In both older and newer homes, the double-hung window is the most common unit, consisting of upper and lower sashes that slide vertically past each other. A friction fit, counterweights or spring balances are used to hold the sashes in fixed position within the window frame. In some windows, the sashes can be removed for cleaning or painting. Some homes have single-hung windows, with the upper sash fixed and the lower sash movable.
In a horizontal siding window, the sashes slide horizontally on separate tracks. Most commonly, the design consists of two sashes, both movable, however in some cases one sash is fixed. In most of these windows, the sash is removable for maintenance such as cleaning or painting.
With casement windows, two or more sashes are hinged at the side and mounted so that they swing outward. To open and close the sashes, a cranking mechanism, a push bar mounted on the frame or a handle fastened to the sash is used. Since sash opens outwards, a storm sash or screen must be attached to the inside of the window.
Jalousie windows are practically adjustable louvers – glass slats several inches wide, held by an aluminum frame at each end. Levers are used to interconnect the levers so that the slats open and close in unison, like venetian blinds. Jalousie windows are crank-operated and provide great ventilation, but the many glass slats make it difficult to wash. Also because of the slats they are not weathertight. Even with a storm sash, cold-air infiltrates around the windows.
Fixed-pane windows, as the name suggests, are stationary and are used only to provide daylight and outdoor views. They can be used either alone or in combination with sliding, double-hung or swinging windows to achieve a custom design.
When it comes to metal frames, they can get quite cold during the winter months, therefore some of the water vapor in the air inside the house tends to condense on the frames. Several manufacturers produce metal-framed windows with a thermal barrier preventing the outside frame from touching the inside frame in order to reduce condensation. However, although this significantly reduces condensation, it does not eliminate it.
Wood windows provide a better insulation than metal windows because wood has a greater thermal resistance, so wood window frames normally do not get cold enough for condensation to form on their inside surface. Wood windows, however, tend to swell or shrink with changes in moisture. If a wood sash absorbs moisture it will expand and bind in the frame, impeding it from operating properly. Wood windows should be treated by the manufacturer with a water-repellent preservative to make them decay- and moisture-resistant. Not all windows were treated with preservatives in the past, therefore rotting sections and binding sashes may occasionally be found during a windows inspection.