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Glossary of Home Improvement Terms

Here is a comprehensive list of terms to help prepare you for your next home improvement project. This terminology is often used by installers, designers and other professionals in our field. We work hard to keep our customers informed and in the loop.

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

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

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

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

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

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

(American Society for Testing and Materials) An organization that creates methods for testing of materials.

An interior molding that is 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 seen on older casements or swinging screens.

Awning window
A window that is hinged at the top and swings outwards for easy ventilation.

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

Balance covers
A snap in covering that hides the EvenForce™ block and tackle balance system within the window frame, keeping dirt and dust out of the chamber.

Basement Window
A basement sash, also known as a cellar sash, that consists of 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 protrude out from the wall of the home. The windows vary in angles and are usually positioned at 30- or 45-degree angles.

Beveled MasterFrame
Some Alside windows feature a unique fusion-welded design that accommodates different installation methods and architectural styles. It is the angled portion of the masterframe profile that adds a three dimensional look 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 upwards and downwards inside a metal chamber. Tension from a heavy duty coil spring at the top of the block creates the proper resistance necessary for smooth function of the window sash.

Bow window
An angled combination of windows in 3-, 4- or 5-lite design. As the windows are connected to each other, they combine to create an arch shape that projects from the wall of the home.

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

Brick Molding
A standard type of milled wood.

B.T.U. – (Btu)
(British Thermal Unit) The heat needed 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, soffit panel, accessory piece or the opposite the nailing slots, which locks onto the existing panel.

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

Cam-action lock and keeper 
The mechanisms that pull and secure the sashes together when placed in the locked position.

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

Casement window 
A window with a side-hinged sash that opens and closes outward by using a crank handle mechanism. Available in continuing mainframe, with multi-lite configurations.

Also known as trim, the 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 used for filling joints and sealing cracks to prevent leakage of water and air; commonly made of silicone, bituminous, acrylic or rubber-based materials.

Abbreviated °C, a centigrade scale of temperature measurement that uses 0° as the freezing point and 100° as the boiling point of water.

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

Check Rail
The horizontal members, of a double-hung window, which come together. It is also known as meeting rail and lock rail.

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

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

The brand name for the insulated glass unit that is used 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 consisting of a Low E surface. It will uses the SST warm edge spacer system and contain either argon or krypton gas.

Coated Glass
A window glass with an outside surface that consists of a mirror reflective surface; the shading coefficient ranges from 20% to 45%.

An architectural style associated with an early American style c. 1730.

Colonial Windows
Windows with small rectangular panes, or divided lites, consisting 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, with the screen panel exposed.

Commercial Standard
A intended 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 collection 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 for example. A heat transfer process created 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. In the case of vertical siding, the panel runs from top to bottom.

Coved glazing beads 
A contoured piece of vinyl that holds the glass in place within the sash that creates an elegant, finished appearance.

Cove Molding
Trim molding that has a concave face.

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

A rectangular opening or groove (with 3 surfaces) cut across the grain of a wood member, into which another board is placed.

A measure of heating demand, based on the difference between the mean daily outside temperature and 65°F. Combined 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 many prescribed extreme weather conditions. The values are helpful for selecting heating equipment and for estimating seasonal energy requirements. Incursion heat loss is a part of the design heat loss.

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

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

Double-hung window 
A window that consists of two vertical operating sashes.

Double Channel Lineal 
A siding accessory that combines two soffit panels.

Double-Strength Glass
Sheet glass whose thickness is between 0.115″ to 0.133″ (3 to 3.38mm).

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

Double Window
Two windows separated by a mullion, creating a unit. Also know as a coupled window.

Drip Cap/Head Flashing 
An accessory installed with vertical siding to ensure that water drips away from panels and does not enter them; it is also used as a vertical base.

Dry Glazing
A type 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 prevents oxidation and corrosion.

Egress Code 
The minimum opening of a window for people to be able to get out of or firefighters to enter a building/dwelling. Different states or regions have different code regulations.

