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

Air infiltration

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

Aluminum-Clad Window

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


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

Argon gas

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


American Society of Heating, Refrigerating & Air Conditioning Engineers, Inc.


American Society for Testing and Materials. Organization that develops methods for testing of materials.


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 window that is hinged at the top and swings outward for ventilation.


A flat material used on the face of the house, between the studs and the siding, to provide a nailable surface for the siding.

Balance covers

A snap in covering that conceals the EvenForce™ block and tackle balance system within the window frame, helping to keep dirt and dust out of the chamber.

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

An angled combination of three windows that project out from the wall of the home. The windows are usually positioned at 30- or 45-degree angles.

Beveled MasterFrame

Some Alside windows feature a unique fusion-welded design that accommodates differing installation methods and architectural styles. It is the angled portion of the masterframe profile that adds a three dimensional appearance to the exterior of the window.

Block and Tackle Balance System

The block and tackle system utilizes a high-density nylon cord pulley action which is attached to a moveable block that travels up and down within a metal chamber. Tension from a heavy duty coil spring at the top of the block creates the proper resistance necessary for smooth operation of the window sash.

Bow window

An angled combination of windows in 3-, 4- or 5-lite configurations. As the windows are joined to each other, they combine to form an arch shape that projects from the wall of the home.

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.


The bottom edge of a siding or soffit panel, or accessory piece, opposite the nailing slots, which locks onto the preceding panel.


A rubber material that seals the glass to the spacer, creating an airtight and water-tight insulated glass unit.

Cam-action lock and keeper

The mechanisms, which pull and secure the sashes together when placed in the locked position.


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 a side-hinged sash that opens and closes outward by a crank handle mechanism. Available in continuous mainframe, with multi-lite configurations.


(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.


The area of the accessory trim or corner post where siding or soffit panels are inserted. Channels also refer to the trim itself, and are named for the letters of the alphabet they resemble (e.g., J-channel, F-channel, etc.).

Check Rail

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

ClarityOne Glass

A chemical treatment that when applied to glass, helps to create a smoother surface that won’t attract or hold dirt and dust. Established by PPG Industries.


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


The brand name for the insulated glass unit that is present in Alside’s insulating glass packages. A ClimaTech unit will contain either two or three panes of glass, with one or two of those panes being a Low E surface. It will utilize the SST warm edge spacer system and contain either argon or krypton gas.

Coated Glass

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


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 cold window glass or frame that is exposed to humid indoor air.


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


A row of panels, one panel wide, running the length of the house from one side to the other, or, in the case of vertical siding, from top to bottom.

Coved glazing beads

A contoured piece of vinyl that holds the glass in place within the sash and adds an elegant, finished look.

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.

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

A window that has two vertical operating sashes.

Double Channel Lineal

A siding accessory that joins two soffit panels.

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/Head Flashing

An accessory installed with vertical siding to ensure that water drips away from panels and does not infiltrate them; it is also used as a vertical base.

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.


Brand name for specially coated, operating hardware that helps to resist oxidation and corrosion.

Egress Code

The minimum opening of a window for people to exit or firefighters to enter a building/dwelling. Different states or regions have different code requirements.

Emergency Exit Window

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

Energy Star®

The Energy Star program is a joint venture between the US Environmental Protection Agency (EPA) and the US Department of Energy (DOE) designed to encourage homeowners to purchase energy-efficient products. Using less energy in our homes reduces the amount of CO2 emissions released into the atmosphere from burning fossil fuels such as coal, oil and natural gas. The advanced components and design in the ClimaTech™ insulated glass package exceed all performance criteria required by the Energy Star program.


Alside’s brand name for its block and tackle balance system. A balance system is a device for holding the vertically sliding sashes in any desired position within the window mainframe.

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.


Refers to the side of a siding or soffit panel that is showing once the panel has been installed.


The action of fastening directly onto the “face” side of a panel (instead of using the nail hem slot). This practice is generally not used in siding installation.

Fascia Board

A board attached to the ends of the rafters between the roofing material and the soffit overhang. Fascia cap is the covering around that board.


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. Also, a window, door or skylight and its associated interior or exterior elements, such as shades or blinds.

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 Lite

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.


A thin, flat material, usually aluminum, positioned under or behind J-channels, corner posts, windows, etc., to keep draining water from penetrating the home.

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.

French patio doors

A two panel glass door where both panels operate and swing either inward or outward.

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.

Furring/Furring Strip

A wooden or steel framing material, usually 1″ x 3″, used to provide an even nailing base. To
“fur” a surface means to apply these strips.


The process of joining materials by melting them together with extreme heat (in most cases over 500°F), resulting in the materials combining into a one-piece unit.

Garden window

Designed much like a bay or bow window, a garden also extends from the wall to the exterior of the home. It is built in a square or rectangular shape at right angles. The two side lights often operate for added ventilation.

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.

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


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.

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


Optional horizontal or vertical lineals installed between the glass panes help to create the appearance of a divided window design.

