Window Terms Glossary

• Air Chambers - Small honeycomb spaces within the sash and frame which help to insulate and strengthen to a window.
• Air Infiltration - The amount of air that passes between a window sash and frame. It is measured in terms of cubic feet or air per minute, per square foot of area. The lower the number, the less air passes through.
• Architectural Shapes – Specially designed windows such as rectangles, triangles, trapezoid, octagons, pentagons, etc., or Radius Geometrics which include half-rounds, quarter-rounds, full-rounds, sectors, ellipses, eyebrows, etc. (See Geometric Shapes)
• Argon Gas - An odorless, colorless, non-toxic gas which is six times denser than air. It is used to replace air between the glass panes to reduce temperature transfer.
• Awning Window - A window that is top-hinged and swings outward for ventilation.

B

• Balance Covers - Covers the balance cavity holding the balance system inside the jamb.
• Balance Spring - A device for counterbalancing a sliding sash, generally associated with a double hung window, so that it can be held open at a position. (See Sash Balance or Balance)
• Balance Shoe – Part attached to bottom of balances that attaches balances to sash. A bar or pin on the sash fits in a “slot” in the shoe.
• Balance System - A device for holding vertically sliding sash in any desired position through the use of a spring or weight to counterbalance the weight of the sash. (See Balance Spring or Sash Balance)
• Basement Sash Unit – (cellar sash unit) a sash unit, usually of the in swinging or hopper sash type, used for basement or cellar sash openings.
• Bay Window - An angled combination of three windows that project out from the wall of the home. U.S. Window can make these units to accommodate a large amount of Jamb and seat sizes.  
• Beveled Exterior - An angled extension from the frame that adds an aesthetically-pleasing dimension to the outside of the window.

• Blind Stop – A sash or window frame member applied to the exterior vertical edge of the side and head jamb in order to serve as a stop for window sash or screens.
• Bottom Rail – The bottom horizontal member of a window sash.
• Bow Window - An angled combination of windows in 3, 4 or 5 lite configurations. U.S. Window can make these units to accommodate a large amount of Jamb and seat sizes.
• Brick Molding – A standard milled trim piece to cover the gap between the window frame and masonry opening. It is found on the exterior of doors and windows.
• BTU – This is the abbreviation of British thermal unit, and is a primary measure of heat. It is the amount of heat needed to raise the temperature of one pound of water 1 degree F.
• Buck – The jamb stud or header; wood buck.
• Buck Opening – The opening in a wall formed by the rough framing members; also stud opening.
• Butt Joint – A joint formed by square edge surfaces (ends, edges, faces) coming together; ends butt joint, edge butt joint.

C

• Cam Lock and Keeper - Mechanisms that pull the sash together when placed in the locked position.
• Casement Window - A window with a side-hinged sash that opens outward for ventilation.
• Casing – 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. (See Trim)
• Caulk – To seal cracks and joints around window and door rames to prevent leakage of water and air.
• 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.
• Circle Top Window – A window having a curved top and a flat bottom. The shape of the window is an exact half-circle with the height being exact one-half of the width. Also called circle head, half-circle and half round.
• Combination Storm Sash & Screen – A frame assembly of stiles and rails containing a half screen and two glass storm panels; in summer the bottom storm panel is stored in the top of the combination frame and replaced by the screen panel; sometimes called “combination storm sash and screen unit” or simply “combination window unit.” (Not recommended for vinyl windows)
• 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.  

Cottage-Style Window – Hung Window (Single or Double) that has larger bottom sash (lite) than the top sash (lite). (See Oriel)

D

• Dead-air space - The space between the panes of glass of an I.G. Unit.
• Deadlite (Flat 4-side sash) - A piece of glass or IGU with a sash profile around it; not set within the main frame of a window unit.
• Desiccant – A drying agent (such as silica gel) used by some manufacturers between the panes of insulating glass (in edge spacer) to prevent “fogging” between the panes.
• Double Hung Window - A window that has two operable sash which slide vertically.
• Double-strength Glass - Glass with a thickness of approximately 1/8". (3 mm)
• Double Window – Two windows separated by a mullion, forming a unit. Also called a twin unit.
• Drip Cap – A horizontal molding to divert water from the top of the window or door unit so that the water drips beyond the outside of the frame.