Emergency Exit Window
Egress Window, also known as a fire escape window, have a large enough opening for a person to climb out; each bedroom should be provided with exit windows.

Energy Star® 
The Energy Star program is a combined 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 decreases the amount of CO2 emissions emitted 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 exceeds all performance requirements by the Energy Star program.

A brand name of block and tackle balance system by Alside. A balance system is a device that holds the vertically sliding sashes in any desired position within the window mainframe.

Extension Jamb
A extension jamb, also known as a jamb lining or a jamb extender, is 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 visible once the panel has been installed.

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

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

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

The placement of window openings in a building wall, one of the crucial elements in controlling the exterior appearance of a building. Also, a window, door or skylight and its associated inside or outside elements, such as shades or blinds.

Finish Casing
Also know as finish trim, the interior trim boards are located around a window unit.

Fire-Escape Window
(Emergency Exit Window) Window which opens onto fire escape; which are intended for emergency exits.

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

Fixed Panel 
A panel of a sliding glass door or slider window that is inoperable.

Fixed Window 
A window with inoperable sashes.

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

Flat Glass
Glass sold in flat sheets and named according to the method used in its manufacture such as window, plate, float, rolled and cylinder.

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

French patio doors 
A two panel glass door where both panels function and swing either outward or inward.

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

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

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

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

Garden window 
Designed much like a bay or bow window, a garden also protrudes from the wall to the outside of the home. It is built in a rectangular or square shape at right angles. The two side lights often function for additional ventilation.

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

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

The sheets of glass or plastic in a window, door or skylight.

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

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

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

A long, narrow cut on the face of a wood member; a groove across the grain is also known as 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 inside its frame.

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

The upper or top part 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 parts at the top of the window frame.

Also known as lintel or beam, a 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 lintel is generally in reference to a steel beam, whereas header is generally in reference to a wood beam,

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

Heat Transfer Coefficient
Also known as 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 numerical equivalent to the ‘inverse of the sum of R-values’ for the construction.

A joint that moves enabling a window to swing open.

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

Hopper Light
Also know as hopper vent or hopper ventilator, the inward opening sash hinged at the bottom.

Hopper Window 
A bottom-hinged sash window that operates by opening inward for ventilation.

Horizontal Sliding Window
Also known as a horizontal slider, windows that slide horizontally.

Hung Window
Window that consists of one or more hanging sashes.

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

Insect Screen
Also known as a window screen, a woven mesh of metal, plastic, or fiberglass stretched over a window opening to permit air to pass through, but not insects.

Inside Casing
Also know as interior casing, interior finish or interior trim, the inside visible molding surrounding the interior of the window frame

Insulating air chambers 
Different chambers within the sash and masterframe, which help insulate and strengthen the window.

Insulating Glass
Double- or triple-glazing with an enclosed, dehydrated, and air-tight sealed air space between the panes; the space is commonly from 3/16″ to 3/4″.

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

The fusion-welding process used on some Alside windows.

Interior Glazes
Glaze that is installed from the inside of building.

Interior Mullion Casing
The inside trim between closest windows.

Jal Awning
Also known as an awning window, a window with several out-swinging, awning-type windows that pivot towards the top of the glass and operate in unison.

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

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

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

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

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

Also known as a catch or lock, a device which holds a window closed, such as the latch at the meeting rail of a double-hung window or one mounted on the stile of casement windows.

Lead Light
Lead light, also known as lead glazing and stained glass, are windows with small panes of glass set in grooved rods of cast lead. The glass can be clear, colored or stained.

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

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

A sheet of glass in a window.

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

Lock Stile
The upright 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 mainly 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 reflection of longwave infrared radiation.

The elevated “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 an arch or moldings.

When the head, sill and jamb sections of a window are combined.

Meeting rail 
The member of a sliding glass door, hung window or sliding window where two panels meet and create a weather barrier.

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

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

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

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

Moisture Barrier
Also known as a vapor barrier, a material which retards the passage of water vapor from one space to another. Polyethylene sheet is frequently used as a vapor retarder.