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.


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.

Head Jamb(Head)

All of the horizontal members at the top of the window frame.


(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.

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.

Hinged patio doors

A two panel glass door where one panel is stationary or fixed, while the other operates and swings either inward or outward.

Hopper Light

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

Hopper window

A bottom-hinged sash window that opens inward for ventilation.

Horizontal Sliding Window

(Horizontal Slider) Windows which slide horizontally.

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.

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

Insulating air chambers

Various chambers within the sash and masterframe, which help to insulate and strengthen the window.

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

Insulated Window

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


The fusion-welding process of some Alside windows.

Interior Glazes

Glazing installed from inside of building.

Interior Mullion Casing

The inside trim between adjacent windows.

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

Jamb Depth

Width of the window frame from inside to outside.


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

Krypton gas

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


To overlap the ends of two siding panels or accessory pieces to allow for expansion and contraction of the vinyl product.


(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.

Lift Handle

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


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


A unit of glass in a window.


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 Stile

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

Low E (Emissivity) Glass

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 longwave infrared radiation.


The raised “ears” or tabs on a siding panel, created by a snaplock punch, which can be used to lock a siding panel into place when the nailing hem has been removed.


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


The combination of the head, sill and jamb sections of a window.

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.


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.


To make a diagonal cut, beveled to a specific angle (usually 45°). Sometimes miter cuts are made into an overlapping siding or soffit panel surface, to provide a neater appearance.

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.


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


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


Vertical member between window units.


(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.


A weatherstripping material that is present where the sash frame meets the masterframe. Adds increased resistance to air infiltration.

Nailing Hem (or Flange)

The section of siding or accessories where the nailing slots are located.

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.


National Fenestration Rating Council.


A rectangular cut across the grain of the wood member at the end of the board.

Obscure glass

Glass that has been made translucent instead of transparent.


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.

Origin II™

The virgin uPVC vinyl used in all Alside windows. The material’s low thermal conductivity makes it the best choice for window manufacturing. Will not rot, peel, blister, swell or deteriorate from corrosion or pitting.

Outside Casing

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

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.


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.

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

A picture window that does not move or operate.


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.


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


A position or measurement that is truly and exactly vertical, 90° from a level surface.

Positive Lock

Area below the nailing hem that the buttlock locks into.

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.


Describes the design of the panel (Clapboard, Dutch lap, Triple 3, etc.)

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.


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

No Glossary Entries For “Q”


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.


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

Rabbetted Joint

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


(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.

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.


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.

Safety Glass

A strengthened or reinforced glass that is less subject to breakage or splintering, such as glass for storm doors and some windows.


Separate from the masterframe, the portion of the window that contains the glass.

Sash Balance

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

Sash and Frame

A window and its casing frame.

Sash limit locks

A feature that allows a window to be safely raised to a certain height.

Sash Stop

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


Running a utility knife blade, a sharpened awl, scoring tool, or other sharp implement across a soffit or siding panel face without cutting all the way through the panel. This weakens the vinyl surface in a specific area and allows the panel to be bent and broken off cleanly.


(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.

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 Sealer) – An enduring sealing agent that resists water.


The horizontal, bottom section of the masterframe.

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 patio doors

A combination of fixed and sliding glass door panels that operate solid brass roller trucks. Available in 2-, 3- or 4- lite configurations with the operable panel available in any position.

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.

Sliding Sash

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

Sliding Window

A window in which the sashes move horizontally. Available in a 2- or 3-lite configurations.


Material used to enclose the horizontal underside of an eave, cornice, or overhang. Some soffit panels may also be used as vertical siding.

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 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.


An object placed between two or more pieces of glass which helps to maintain a uniform width between the glass, and prevent sealant distortion.


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.


A measurement of siding. One square equals 100 square feet (10 x 10 wall).

SST non-metal 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.

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.

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.


(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.


A flexible framing material used to even a surface prior to installation.

Starter Strip

An accessory applied directly to the surface of the building and used to secure the first course of siding to the home.

Sweep Lock

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

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.


(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.

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.

TrueCapture™ Sloped Sill

The sill of some Alside double-hung windows that has a downward slope toward the outside with a capture dam that helps to keep water from infiltrating the base of the bottom sash. Sloped sill assists water drainage to the exterior of the window.


Weather-resistant material placed under vinyl siding panels.

UV (Ultraviolet light)

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.

UV 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.

UV reflection

The percentage of ultraviolet rays being blocked rather than being transmitted through the window’s glass unit. The higher the number, the lower the percentage of ultraviolet rays being transmitted through the window.

U-value (U-factor)

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 (W/sq m-°C). 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.

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.

Visible Light

The portion of the electromagnetic spectrum that produces light that can be seen. Wavelengths range from 380 to 720 nanometers.

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.

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.

Warm-edge technology

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


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.

Weep Holes

Openings cut into siding or accessories to allow for water runoff.


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

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.

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