A
Annealed glass
Ordinary float glass that has been cooled slowly so it holds no internal stress. It is the default glass in most windows; it breaks into large, sharp pieces, which is why some locations and sizes call for tempered glass instead. See when glass must be tempered.
Argon fill
An inert gas sealed into the space between the panes of an IG unit in place of air. It conducts heat less than air, so it lowers the window's U-factor. The gas fill is part of the configuration an NFRC rating covers.
Awning window
A window hinged at the top that opens outward from the bottom, so it can stay open in light rain.
B
Bill of materials (BOM)
The list of every part needed to build one window: frame and sash profiles with their cut lengths, glass, hardware, screens and reinforcement. In window software the BOM is generated from the configuration with formulas, since every window is a different size.
C
Casement window
A window hinged on one side that swings outward, usually opened with a crank.
Centerline size
The size of one opening in a mulled unit, measured to the middle of each mull post beside it. The openings' centerline sizes add up to the overall size; it is usually the size a mulled opening is priced on. See the mulled window size calculator.
Condensation resistance (CR)
An optional NFRC rating from 1 to 100 of how well a product resists condensation on its inside surface; higher is better.
CPD number
The identifier of a product configuration in the NFRC's Certified Products Directory. It is printed on the NFRC label and ties the label's ratings to one exact combination of frame, glass, gas and grids.
Cut list
The lengths to cut from each profile for a batch of windows, usually grouped by profile and ordered for the saw. See cut lists in sticks and feet.
D
Dealer level (price level)
The pricing a manufacturer gives a dealer, agreed as a percentage off list and usually stored as a multiplier: 45% off is a multiplier of 0.55. See the dealer price calculator.
Double hung window
A window with two sashes that both slide up and down. In a single hung only the bottom sash moves.
E
Eyebrow (arch top)
A shaped window or shape top whose head is a shallow arc, rising less than half its width. An arch that rises a full half of its width is a half round.
F
Frame size
The size of the window actually built. In a mulled unit it is the centerline size less half a mull post on each shared side; cut lengths, glass and labels are worked from it.
G
Grids (grilles, muntins)
Bars that divide a window's glass into smaller panes, either sealed between the glass or fixed to its surfaces. Grids between the glass slightly lower a window's SHGC and visible transmittance, so they change its NFRC rating.
H
Half round
A semicircular window or shape top, as tall as half its width. A shallower arc is an eyebrow.
I
IG unit (insulating glass unit, sealed unit)
Two or more panes of glass held apart by a spacer and sealed around the edge, often with argon between them. It is the glass assembly that goes into one opening of a window. A single hung has two. See one label per sealed unit.
K
Kerf
The width of material a saw blade removes with each cut, often around an eighth of an inch. A cutting plan has to allow for it, or pieces that fit on paper won't fit on the stick.
L
Lite
A single piece of glass, or one glazed opening in a window. A window divided by grids is still the same number of lites unless the glass itself is divided.
Low-E coating
A microscopically thin, transparent metal-oxide coating on glass that reflects infrared heat. It lowers a window's U-factor and, depending on the coating, its solar heat gain.
M
Mull, mullion, mulled unit
A mullion (mull post) is the member that joins two windows side by side or one above another. A mulled unit is the assembly they make, sold to one overall size and split into openings. See how to size and price mulled windows.
N
NFRC
The National Fenestration Rating Council, the nonprofit that runs the US certification program for the energy performance of windows, doors and skylights. Its label shows U-factor, SHGC and visible transmittance, and optionally air leakage and condensation resistance. See what goes on an NFRC label.
O
Oriel (cottage) window
A hung window whose two sashes are unequal in height, most often with a shorter top sash. The split is usually stated as the top sash's share of the height.
P
Picture window
A fixed window that does not open, usually large, chosen for the view. It has one IG unit.
S
Sash
The frame that holds the glass in the operable part of a window: the part that slides, swings or tilts.
Solar heat gain coefficient (SHGC)
The share of the sun's heat that passes through a window, from 0 to 1. Lower keeps heat out in hot climates; higher lets winter sun warm a house. One of the three mandatory NFRC ratings.
Single hung window
A window with two sashes where only the bottom one slides; the top is fixed. In a double hung both move.
Slider (XO, XOX)
A window whose sash slides sideways. The letters describe it from outside: X is an operating sash, O a fixed one, so XO is a two-lite slider and XOX a three-lite slider with a fixed center.
Spacer
The strip that holds the panes of an IG unit apart at the edge, usually holding a desiccant to keep the space dry. "Warm-edge" spacers conduct less heat than metal ones.
Stick (lineal)
A full length of extruded profile as it is bought and racked, often 16 feet for vinyl. Bills of material count profile in feet; the rack counts sticks. See the vinyl stick estimator.
T
Tempered glass
Glass heat-treated to be several times stronger than annealed glass, which breaks into small, blunt pieces. Building codes require it in hazardous locations, and suppliers require it above certain sizes. See the tempered glass check.
U
U-factor
How quickly a whole window lets heat escape, in Btu per hour per square foot per °F. Lower is better. One of the three mandatory NFRC ratings.
United inches (UI)
A window's width plus its height, in inches. Many price books and charges, such as color upcharges, are priced by united inches.
V
Visible transmittance (VT)
The share of visible light that passes through a window, from 0 to 1. Higher means more daylight. One of the three mandatory NFRC ratings.