Window Terms

Aluminum-Clad Window
A window with wood construction covered with aluminum sheet having a factory-applied finish (to provide a longer maintenance-free life).

Angle Bay Window
See Bay Window.

The molded frame or ornament surrounding a window, door or other rectangular opening.

American Society of Heating, Refrigerating & Air Conditioning Engineers, Inc., 1791 Tullie Circle, N.E. Atlanta, GA 30329. Toll-free for Customer Service: (800) 527-4723 (U.S. and Canada only). Phone: (404) 636-8400. Fax: (404) 321-5478

American Society for Testing and Materials, ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA, 19428-2959 USA. Phone: (610) 832-9585. Fax: (610) 832-9555.

Astragal An interior molding attached to one of a pair of doors or side-hinged windows in order to prevent swing through; also used with sliding doors to ensure a tighter fit where the doors meet; often found on older casements or swinging screens.

Awning Window
A top-hinged sash. See Jal-Awning Window. Introduced in the 1950s.

See Sash Balance.

See Muntin.

Basement Window (Basement Sash, Cellar Sash)
A wood or metal in-swinging sash that is hinged at either the top or the bottom.

Bay Window
Windows that project out from the wall and extend to the ground. An Angle Bay Window refers to the angle departure from the plane of the wall. See also Bow Window.

Bottom Rail
The bottom horizontal member of a window sash.

Bow Window (Compass Window, Radial Bay Window)
A rounded bay window that projects from the wall in the shape of an arc; commonly consisting of five sash. See also Bay Window.

Boxed Mullion
A hollow mullion between two double-hung windows to hold the sash weight.

Brick Molding
A standard milled wood.

B.T.U. (Btu)
An abbreviation for British Thermal Unit, commonly shown as “Btu”; the heat required to increase the temperature of one pound of water one degree Fahrenheit, about the heat from burning one wooden match.

Butyl Tape
See Sealant.

A window sash which swings open on side hinges; in-swinging are French in origin while out-swinging are from England.

Casement Window
A window with one or more casements.

Casing (Trim)
Exposed molding or framing around a window or door, on either the inside or outside, to cover the space between the window frame or jamb and the wall.

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 materials.

A centigrade scale of temperature measurement based on 0° as the freezing point and 100° as the boiling point of water. Abbreviated °C.

Check Rail (Meeting Rail, Lock Rail)
The horizontal members (of a double-hung window) which come together.

Circle Top Transom
See Fanlight.

Clerestory (Clearstory, High-Light Window)
A window in the upper part of a lofty room that admits light to the center of the room.

Coated Glass
A window glass with an outside surface provided with a mirror reflective surface; the shading coefficient ranges from 20% to 45%. See Shading Coefficient.

An architectural style associated with an early American period; Early American style c. 1730.

Colonial Windows
Windows with small rectangular panes, or divided lites, designated as 12-lite, 16-lite, etc.

Combination Window Unit (Combination Storm Sash and Screen)
A window assembly containing a half screen and two glass storm panel; in frame, exposing the screen panel.

Commercial Standard
A voluntary set of rules and regulations covering quality of product (or installation), method of testing, rating of the product, certification, and labeling of manufactured products.

The deposit of water vapor from the air on any cold surface whose temperature is below the dew point, such as a window glass or frame that is exposed to cold outdoor air. See Dew Point Temperature.

See Natural Convection and Forced Convection. A heat transfer process resulting from the circulation or movement of fluids, such as air.

Cove Molding
Trim molding with a concave face.

Crack Perimeter
The total length of the crack around a sash through which outdoor air could leak into the room. In a double-hung window, the total crackage is 3 times the width plus 2 times the height of the sash.

A rectangular slot or groove (with 3 surfaces) cut across the grain of a wood member, into which another board is fitted. See also Plough.

A measure of heating demand, based on the difference between the mean daily outdoor temperature and 65°F. Cumulative totals for the month or heating season are used by engineers for estimating heating energy requirements.

Design Heat Loss
The calculated values, expressed in units of Btu per hour (abbreviated Btuh), for the heat transmitted from a warm interior to a cold outdoor condition, under some prescribed extreme weather conditions. The values are useful for selecting heating equipment and for estimating seasonal energy requirements. Infiltration heat loss is a part of the design heat loss.

A drying agent, such as silica gel, used by some manufacturers between the panes of insulating glass to prevent fogging between the panes.

Dew Point Temperature
The temperature of the air at which the water vapor in the air starts to condense in the form of liquid or as frost.

Double-Hung Window
Window with two vertically moving sashes, each closing a different part of the window.

