Visible transmittance (VT)

Other terms

Air infiltration

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

Annealed glass

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

Argon

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

An astragal is a piece of hardware that is used on a pair of doors to seal the gap between the doors when they are closed. Astragals are different from a mullion because the astragal is fastened to the door itself. A mullion is attached to the frame and the doors stop against it..

Awning

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

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.

Balance

A mechanical device (normally spiral, spring-loaded) used in single- and double-hung windows as a means of counterbalancing the weight of the sash during opening and closing. Other types include block and tackle, cords with weights, springs and coils.

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.

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, usually consisting of 4 or more windows.

Brick molding

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

Cam Lock

A single point locking mechanism that uses a cam action to lock and pull the window sash against the frame to form a tighter seal. Large windows may have more than one cam lock.

Casement

A window sash that swings open on side hinges.

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.

Caulk, 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.

Center Hinge Door

A 2 panel patio door system with the operable panel hinged from the center of the door.

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.

Condensation

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

Conduction

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 one.

Convection

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.

Daylight Opening

The visible area of glass in a window or door.

Dewpoint

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

Divided Lite

A window opening divided into smaller sections by a grid system on the interior or exterior of the glass, or between the glass panes.

Double glazing

In general, two thicknesses of glass separated by an air space within an opening to improve insulation against heat transfer and/or sound transmission. In factory-made double glazing units, the air between the glass sheets is thoroughly dried and the space is sealed airtight, eliminating possible condensation and providing superior insulating properties.

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" (3-3.38 mm) thick.

Egress window

Fire escape 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 building codes.

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

Windows with a shallow arched head with the arch using only a section of a circle radius. These are different from an ellipse or a half-circle. Can be made with or without extended legs.

Fixed panel

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

Fixed window

A window with no operating sashes. See Picture Window.

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.

French Door

A style of door in which 2 panels open to provide a clear opening almost as wide as the door frame.

Gas fill

A gas other than air, usually argon or krypton, placed between window or skylight glazing panes to reduce the U-factor by suppressing conduction and convection.

Geometric Window

Windows of various shapes including: triangles, trapezoids, octagons, and pentagons.

Glass

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

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.

Header

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

Heat gain

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

Heat loss

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

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.

Hinged windows

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

Horizontal slider

A window with a movable panel that slides horizontally.

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.

Interlock

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. Also the check rails of single and double hung windows.

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.

Level

A condition that exists when a surface is exactly horizontal.

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.

Lintel

A horizontal member above a window or door opening that supports the structure above.

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.

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.

Mulled Unit

2 or more windows structurally joined together to fit into one rough opening

Mullion

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

NFRC

National Fenestration Rating Council.

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.

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.

Operator

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

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.

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 Bars (pins)

The component on the bottom corner of an operable sash in a single or double hung window that allows the sash to be tilted in.

Plate glass

A rolled, ground, and polished product with true flat parallel plane surfaces affording excellent vision. It has been replaced by float glass.

Plumb

A condition that exists when a surface is exactly vertical.

Polyvinylchloride (PVC)

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

R-value

A measure of how well a barrier such as a layer of insulation, a window or a complete wall or ceiling, resists conductive flow of heat. It is the inverse of the U-factor (R=1/U). R-values measure the thermal resistance of a barrier's exposed area. The greater the R-value, the greater the resistance, and so the better the thermal insulating properties of the barrier. R-values are used in describing effectiveness of insulating material and in analysis of heat flow across assemblies (such as walls, roofs, and windows) under steady-state conditions.[6] Heat flow through a barrier is driven by temperature difference between two sides of the barrier, and the R-value quantifies how effectively the object resists this drive.

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.

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.

Reveal

The space between the window sash or door panel and the frame.

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.

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.

Screen

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

Sealant

A compressible plastic material used to seal any opening or junction of two parts, such as between the glass and a sash, commonly made of silicone, butyl tape, or polysulfide.

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.

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" (2.16-2.57 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.

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.