bottom of a sliding glass door or swinging door; the sill of a doorway.

Tilt window. A single- or double-hung window whose operable sash can be tilted into the room for interior washability.

Tinted glass. Glass colored by incorporation of a mineral admixture. Any tinting reduces both visual and radiant transmittance.

Transmittance. The percentage of radiation that can pass through glazing. Transmittance can be defined for different types of light or energy, e.g., visible light transmittance, UV transmittance, or total solar energy transmittance.

Transom. A horizontal transverse beam or bar in a frame; a crosspiece separating a door or the like from a window or fanlight above it. Also, a window above a door or other window, built on and commonly hinged to a transom.

Transom window. The window sash located above a door. Also called transom light.

Triple glazing. Three panes of glass or plastic with two air spaces between.

Tubular Daylighting Device (TDD). A non-operable device primarily designed to transmit daylight from a roof surface to an interior ceiling surface via a tubular conduit. The device consists of an exterior glazed weathering surface, a light transmitting tube with a reflective inside surface and an interior sealing device, such as a translucent ceiling panel. See also Hybrid Tubular Daylighting Device.

Back to top


U-factor (U-value). A measure of the rate of non-solar heat loss or gain through a material or assembly. It is expressed in units of Btu/hr-sq ft-°F (US) or W/sq m-°K (European metric). Values are normally given for NFRC/ASHRAE winter conditions of 0° F (18° C) outdoor temperature, 70° F (21° C) indoor temperature, 15 mph wind, and no solar load. The U-factor may be expressed for the glass alone or the entire window, which includes the effect of the frame and the spacer materials. The lower the U-factor, the greater a window's resistance to heat flow and the better its insulating value.

To convert the U-factor from US (imperial/IP) to European (metric/SI), multiply the imperial number by 5.678. For example, If U=0.35 Btu/hr-sq ft-°F in imperial units, then 0.35*5.678 = 1.9873. The U-factor in metric units will be 1.9873 W/sq m-°K.

UBC. 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.

Vent. The movable framework or sash in a glazed window that is hinged or pivoted to swing open.

Vinyl. Polyvinyl chloride material, which can be both rigid or flexible, used for window frames.

Vinyl-clad window. 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 the glazing.

Back to top


Warm-edge technology. The use of low-conductance spacers to reduce heat transfer near the edge of insulated glazing.


Weep hole. A small opening in a wall or window sill member through which water may drain to the building exterior.

Window. A glazed opening in an external wall of a building; an entire unit consisting of a frame sash and glazing, and any operable elements.

Window hardware. Various devices and mechanisms for the window including catches, fasteners and locks, hinges, pivots, lifts and pulls, pulleys and sash weights, sash balances, and stays.