Double-Strength Glass
Sheet glass with a thickness between 0.115” to 0.133” (3 to 3.38mm).

Double Windows (Double Glazing)
Two windows, such as a regular window plus a storm sash; also an insulating window with air space between panes.

Double Window
Two windows separated by a mullion, forming a unit. Also called a coupled window.

Drip Cap
A horizontal molding to divert water from the top casing so that the water drips beyond the outside of the frame.

Dry Glazing
A form of glazing in which the glass is secured in the frame with a dry gasket, wood stops, or metal stops, instead of by a glazing compound. See also Reglet.

Emergency Exit Window (Egress Window)
Fire escape window; large enough for a person to climb out; each bedroom should be provided with exit windows.

Extension Jamb (Jamb Lining, Jamb Extender)
A board used to increase the depth of the jambs of a window frame to fit a wall of any given thickness.

Exterior Casing
See Casing.

A temperature scale based on 32° as the freezing point and 212° as the boiling point of water at sea level. Abbreviated °F.

Fanlight (Sunburst Light, Fan Window, Circle-Top Transom)
A half-circle window over a door or window, with radiating bars. See Lunette.

Devices for jointing two parts together, such as screws nails and bolts.

The placement of window openings in a building wall; one of the important elements in controlling the exterior appearance of a building.

Finish Casing (Finish Trim)
Interior trim boards around a window unit.

Fire-Escape Window (Emergency Exit Window)
Window which opens onto fire escape; window designed for emergency exit.

Fixed Light (Fixed Sash)
Window which is non-operative (does not open).

Sheet metal provided for drainage of water and to prevent water penetration into building.

Flat Glass (Window Glass, Plate Glass, Float Glass, Rolled Glass, Cylinder Glass)
Glass sold in flat sheets and named according to the method used in its manufacture.

Float Glass
Smooth glass formed on the top of molten tin surface; a flat glass sheet.

See Window Frame.

French Window
Two casement sash hinged on the sides to open in the middle; the sash extends to the floor and serves as a door to a porch or terrace.

Forced Convection
A heat transfer process, aided by mechanical circulation of a liquid (such as water) or a gas (such as air). This applies to natural wind flow over a window.

An elastic transparent material composed of silica (sand), soda (sodium carbonate), and lime (calcium carbonate) with small quantities of aluminum, boric, or magnesia oxides. See Flat Glass.

The installation of glass in a window opening; also the fenestration or windows.

Glazing Bead (Glass Stop, Wood Stop, Sill Bead)
A removable trim that holds the glass in place.

Glazing Channel
A groove cut into the sash for the mounting of glass.

Glazing Rabbet
See Rabbet.

See Muntin.

A long, narrow cut on the face of a wood member; a groove across the grain is a Dado; one parallel with the grain is a Plough. A groove exposes three surfaces, in contrast with the two surfaces exposed in a Rabbet or Notching. See Rabbetted Joint and Plough.

Mounting a sash in its frame.

Hanging Sash (Hung Sash)
Sash hung on a cord connected to a counterweight.

The top or upper member of any element or structure; in windows, it refers to the top of the frame, as in Round Head Window. See Head Jamb.

Head Jamb(Head)
All of the horizontal members at the top of the window frame.

Header (Lintel, Beam)
Supporting member or beam above window opening which transfers building weight above to the supporting wall structure on each side of the window. The term header is generally in reference to a wood beam, whereas Lintel often refers to a steel beam.

Head Rail - See Rail.

Heat-Absorbing Glass (Tinted Glass)
Window glass containing chemicals (with gray, bronze, or blue-green tint) which absorb light and heat radiation, and reduce glare and brightness. Shading coefficient of this glass varies from about 50% to 70%.

Heat Transfer Coefficient (U-value)
A value indicating the rate of heat flow through a building construction, expressed in units of 'Btuh per square foot of surface per degree F. difference between indoor and outdoor air temperature.' This is numerically equal to the 'inverse of the sum of R-values' for the construction.

A movable joint enabling a window to swing open.

Hopper Light (Hopper Vent, Hopper Ventilator)
Inward opening sash hinged at the bottom.

Horizontal Sliding Window (Horizontal Slider)
Windows which slide horizontally.

Hung Sash
See Hanging Sash.

Hung Window
Window with one or more hanging sashes.

The ability to 'give off' and 'take on' moisture, as in wood exposed to changes in relative humidity of air.

Leakage of outdoor air into a house, such as through cracks around sash or window frame. See also Crack Perimeter.

