Your complete reference for window terminology. Everything from frame materials and glass types to energy ratings — clearly explained.
A
Air Infiltration
The amount of air that passes through a closed window. Lower values indicate better energy performance and a tighter seal. Measured in cubic feet per minute per square foot (cfm/ft²).
Annealed Glass
Standard, unstrengthened glass that breaks into sharp shards. Often used in interior applications where safety glazing is not required.
Argon Gas
An inert, odorless gas used between panes of insulating glass. Argon is denser than air and improves the window's thermal performance by reducing heat transfer.
Awning Window
A window hinged at the top that opens outward from the bottom. Awning windows provide ventilation even in light rain, making them popular for bathrooms and kitchens.
B
Bay Window
A multi-panel window that projects outward from the wall, creating a small alcove or extra floor space inside. Typically composed of a large center window flanked by two angled side windows at 30° or 45°.
Bow Window
Similar to a bay window, but with a rounded, curved projection using four or more panels set at equal angles to create a gentle arc.
Brick Mould
The exterior casing or trim that surrounds a window frame and covers the gap between the frame and the exterior wall surface.
BTU (British Thermal Unit)
A unit of heat energy. In window context, BTU ratings indicate how well a window prevents heat flow between indoors and outdoors.
C
Casement Window
A window hinged on one side (left or right) that opens outward like a door. Casement windows provide excellent ventilation and seal tightly when closed.
Caulk / Caulking
A flexible sealant used around window frames to close gaps and prevent air and water infiltration. Should be inspected and replaced every 5–10 years.
Condensation
Moisture that forms on a window surface when warm, humid indoor air contacts a cold glass surface. Condensation on the inside of a window indicates poor insulation.
Conduction
The transfer of heat through direct contact between materials. Window frames and glass panes can conduct heat, which is why insulating materials (vinyl, argon gas) are used to reduce it.
D
Double Glazing
A window unit with two panes of glass separated by an air or gas-filled space. Also called "double-pane" or "insulating glass." Significantly more energy-efficient than single-pane windows.
Double-Hung Window
The most common window style. Both the upper and lower sashes slide vertically, allowing ventilation from either the top or bottom. The sashes typically tilt in for easy cleaning.
Desiccant
A moisture-absorbing material placed inside the spacer bar of an insulating glass unit. It prevents fogging between the panes by absorbing any moisture that enters the sealed unit.
E
Egress Window
A window that is large enough to serve as an emergency exit. Building codes require egress windows in bedrooms, especially in basements. Minimums typically include a 5.7 sq ft opening.
Energy Star
A U.S. government-backed program certifying products that meet strict energy efficiency standards. Energy Star windows are independently tested and verified to reduce home energy costs.
F
Fixed Window
A window that does not open. Fixed windows (also called "picture windows") are used to maximize light and views. They provide an excellent seal since there are no moving parts.
Flashing
Thin metal or rubber strips installed around the window frame to direct water away from the window opening and prevent leaks. Proper flashing is critical to preventing rot and water damage.
Frame
The structural surround of a window that holds the glass and sash. Vinyl frames are the most popular today due to their durability, energy efficiency, and low maintenance.
G
Garden Window
A window that projects outward like a small greenhouse, with a top panel and side panels of glass. Used primarily in kitchens to display plants and let in abundant light.
Glazing
The glass portion of a window. Also refers to the process of installing glass into a window frame. "Single glazing," "double glazing," and "triple glazing" indicate 1, 2, or 3 panes of glass.
Grilles / Grids
Decorative bars or dividers that create the appearance of multiple smaller panes within a single window. Can be internal (between glass panes) or external. Also called "divided lites" or "muntins."
H
Head
The horizontal top member of a window frame or rough opening.
Hopper Window
A window hinged at the bottom that opens inward from the top. Commonly used in basements for ventilation.
Hung Window
See "Single-Hung Window" or "Double-Hung Window." A window style where sashes slide vertically.
I
Infiltration
The unintended movement of air into or out of a building through cracks, gaps, or poorly sealed window frames. High infiltration increases energy costs and reduces comfort.
Insulating Glass Unit (IGU)
Two or more panes of glass sealed together with a spacer between them to create an insulating airspace. The most common window glass type sold today.
J
Jamb
The vertical sides of a window frame or door frame. The jamb depth must match the wall thickness for proper installation.
Jamb Extension
An added piece of material that extends the jamb to the interior wall surface when the window depth is shallower than the wall thickness.