E

• Egress Code - A code that requires a minimum opening of a window for persons to exit or firefighters to enter a building.
• Energy Star® - ENERGY STAR® is an independent U.S. government program establishing a standard set of guidelines to recognize the energy efficiency of various products. ENERGY STAR® guidelines are used in conjunction with a variety of building materials, including windows and patio doors. Over the past ten years, ENERGY STAR® guidelines have helped double the efficiency of windows they endorse.
• Etched Glass - Patterns and designs are sandblasted directly onto the glass giving a subtle accent.
• Extension Jambs – Flat parts made of vinyl, wood or other materials which are attached to the inside edges of a window jamb to extend it in width to adapt to a thicker wall.
• Eyebrow Window – Today used to identify certain arch-topped or radius-topped windows that have a curved top like the shape of the human eyebrow. The overall height of an eyebrow window will always be less than ½ of the width.

F

• Fenestration – The placement of window openings in a building walls; one of the important elements in controlling the exterior appearance of a building, its ventilation, light, etc…
• Fixed Light (Fixed Sash) – Window, which is non-operative (does not open).
• Flat 4 Side – A piece of glass or IGU with a sash profile around it, not set within the main frame of a window unit.(See Deadlite)

• Foam Filling - A polyurethane foam that is used to fill the spaces in a sash and frame. This foam can increase the insulating value of a window up to 30%.
• French Door – An interior or exterior door consisting of stiles, top and bottom rail and divided glass panels or lites.
• Frame – Outside member of a window (or door) unit, this encloses the sash. Composed of side jams, head jamb and sill
• Friction Hinge – A window hinge remains open at any position by means of friction in the hinge.
• Fusion-welded - Materials joined together by melting them with extreme heat, resulting in the materials uniting into a one-piece unit.

 

G

• Geometric Shapes (Also Architectural Shapes) - Specially designed windows such as rectangles, triangles, trapezoid, octagons, pentagons, etc., or Radius Geometrics which include half-rounds, quarter-rounds, full-rounds, sectors, 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.
• Glass (Insulating) – Two sheets of glass bonded together in a unit to enclose a captive air space.
• Glass Sheet – glass with a thickness between 0.1” or approximately 3/16”. Used in very large window units.  
• Glazing - The process of sealing the glass to the sash.
• Glazing Bead - A strip of vinyl which surrounds the edge of the glass and holds it in place in combination with other sealants.
• Glazing Block – A small hard rubber block placed around the edges of the glass unit in a window sash to position the glass and prevent it from shifting.
• Glazing Compound – A pliable substance applied between the window sash and the lites of the glass to seal against the element and sometimes to adhere the glass to the sash.
• Glazing Tape – Double sided tape used to adhere glass to sash and form an airtight, watertight seal.
• Grids (Also Grilles) - Decorative horizontal or vertical bars that are installed between the glass panes creating the appearance of divided smaller lites of glass.
• Grilles (Also Grids) – Ornamental or simulated muntins or bars, which don’t actually divide the lites of glass. Generally made of vinyl and fit on the inside of the sash against the glass surface for easy removal. Grilles or grids between the glass are usually made of aluminum and sealed inside insulating glass unit.

H

• Half Screen – A screen which does not cover the entire opening of a window. Used on the bottom half of a window or on the operating side of a slider.
• Head - The horizontal top portion of the main frame.
• Head Board – A flat board cut to fit the contour of a bay or bow window and used to affix the top of the window units.
• Head Expander - A vinyl shape cut the width of a product and placed on the head, fitting over it tightly. This piece is used as a filler to expand or lengthen the unit from the head and take up the gap in the opening between the unit and the opening in the unit.  
• Heat Mirror – A thin transparent insulating film that is inserted between double glazing which permits transmission of visible light but reflects far-infrared radiation.
• Heat Transfer Coefficient (U-Value) – The value indicating the rate of heat flow through a building construction, expressed in units of “Btu/h 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 o r-values’ for the construction.
• Hinge – A movable joint enabling a window or door to swing open.
• Hopper - Known as basement windows. A window with a bottom-hinged sash that opens inward for ventilation.
• Hung Window – Windows with one or more hanging sashes.