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

A relatively narrow strip of wood used to hide a joint or to emphasize ornamentation of a structure.

Mould is the British spelling of mold and molding.

Vertical part between window units.

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

Muntin Grilles
Grilles consisting of wood, plastic or metal.

Multi-Lite Sash
A sash that is divided into many lites.

A weatherstripping material that is present where the sash frame touches the masterframe. This weatherstripping increases resistance to air infiltration.

Nailing Hem (or Flange) 
The portion of siding or accessories where the nailing slots are located.

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

National Fenestration Rating Council

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

Obscure glass 
Glass that is created to be translucent and not transparent.

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

Oriel Window
A window that projects from the wall and is carried on brackets, corbels or a cantilever. Unlike a Bay Window, the oriel’s projection does not extend all the way to the ground.

The placement of a window, room or building with respect to the sun, wind, earth, access or view.

Origin II™
The virgin uPVC vinyl utilized in all Alside windows. The material’s low thermal conductivity makes it the perfect choice for window manufacturing. This vinyl will not rot, peel, blister, swell or deteriorate from corrosion or pitting.

Outside Casing
Also known as outside facing, outside trim or exterior casing, the portion of the window frame which is exposed to the outdoors.

Outside Glazing
Glazing that is installed from the outside.

A sheet of glass used for glazing a window. After installation, the pane is also know as a ‘light’ (lite) or ‘window light.

A large 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 can  be fixed or sliding.

Parting Bead
Also know as parting strip and 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 pattern; used for privacy and light diffusion.

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

Picture window 
A picture window that does not open.

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

Also know as a plow, a rectangular groove or slot (with 3 surfaces) cut parallel with the grain.

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

Positive Lock 
Area below the nailing hem which the buttlock locks into.

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

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

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

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

Psychrometric Chart
A chart which shows the 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 displayed.

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

Heat transfer 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 the same way.

A rectangular notch which consists of two surfaces that cut parallel with the grain of wood along the edge.

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

The horizontal part of a window sash which can come in head, top, bottom or meeting rails.

Reflective Glass
Window glass that is 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 equal groove for a spline-type gasket to hold window glass in place.

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

A measurement consisting 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 displayed in units of hr-sq ft-°F/Btu. A high-R-value window has a higher resistance to heat flow and a higher insulating value than one with a low R-value.

Safety Glass
A reinforced or strengthened glass that is less subject to breaking or splintering and is often used in storm windows.

Separate from the masterframe, a sash is the portion of the window that holds the glass.

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

Sash and Frame
A sash and frame consist of a window and its casing frame.

Sash limit locks 
A feature that allows a window to be safely raised to a particular height.

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

Running a utility knife blade, scoring tool, a sharpened awl or other sharp implement across a siding panel or soffit 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 preventing a mess.

A close-mesh woven screening material consisting 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 junction or opening of two parts, such as between the glass and a metal sash, often made of silicone, butyl tape or polysulfide.

Sealed Double Glass
Two panes of glass that are separated by a sealed space.

Shade Screen
Also know as sun screen, a specially created 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 very small insects can pass through. Also referred to as an awning with fixed louvers of metal or wood construction.

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

Shatter-Proof Glass
Also known as laminated glass, two sheets of glass with a transparent plastic sheet sandwiched between to create a pane resistant to shattering.

Side Light
Also know as a margin light, a fixed, often narrow, glass window next to a door opening or window.

An enduring sealing agent that protects against water.

The horizontal, bottom part of the masterframe.

Single-Hung Window
A window that is similar to a double-hung window except that the top sash is does not move.

Single-Strength Glass
A glass that has a 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 allows solid brass roller trucks to operate. Available in 2-, 3- or 4- lite options with the operable panel available in any position.

Sling Psychrometer
A measuring instrument with double 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
Also know as a sliding window, a window which moves horizontally in tracks or grooves.

Sliding Window 
A window whose sashes move horizontally. Available in a 2- or 3-lite options.