Infiltration Heat Loss
The heat loss, expressed in units of Btu per hour (Btuh), resulting from leakage of outdoor air into a structure and the escape of indoor air. The loss depends upon the indoor and outdoor temperatures, the crack perimeter, and the rate of air leakage per foot of crack (See also Design Heat Loss).

Insect Screen Window Screen, Screen)
Woven mesh of metal, plastic, or fiberglass stretched over a window opening to permit air to pass through, but not insects.

Inside Casing (Interior Casing, Interior Finish, Interior Trim)
The inside visible molding surrounding the interior of the window frame, See Casing.

Inside Trim
See Inside Casing.

Inside Sill
See Stool.

Insulating Glass
Double- or triple-glazing with an enclosed, dehydrated, and hermetically sealed air space between the panes; the space is commonly from 3/16" to ¾".

Insulated Window
A window with multiple glazing that provides one or more air spaces between layers of glazing.

Interior Finish
See Inside Casing.

Interior Glazes
Glazing installed from inside of building.

Interior Mullion Casing
The inside trim between adjacent windows. See Mullion.

Interior Trim
See Inside Casing.

Jal Awning (Awning Window)
Window with several out-swinging, awning-type windows that pivot near the top of the glass and operate in unison.

A vertical member at the side of the window frame; also refers to the horizontal member at the top of the window frame, as in Head Jamb and Window Jamb.

All of the vertical members at the side of the window frame.

Jamb Depth
Width of the window frame from inside to outside.

Jamb Extender
See Extension Jamb.

Not assembled; parts for a window frame pre-manufactured for assembly at a later date on the job site.

Laminated Glass
See Shatter-Proof Glass.

Latch (Catch, Lock)
A device which holds a window shut, such as the latch at the meeting rail of a double-hung window or one mounted on the stile of casement windows, often referred to as a lock.

Lead Light (Lead Glazing, Stained Glass)
Window with small panes of glass set in grooved rods of cast lead (or came). The glass can be clear, color, or stained.

Light (Lite)
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.

Horizontal member (wood, steel, or stone) over a window opening to support the weight of the wall above (See also Header).

A fastening device in which a bolt is secured and can be operated by a key. Commonly used to refer to Latches or Catches.

Lock Rail
See Meeting Rail.

Lock Stile
The vertical member (stile) of a casement sash which closes against the surrounding frame.

Low-Emissivity Coating (Low-E)
Coating for glass surfaces which reflects radiant heat energy rather than allowing that energy to radiate through the glass surface. The lower the emissivity of the glass the lower the heat transfer coefficient.

A crescent-shaped window framed by moldings or an arch.

Meeting Rail (Lock Rail)
One of the two horizontal members of a double-hung sash which come together See Check Rail.

Any structural part of a window, such as a rail, stile, or lintel.

Window sash and other wood products made in a wood-working plant.

Miter Joint
Two members joined at an angle, commonly 45 degrees.

Moisture Barrier (Vapor Barrier)
A material which retards the passage of water vapor from one space to another. Polyethylene sheet is commonly used as a vapor retarder.

Moisture Content
Percentage of dry weight of material which is composed of water, such as in wood.

Mold (Molding)
A relatively narrow strip of wood used to conceal a joint or to emphasize ornamentation of a structure.

Mould (Moulding)
British spelling of mold, and molding.

Vertical member between window units.

Muntin (Sash Bar, Window Bar, Glazing Bar)
A secondary framing member (horizontal, vertical, or slanted) to hold the window panes in the sash. This term is often confused with Mullion.

Muntin Grilles
Wood, plastic, or metal grilles.

Multi-Lite Sash
A sash divided into many lites.

Trade name for a clear, durable plastic sheet used for covering an inside storm panel or for removable, roll-up glazing over an entire window frame.

Natural Convection
A heat transfer process involving motion in a fluid (such as air) caused by difference in density of the fluid and the action of gravity This is an important part of heat transfer from the glass surface to room air. See Forced Convection.

A rectangular cut across the grain of the wood member at the end of the board (See Rabbet).

Obscure Glass (Visionproof Glass)
Any textured glass (frosted, etched, fluted, ground, etc.) used for privacy, light diffusion, or decorative effects.

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

Oriel Window
A window projecting from the wall and carried on brackets, corbels, or a cantilever. Unlike a Bay Window, the projection of an Oriel does not extend all the way to the ground.

The placement of a room, window, or building with respect to sun, wind, earth, access, or view (See also Solar Orientation).

Outside Casing (Outside Facing, Outside Trim, Exterior Casing)
That portion of the window frame which is exposed to the outdoors, See Casing.

Outside Glazing
Glazing installed from the outside.