I

• I.G. Unit (Insulating Glass Unit) - Two or more lites of glass separated by a spacer and hermetically sealed at the glass edges.
• Infiltration – Leakage of outdoor air into a house, such as through cracks around sash or window frame.
• Installation Fin Or Flange – A vinyl or metal flange inserted into or an integral part of the side and head jambs of the side and head jambs of a window unit, which is used for installing it in an opening. It also affords a weather seal or flashing around the perimeter of the window frame. (See Nailing Fin)
• Insulating Glass – Double or triple glazing with an enclosed dehydrated, and hermetically 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.

 

J

• J-channel Frame - Integral extension on the outside of a new construction window that eases installation on siding applications.
• Jamb - Vertical sections of the main frame.

K

• Keeper – The part of the window lock, mounting on the opposing surface of the window, that the lock arm locks under or into to pull the sash into a locked position and fully releases it when opened.
• Keeper Rail - The horizontal section of the sash where the keeper is attached.
• Keeper Stile - The vertical section of the sash where the keeper is attached.
• Knocked-Down – Not assembled parts for a window or doorframe pre-manufactured for assembly at a later date on the job site.

L

• Latch – A device which holds a window shut, such as the latch at the meeting of a double hung window or one mounted on the stile of casement windows, often referred to as Lock.
• Lift Rail - A handhold that is continuous across the sash to raise and lower the sash.
• Lite - A unit of glass in a window.
• Lock Rail - The horizontal section of the sash where the cam lock is attached.
• Lock Stile - The vertical section of the sash where the cam lock is attached.
• Low E (Emissivity) Glass - Glass with a transparent metallic oxide coating applied onto or into a glass surface. The coating allows short-wave energy to pass through but reflects long-wave infrared energy which improves the U-value.

M

• Main Frame - The head, sill and jambs sections of a window.
• Masonry Opening – The opening in a masonry wall to accept a window or door unit, the same as a rough opening in a frame wall.  
• Mechanically Fastened Frame / Sash - Refers to frames or sash fastened with screws.
• Meeting Rail - The horizontal sections of a pair of sash that meet when the sash are closed.
• Meeting Stile – The vertical member in a pair of stiles, as in abutting casement windows.
• Miter – Molding returned upon itself at right angles. In joinery the ends of two pieces that are cut at a 45 degree angle abutting upon one another so as to form a right angle.
• Mullion - A vertical or horizontal connecting unit between two or more windows.

N

• Nailing Fin - An extrusion that attaches to the window frame and used to secure the unit to the rough opening. (See Installation Fin)
• National Fenestration Rating Counsel (NFRC) – Organization, which develops procedures for rating the energy performance of windows.
• Night Latch - Latch mechanism on the inside of the top sash that retains the window in a partially open position for ventilation.  

O

• Obscure Glass - Glass that has been made translucent instead of transparent. Used when privacy is an issue such as bathrooms.
• Operable Window – Window, which can be opened for ventilation.
• Operator – Crank-operated device for opening and closing casement or Jalousie windows.
• Oriel Style Window (See Cottage) - A window with the meeting rail located off center of the frame.
• Outside Casing – Trim or molding around a window or door on the exterior of the house. Casing usually refers to a flat board trim, typically 3” to 4” in width.

P

• Pane – A Sheet of glass for glazing a window. After installation the pane is referred to as a lite.
• Parting Bead – A vertical strip on each jamb that separates the sashes of a double hung window. (See Blind stop)
• Patio door - A glass door that slides back and forth on adjustable tandem rollers. Available in 2 or 3 lite configurations with the operable panel available in any position.
• Picture Window - A window that does not have a moveable sash.
• Pocket Sill – A deep sill (frame) design that completely surrounds the bottom edge of the sash in the closed position.
• Projected Window – An awning type window that swings either inward or outward at the top or bottom.
• Push Bar – Used on an awning and other projecting sash windows for opening and closing instead of a crank operator.
• PVC (Polyvinylchloride) – An extruded or molded plastic material used for window framing and as a thermal barrier for aluminum windows.

Q

• Quarter Round Window – Stationary or operating window with glass shaped as a quarter circles; it is often divided into separate panes by a removable grille, installed on the interior or grille between the glasses.