K
Krypton Gas
A denser inert gas than argon, used in high-performance triple-pane or very thin window units for superior insulation. More expensive than argon gas.
L
Laminated Glass
Safety glass made by bonding two panes together with a plastic interlayer. When broken, the glass stays together instead of shattering. Required for skylights and some impact-resistant applications.
Lite
A single pane of glass in a window. Also spelled "light." A six-lite window has six individual glass panes.
Low-E Glass
Glass with a thin metallic coating that reflects infrared and ultraviolet light while allowing visible light to pass through. Low-E glass significantly reduces heat transfer and UV fading.
M
Muntin
The strips of material (wood, vinyl, or aluminum) that divide a window into multiple panes or simulate divided panes. Also called grilles or grids.
N
NFRC Rating
National Fenestration Rating Council — an independent organization that certifies the energy performance of windows and doors. Look for the NFRC label to compare windows accurately.
O
Obscure Glass
Glass with a frosted, patterned, or textured surface that allows light through while obscuring visibility. Commonly used in bathrooms and privacy applications.
Operating Window
Any window that opens, as opposed to a fixed/picture window. Includes single-hung, double-hung, casement, awning, and sliding windows.
P
Picture Window
A large fixed window that does not open. Provides maximum light and views. Excellent energy efficiency since there are no moving parts or seals that can deteriorate.
Profile Depth
The measurement from the exterior to the interior face of a window frame. Must match wall thickness for a proper fit without jamb extensions.
R
R-Value
A measure of a material's resistance to heat flow. Higher R-values indicate better insulation. Not commonly used for windows — U-factor is the preferred metric.
Rough Opening (R.O.)
The framed hole in the wall where a window is installed. Typically 1/2 inch larger than the window on all sides to allow for shimming and leveling.
S
Sash
The movable part of a window that holds the glass. In a double-hung window, both the upper and lower sashes are separate movable units.
Simulated Divided Lites (SDL)
A window appearance option where grilles are adhered to the outside and inside of the glass to mimic the look of true divided lites (individual panes) while maintaining the energy benefits of a full-pane IGU.
Single-Hung Window
A window where only the bottom sash moves up and down. The upper sash is fixed. More economical than double-hung windows with similar appearance.
Sliding Window
A window where the sash slides horizontally left or right along a track. Also called a "gliding window." Ideal for wide openings and easy operation.
Solar Heat Gain Coefficient (SHGC)
A measure of how much solar heat passes through the window. Ranges from 0 to 1. Lower values mean less solar heat gain — desirable in hot climates or south/west-facing windows.
Spacer Bar
The material (typically aluminum or "warm-edge" foam) that separates the two panes in an insulating glass unit, maintains the gap, and contains the desiccant.
T
Tempered Glass
Safety glass that has been heat-treated to be 4–5x stronger than standard glass. When broken, it crumbles into small rounded pieces instead of sharp shards. Required near doors, in low sills, and in bathrooms.
Thermal Break
An insulating material built into a window frame to interrupt the path of heat conduction. Vinyl frames provide a natural thermal break because vinyl is a poor conductor of heat.
Tilt-In Sash
A sash that can be tilted inward for easy cleaning of the exterior glass surface from inside the home. A standard feature on most modern double-hung vinyl windows.
Triple Glazing
A window unit with three panes of glass and two insulating airspaces. Provides superior thermal and acoustic performance. Most common in cold northern climates.
U
U-Factor (U-Value)
The primary measure of a window's insulating ability. Measures heat loss in BTU/hr·ft²·°F. Lower U-factors = better insulation. Energy Star minimum is 0.30 for most northern climates.
V
Vinyl Frame
A window frame made from PVC (polyvinyl chloride). Vinyl is the most popular frame material today because it is energy-efficient, durable, virtually maintenance-free, and never needs painting.
Visible Transmittance (VT)
A measure of how much visible light passes through a window. Ranges from 0 to 1. Higher values mean more natural light. Coatings and tints reduce VT.
W
Warm-Edge Spacer
A spacer bar made from low-conductivity material (foam, fiberglass, or composite) instead of aluminum. Reduces heat loss at the edge of the glass and reduces condensation risk.
Weep Hole
Small holes or slots in the window frame sill that allow water (from rain or condensation) to drain out and prevent water from pooling and causing rot or leaks.
Weatherstripping
Sealing material applied around the movable parts of a window to prevent air and water infiltration when the window is closed. Should be inspected and replaced when worn.
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