A sheet of glass for glazing a window. After installation, the pane is referred to as a 'light' (lite) or 'window light.'

Parting Bead (Parting Strip, Parting Stop)
A vertical strip on each jamb that separates the sashes of a double-hung window.

Patterned Glass
One or both surfaces of glass with a rolled design; used for privacy and light diffusion.

Percentage Humidity
Weight of water vapor in air divided by weight of vapor contained in saturated air, expressed as a percentage.

Picture Window
Large fixed windows; introduced in the 1940s.

Artificial substances made of organic polymers that can be extruded or molded into various shapes, some of which have been adapted to windows. The material is commonly stiffer than rubber.

Plough (Plow)
A rectangular groove or slot (with 3 surfaces) cut parallel with the grain (See Dado).

See PVC.

Prime Sash
The balanced or moving sash of a window unit.

Prime Window
Window with single or multiple glazing to which storm sash may be installed.

Projected Window
An awning-type window that swings either inwards or outwards at the top or the bottom. The 'PIB' or 'project in at bottom' window can be cleaned from the inside.

Psychrometric Chart
A chart which shows dry bulb and wet bulb temperatures used to determine the relative humidity of air and the dew point temperature. Other engineering data referring to moisture in air are also shown.

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

The transmission of energy through space without heating the air between, as in Solar Radiation.

A rectangular notch (consisting of two surfaces) cut parallel with the grain of wood along the edge. See Notching.

Rabbetted Joint
The joint formed by two boards with rabbetted ends, as in some window frames.

Rail (Head Rail, Top Rail, Bottom Rail, Meeting Rail)
Horizontal member of a window sash.

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

A plastic or wood molding placed in a concrete or masonry opening to provide a uniform groove for a spline-type gasket to hold window glass. See Dry Glazing.

Relative Humidity
Weight of water vapor in air divided by the weight of water vapor in completely saturated air at the same temperature, expressed as a percentage.

R-Value (Thermal Resistance)
A measure of resistance to heat flow of a material or construction; a higher value indicates a better heat insulating property. The R-value of an ordinary single-pane sash with a 15 mph wind on one side is about 0.9.

Safety Glass
A strengthened or reinforced glass that is less subject to breakage or splintering, such as glass for storm doors and some windows. See Tempered Glass, Shatterproof Glass, and Plexiglas.

Sash - (Window Sash)
Framework of stiles and rails in which the lights of a window are set.

Sash and Frame
A window and its casing frame.

Sash Balance
A device for counter-balancing a sash of a double-hung window to hold it in the up position.

Sash Stop
A molding that covers the joint between window sash and the jamb.

See Insect Screen.

Screen (Wire Cloth)
A close-mesh woven screening material of metal, plastic, or fiberglass for a window screen, to block the entry of insects but permit light, air, and vision through the screen.

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.

Sealed Double Glass
Two panes separated by a sealed space. See also Insulating Glass.

Shade Screen (Sun Screen)
A specially fabricated window 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, an awning with fixed louvers of metal or wood construction.

Shading Coefficient
A decimal value which is the solar gain of a window, divided by the solar gain for a clear single-glass window of the same size. The shading coefficient of clear, double-glazing is about 0.85 to 0.9.

Shatter-Proof Glass (Laminated Glass)
Two sheets of glass with a transparent plastic sheet sandwiched between to form a pane resistant to shattering.

Side Light (Margin Light)
A fixed, often narrow, glass window next to a door opening (or window).

Silicone (Silicone Sealer)
An enduring sealing agent that resists water. See Sealant.

Sill (Sill Plate, Inside Sill, Outside Sill)
The horizontal members at the bottom of the window frame; a masonry sill or sub-sill can be below the sill of the window unit.

Single-Hung Window
A window that is similar to a double-hung window except that the top sash is stationary.

Single-Strength Glass
Glass with thickness between 0.085 to 0.100" (2.16 mm to 2.57 mm).

Sliding Sash (Sliding Windows)
A window which moves horizontally in grooves or tracks.

Sling Psychrometer
A measuring instrument with two thermometers (dry-bulb and wet-bulb) used for determining the dewpoint and relative humidity of air; its relation to windows is ascertaining the point at which moisture will condense on the inside surface of the glass. See Psychrometric Chart.

Solar Heat Gain
Heat from solar radiation that enters a building.

Solar Orientation
A building placed on a lot so that the long dimension faces south and a majority of the windows are south-facing.

Sound-Insulating Glass (Sound-Resistive Glass)
Double glass fixed on resilient mountings and separated so as to reduce sound transmission.