R

• R-value - Resistance a material has to heat flow. The higher the R-value, the greater the resistance.
• Radiation - Wave energy transmitted directly from one object to another through the atmosphere or through transparent or translucent materials. The energy radiated is transmitted, absorbed, reflected or a combination of all three.
• Rail – Horizontal members of a window sash or door panel.
• Raised Exterior - An angled extension from the frame that adds an aesthetically pleasing dimension to the exterior of the window.
• Resin Art Glass - A look of stained glass is handcrafted directly onto the glass during the manufacturing process. 
• Rough Opening – The opening left in a frame wall to receive a window or a door unit. The jack studs on each side, which supports the header across the top, form it. Cripples support the rough sill at the bottom. The rough opening generally allows 1/2” or more in each dimension in excess of the window or door unit dimension. Openings in brick walls are known as masonry openings.
• Rough Sill – The horizontal rough framing member, usually 2 X 4, which forms the bottom of the rough opening.

S

• SafeGard® glass - Two panes of glass are bonded to a durable plastic interlayer, providing increased safety, UV protection and noise reduction. If a window gets broken the glass will adhere to the plastic interlayer - preventing glass fallout in the home.
• Safety Glass – A strengthened or reinforced glass that is less subject to breakage or splintering, such as glass for storm doors and some windows.
• Sash - The part of the window which contains the glass.
• Sash Balance – A device for counter balancing a sash of a double hung window to hold it in an up position. There are four basic types:
• Block & Tackle - A type of balance that employs a block and tackle apparatus and coiled spring. This type balance allows the sash to be easily removed from the window frame. A block and tackle balance can normally carry a heavier load than a spiral or friction balance.
• Spiral – A balance using a spirally-wound spring.
• Spring – A balance using a spring for counter-balancing; introduced in the 1980’s.
• Constant Force – A coiled steel tape under spring tension for balancing the sash, located in the head jamb of the window frame.
• Counterweight – The most common type of balancing system in older homes, utilizing a weight held by a sash rope or chain over a pulley.
• Sash Cord – In older windows, the rope used to connect the sash with its counter weights.
• Sash Crack – The opening between the operating sash and the frame of the window. Its thickness or measurement is equal to ½ the difference between the inside window frame dimension and the outside sash width. Crack foot length is used in calculating total air infiltration on a window unit.
• Sash Lift- A handle for raising the lower sash.
• Sash Lock – Generally a cam-action type lock applied to the rails of a window or at the open edges of a projecting window to pull rails tightly together or to seal the sash tightly to the frame, both for security and weather tightness.
• Sash Stop – A molding that covers the joint between window sash and the jamb. Often limits sash travel.
• 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.
• Sealant – A compressible plastic material used to seal any opening or junction of two parts, such as between the glass and its sash, commonly made of silicone, butyl tape or polysulfide.
• Seal Failure (Hermetic Seal Failure) – The failure of the seal or seals in an insulated glass lite. Evidence of seal failure is usually dirt, dust or the appearance of water vapor within the insulated glass unit.
• Seat Board – A flat board cut to fit the contour of a bow or bay window and installed between the sill and the flat wall surface, providing a seat of shelf space.
• Shading Coefficient - The ratio of solar heat that is transferred through a glazing material relative to the solar heat transferred through 1/8" clear glass. The lower the number the more efficient the window is at reducing solar heat gains.
• Shims – Wood wedges (often wood shingles) used to position on the window or door unit in a rough or masonry opening in a square, level and plumb position during (and after) installation.
• Side Jamb – The upright vertical member forming the sides of the frame of a window or door unit.
• Side Lite – A fixed often narrow glass window next to a door opening (or window).
• Sill - The horizontal, bottom section of the main frame.
• Sill Course – The row of bricks, cement blocks or stones laid across the bottom of a masonry opening which lie under the outside edges of the window sill.
• Single Hung - A window in which one sash slides vertically and the other sash is fixed.
• Single-Strength Glass - Glass with a thickness of approximately 3/32" (2.5 mm).
• Sliding Sash – A window, this moves horizontally usually in grooves or tracks.
• 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.
• Snap-In-Grid – Wood or plastic removable divided-lites for sash and windows.
• Solar Energy – Thermal radiation from the sun, as measured by short radiation wavelengths.
• Solar Heat Gain - The percentage of heat gained from both direct sunlight and absorbed heat. The smaller the number, the greater the ability to reduce solar heat gain.
• Spacer - Material placed between two or more pieces of glass in order to maintain a uniform width between the glass, and prevent sealant distortion.
• Spline – For screening, a small strip of metal or flexible tubing or gasket material that fits into a groove in the screen frame to secure the screen cloth.
• Stacked Window Units – A vertical grouping of an awning, hopper, casement, or non-operating windows to form a large, multiple units.
• Stationary Sash – A fixed sash; also refereed to as a picture, studio vista or view sash.
• Stepped Sloped Sill - An exclusive, sloped sill design with a defined step to aid in the prevention of air and/or water.
• Stile – The upright or vertical outside members of a window (sash) or door panel, blind, screen or shutter.
• Stool – The inside horizontal trims member at the bottom of the window from which rests on the sill. The shelf likes part that projects into the room. This is a common inside trim method used on wood double hung windows and sometimes called inside sill.
• Stop – A trim member attached to the window frame to stop the same of a projecting window when closed to prevent it from swinging through the opening. It also covers the perimeter crack between the sash and the window frame in double hung and sliding windows and prevents sash from coming out of the frame. Stops used at the top and bottom of the balance channel prevent the sash in hung windows from hitting when opened.
• Stress Crack – A stress crack in an insulated glass unit is a crack in the glass that occurs only on the inside lite of the piece of insulating glass, usually from a rapid change in temperature. A stress crack starts as a single crack from near the edge of the unit, which will often continue to “walk” across the glass.
• Studs – Vertical wood framing members, which form a wall.
• Super Spacer® - A solid silicone foam spacer covered with Mylar. It is sealed to the edge of the glass and then sealed with butyl for greater energy efficiency.

