Windows come in many sizes, shapes, and kinds, sometimes the variety and the terminology describing that variety--is confusing. The following definitions may help you see it all more clearly.
Awning window: A unit in which the bottom of the sash swings outward.
Bay window: A composite of three windows, usually made up of a large center unit and two flanking units at 30 or 45 angles to the wall.
Bow window: A composite of four or more window units in a radial or bow formation.
Casement window: A window unit in which the single sash cranks outward.
Circlehead: A generic term referring to any variety of window units with one or more curved frame members, often used over another window or door opening.
Clerestory window: Venting or fixed window above other windows or doors on an upper outside wall of a room.
Cottage double hung: A double-hung window in which the upper sash is shorter than the lower sash.
Double hung: A vertically operable window in which both sash operate
French doors: Doors which are primarily windows but have wide panel members around the glass. These may be hinges or sliding.
Hopper: A window unit in which the top of the sash swings inward.
Louver: A window made of slats of glass that overlap when closed
Muntin: Short bar, either vertical or horizontal, used to separate glass in a sash into multiple lights (i.e. windowpane divider or grille).
Oriel: A window with unequal sash.
Palladian window: A large, arch top window flanked by smaller windows on each side
Roof window: Operable window installed in the roof within easy reach.
Single hung: A double-hung window with the top sash fixed or inoperable.
Skylight: A fixed or ventilating window in the roof which cannot be reached directly. This includes systems which refract light to the inside through tubes.
Window Terms
A window must be energy efficient and durable. Below are a few of the many terms used by builders and manufacturers to evaluate these qualities:NFRC: The National Fenestration Rating Council has developed a new window efficiency rating system (appearing in 1998) using numbers between 0 and 10 to represent a window’s efficiency for each of the heating and cooling seasons.
ER: The North American Energy Rating assesses the overall performance of the glass, the frame, resistance to air infiltration, and beneficial solar gain
R-Value: How the window resists heat transfer. The higher the R-value, the better the insulating effect and the lower the rate of heat flow
U-Value: How well the window conducts heat. The lower the U-value, the better the insulating effect.
Low-E: A transparent coating fused into the glass surface. Standing for “low emissivity,” it allows the sun’s shortwave energy in from the outside while resisting heat transfer from inside to outside (good during cold weather). Longwave energy from the outside, e.g. reflected heat from sidewalks, is reflected off the glass (good during hot weather, also reduces harmful ultraviolet rays).
Insulating Glass System
Double or triple glazing (i.e. 2 or 3 panes of glass) with a scaled space in between. The space may be filled with a gas (often Argon) to provide greater insulation. They insulating ability of the spacers separating the panes also greatly affects the window’s performance
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