Material used to encase the horizontal underside of a cornice, eave or overhang. Some soffit panels can also be used as part of vertical siding.

Solar Heat Gain Coefficient (SHGC) 
The fraction of solar radiation administered through a window or skylight, both directly transmitted and absorbed and afterwards released inward. The solar heat gain coefficient replaced the shading coefficient as the basic indicator of a window’s shading ability. It is displayed as a number between 0 and 1. The lower a window’s solar heat gain coefficient, the decrease in solar heat it transmits and the greater its shading ability. SHGC can be diplayed in terms of the glass alone or can refer to the entire window assembly.

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

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

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

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

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

Known as measurement of siding. One square is also known as 100 square feet (10 x 10 wall).

SST non-metal spacer 
A solid silicone foam spacer that is covered with Mylar. It is sealed to the edge of the glass and then sealed with butyl for additional energy efficiency.

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

Stained Glass Window
A window with a painted scene or design that has been fired into the glass. Windows with plain colored glass set in lead are also known as stained glass.

Stationary Sash
A sash that does not move; also referred to as a picture, studio, vista or view sash.

The vertical-edge parts of a window sash.

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

(Bead, Side Stop, Window Stop, Parting Stop) The molding located 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
Also know as storm window, an extra window on the outside to protect the current window, but mainly to add thermal resistance of the window.

A flexible framing material used to even a surface before the installation.

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

Sweep Lock
A sash fastener that can be found 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 that breaks into pebble-sized particles but not into slivers.

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

Thermal Barrier
(Thermal Break) – A material that has a high thermal resistance and is placed between two metal sash, or installed between adjoining metal framing of metal windows, in order to decrease thermal conduction from indoors to outdoors.

Thermal Conduction
Heat transfer through a material by contact of one molecule another. Heat moves from a high temperature area to one of lower temperature.

Thermal Conductivity
Heat transfer property of materials displayed in units of ‘Btu per hour per inch of thickness per square foot of surface per one degree F. temperature difference.’ Also known by the letter ‘k.’

Thermal Conductance
Thermal Cconductance is the same as thermal conductivity except thickness is ‘as stated’ rather than one inch. Also know by the letter ‘C.’

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

Thermal Resistance
Also know as R-value, the property of a substance or construction which retards the flow of heat.

Also known as a transom bar, a horizontal member dividing a door from a window panel above the door, or separating one window above another.

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

the molding surrounding a window opening that can be seen.

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

Triple Window
A term generally known as any tripartite group of windows with square heads. These are frequently used 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 entering the base of the bottom sash. Sloped sills help water drainage to the exterior of the window.


Weather-resistant material placed underneath the vinyl siding panels.

UV (Ultraviolet light)
The invisible rays of the spectrum that is located outside of the visible spectrum at its short-wavelength violet end. Ultraviolet rays are found in everyday sunlight and can cause paint finishes, carpets and fabrics to fade.

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 carpets and draperies..

UV reflection
The percentage of ultraviolet rays being blocked rather than being traveled through the window’s glass unit. The larger 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 increase or decrease through a material or assembly. It is stated in units of Btu/hr-sq ft-°F (W/sq m-°C). Values are usually 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 entire window or the glass alone, 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 the insulating value is

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

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

Visible Light 
The part of the electromagnetic spectrum that creates light that can be seen. Wavelengths vary from 380 to 720 nanometers.

Visible Spectrum
The part of the total radiation that is seen by the human eye and lies between the ultra-violet and the infra-red portions of the electromagnetic spectrum. The colors that consist of the visible spectrum range from violet, indigo, blue, green, yellow, orange, through red.

Visible transmittance (VT)
The fraction or percentage 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 decrease heat transfer near the edge of insulated glazing.

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

A type of seal that prevents the entry of air and precipitation into the structure.

Weep Holes 
Openings cut into siding or accessories that allows water to runoff.

A glazed opening in an external wall; an entire that includes the frame, sash and glazing and any operable elements.

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

Window Hardware
Different 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 entire window with sash and frame.

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

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