T

• Tape Glazing - Two-sided tape used to secure and seal the glass to the sash.
• Tempered Glass – Hardened glass use to meet specific building codes. Usually related to impact and glass breakage.
• Thermal Barrier – A strip of non-conducting material, such as wood, vinyl or foam rubber, which is used to separate the inside and outside surfaces of a metal window sash or frame, or a metal door or sill to stop the conduction of heat to the outside (which results in a cold inside surface). Vinyl windows, because of their thermal efficiency, do not require a thermal barrier.
• Tilt Latch - Mechanism that unlocks the sash and allows it to tilt in from the main frame.
• Tilt Pin – A metal or hard plastic nail-like pin fastened to the sash to allow it to engage the balance shoe and from which the sash may tilt or pivot in.
• Tinted Glass – Glass that has been given a slight shading or coloring to it, usually to control sunlight in very bright, high intensity sun locations. There are many colors available today such as bronze, gray, green, blue, etc.
• Total Heat Loss – The total heat loss from a structure equal to the sum of the heat loss (transmission) and infiltration (air leakage).
• Total Unit U- and R-values - The U- and R-values of the window calculated from the average of the center of glass, edge of glass and frame U- and R-values. It is the reciprocal of the R-value.
• Transom – Generally refers to an opening or stationary sash above a door or window which serves a similar purpose to a side lite. A transom joint is the horizontal joining area between two window units, which are stacked one on top of another.
• 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. (See Casing)
• Twin Unit (Window) – Two windows separated by a mullion, forming a unit. Also called a twin unit. See Double Window.

U

• U-Value - Amount of heat transferred through a material. The lower the U-value, the slower the rate of heat flow and the better the insulating quality.
• Ultra-Violet – Type of radiation with wavelengths shorter than those of visible light and longer than those of X-rays. Causes sunburn, fading and breakdown of fabric, wood furniture and other exposed surfaces.
• United Inches – The sum in inches of the width and height of a window unit. Common “call size” for replacement windows.

• 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.

V

• Venting Window – Operating window such as a venting, awning, etc. One that opens for ventilation.
• Vertical Sliding Window – One or more sashes that move in a vertical direction.
• V-Groove – A V-Shaped groove cut into the surface of a glass pane for decorative purposes.
• Visible Light Transmittance - 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.

W

• Warm Edge Itercept® Spacer - A unique, u-channel stainless steel piece is placed between the two panes of insulated glass to create an effective thermal barrier. This sealed, one-piece design helps to reduce conducted heat loss through the window.
• Weatherstripping - Material used to form a weather-resistant seal around the operable sash.
• Weep Holes - Small openings at the bottom designed to allow water to escape that might otherwise accumulate in a window's sill.
• Window Frame – The fixed frame of a window, which holds the sash as well as the operating hardware for the window.
• Window Screen – Woven mesh of metal, plastic, or fiberglass stretched over a window opening to permit air to pass through, but not insects.