Sound-Resistive Glass
See Sound-lnsulating Glass.

The linear object that separates and maintains the space between the glass surfaces of insulating glass.

Spiral Balance
See Sash Balance.

A rectangular strip of wood or metal inserted between two boards, which have been slotted to receive it.

Spring Bolt
A fastener for holding the sash in a fixed location by means of a spring-loaded bolt in the stile entering a hole in the jamb.

Stacked Window Units
A combined grouping of awning, hopper, casement, or non-operative windows to form a large glazed unit.

Stained Glass Window
A window with a painted scene or pattern that has been fired into the glass. Windows with plain colored glass set in lead are also called stained glass. See Lead Light.

Stationary Sash
A fixed sash; also referred to as a picture, studio, vista, or view sash.

The vertical-edge members of a window sash.

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

Stop (Bead, Side Stop, Window Stop, Parting Stop)
The molding on the inside of the window frame against which the window sash closes, or in the case of a double-hung window, the sash slides against the stop.

Storm Sash (Storm Window)
An extra window on the outside to protect an existing window, but mainly to increase the thermal resistance of the window.

Sunburst Light
See Fan Light.

Sun Screen
See Shade Screen.

Sweep Lock
A sash fastener located at the meeting rails of a double-hung window, which rotates and clamps the two rails closer together.

Tape Balance
See Sash Balance.

Tempered Glass
Special heat-treated, high-strength safety glass which shatters into pebble-sized particles but not into slivers.

In technical usage, the term is a convenient measure of heating value, namely 100,000 Btu. One therm is roughly equivalent to the heating value of 100 cubic feet of natural (methane) gas.

Thermal Barrier (Thermal Break)
A material of high thermal resistance placed between two metal sash, or installed between adjoining metal framing of metal windows, in order to reduce thermal conduction from indoors to outdoors.

Thermal Conduction
Heat transfer through a material by contact of one molecule to the next. Heat flows from a high temperature area to one of lower temperature.

Thermal Conductivity
Heat transfer property of materials expressed in units of 'Btu per hour per inch of thickness per square foot of surface per one degree F. temperature difference.' Referred to by the letter 'k.'

Thermal Conductance
Same as Thermal Conductivity except thickness is 'as stated' rather than one inch. Referred to by the letter 'C.'

Thermal Insulation
A material that resists heat flow. Material having a high R-value.

Thermal Resistance (R-value)
A property of a substance or construction which retards the flow of heat; one measure of this property is R-value. See Heat Transfer Coefficient.

Tinted Glass
See Heat-Absorbing Glass.

Top Rail
See Rail.

Transom (Transom Bar)
A horizontal member separating a door from a window panel above the door, or separating one window above another.

Transom Light (Transom Window)
The window sash located above a door.

Visible molding surrounding a window opening. See Casing.

Triple Glazing
Three panes of glass with two air spaces between, commonly consisting of an insulating glass with a separate storm sash. Also available as an Insulating Window in a single frame.

Triple Window
A term generally referring to any tripartite group of windows with square heads. These are frequently found on Colonial Revival houses; they suggest Palladian Windows but are less expensive to build.

Ultra-violet Radiation
Extremely short wave length invisible radiation, which is a component of solar radiation, and merges into the visible spectrum; attributed as a source of skin sunburn and color fading of draperies and carpeting.

A value indicating the rate of heat flow through a building construction, expressed in units of 'Btuh per square foot of surface per degree F. difference between indoor and outdoor air temperature.' This is numerically equal to the 'inverse of the sum of R-values' for the construction. Also see Heat Transfer Coefficient.

Vapor Barrier (Vapor Retarder)
A membrane or coating which resists passage of water vapor from a region of high vapor pressure to low pressure, more accurately called a Vapor Retarder.

Vertical Sliding Window
One or more sashes that move in a vertical direction.

See PVC.

Visible Spectrum
That portion of the total radiation that is visible to the human eye and which lies between the ultra-violet and the infra-red portions of the electromagnetic spectrum. The colors associated with the visible spectrum range from violet, indigo, blue, green, yellow, orange, through red.

A strip of resilient material for covering the joint between the window sash and frame in order to reduce air leaks and prevent water from entering the structure.

Sealed to prevent entry of air and precipitation into the structure.

Small holes drilled along the bottom edge of storm sash or combination storm-screens to permit moisture condensation or wind-driven rain to drain away from the sill to the outdoors.

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

Window Bar
See Muntin.

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

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

Window Unit
A complete window with sash and frame.

Wind Pressure
The pressure produced by stopping the wind velocity; the main cause of air infiltration.