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Windows provide natural light and ventilation and are classified by the method by which they open and close (e.g. double-hung window). Windows are made from wood, steel, aluminum, vinyl, vinyl-clad wood or aluminum. Some modern windows have a thermal break, usually Bakelite, between the interior and exterior part of the window to prevent condensation during the winter months.


  • Pane (or glazing): the plate of glass (usually 1/8 inch thick for single pane.)
  • Sash: the portion of the window that slides or pivots when you open and close the window unit. The sash includes the glass, its supporting framework, locks and lifts.
  • Rails: top and bottom part of the sash.
  • Stiles: sides of the sash.
  • Stops: hold the sashes in place when sliding or stops a closing window that pivots.
  • Jamb: the side of the window frame.
  • Sill: the bottom of the window frame.
  • Head: the top of the window frame.
  • Muntins or Mullions: used to divide the pane into several sections (not shown).
  • Casing or Trim: the decorative material (usually wood) that covers from the edge of the window frame to the finished wall (not shown).

Most insulated glass units are double or triple-pane windows that are sealed with an epoxy to create an air gap between each plate of glass. The window frame is perforated on the inside and filled with a desiccant material that absorbs the moisture vapors from between the glass plates. The wider the dry air space, the greater the insulating value of the unit. Manufacturers also use a coating over the glass for greater energy efficiency. The coating is often called low-emissivity or low E glass. Low E glass reflects radiant heat in the summer and retains interior heat in the winter. Very few manufacturers hermetically seal or create a dry air gap between the panes and then use an air tight seal. Anderson Windows switched from hermetically sealing the window to using the epoxy and desiccant method.

The double-paned window uses 1/8 inch or 3/16 inch glass separated by a 1/2-inch, 3/4-inch or 1-inch air space. A double-paned window is different from an insulated glass unit, because the glass panes are not sealed to create an insulated air space. Consequently, the panes can be replaced without replacing the entire sash.

Window Types

There are six basic window designs — double-hung and single-hung, sliding, casement, awning, fixed and skylight.

Hung Windows

The double-hung window, which has been used since the 1700s in the United States, consists of a lower and upper sash, each working independently of the other. The sashes move up and down and are often balanced by weights hung on ropes, chains or other balanced devices. The weights counter-balance the weight of the sash and make it possible to set the window open at any position. A modern, double-hung window uses springs on each side of the sash to hold the sash in place. A variation of the double-hung window is a single-hung window, where the top sash is fixed and only the bottom sash is moveable.

The double-hung window generally allows a maximum of 50% of the total window area to be used for ventilation. In other words, when the window is open as far as it will go, the two sashes are doubled up and only half of the window space is actually allowing air to pass through the opening.

Casement Windows

A window that is becoming extremely popular in today’s construction is the casement window. These units are hinged at the sides and swing out and in, like a door. The sashes are opened and closed either by a crank, a push bar mounted on the frame, or a handle fastened to the sash. The benefit of using a casement window is that 100% of the total window area is available for ventilation. If the casement unit pivots outward, the screens are located on the interior portion of the window frame.

Casement windows that are 4 to 6 feet or higher tend to have more problems than shorter/smaller windows. The single operator at the bottom may have difficulty opening or closing the window properly at the top if there is a small amount of friction.

Sliding Windows

Another type of window design is the sliding window unit, in which two or more window panes pass each other on a horizontal track. Sometimes, one of the two sashes is fixed, while the other sash is moveable. Much like the double-hung unit, only 50% of the total window area is available for ventilation. Generally, the sliding windows are of aluminum construction and found in the lower end of the sales or construction market. A major disadvantage of the aluminum windows is difficulty in obtaining replacement parts, such as rollers or corner retainers.

Awning Windows

Awning windows contain one or more top-hinged, outward swinging sashes. When open, the sashes extend out at an angle and resemble an awning. There are four variations of awning design: (1) the top-hinge window, similar to an awning window except that it opens inward instead of outward; (2) the utility window, sometimes called a “hopper” window, hinged at the bottom and usually found in basement windows; (3) the jalousie window, which contains a series of horizontal glass panes that open outward; and (4) the transom window, which is sometimes found above interior doors. All of the awning windows provide 100% of the total sash area for ventilation.

Fixed Windows

Another window design is the fixed window, which consists of a frame and does not contain movable sash components. Examples of fixed windows are: picture windows, decorated bay windows, and the 3/4 round window often found in the gable ends of older houses on either side of the chimney. Fixed windows, as the name implies, provide no ventilation.


The roof window, or skylight, can be fixed with a flat pitch, vaulted, ridge, pyramid or dome design, or a vented window equipped with a manual or power-operated sash. To keep rain out of the building, the vented skylight does not open as wide as the awning windows. Therefore, the effective ventilation area of the vented skylight is considered to be no more than 50% of the total skylight area. And in many cases, it is less than 30%. Skylights should be installed on curbs with metal step and counter flashing.

Window Defects

  • Broken glass
  • Glazing – Glazing compound is the putty-like material that is used to secure panes of window glass in place. Older windows that exhibit deferred maintenance (neglect) should be inspected carefully. Scraping, cleaning and re-glazing can cost as much as painting the window. Of course, this will depend on the condition of the glazing.
  • Broken sash cords – An examination of older wood, double-hung windows should be made for broken sash cords. The newer double-hung windows usually have either a spring device or a small, nylon cord, such as appears in Anderson windows.
  • Malfunctioning window crank mechanisms – These mechanisms are located in casement, awning, and jalousie windows. The hardware for some older windows may not be available, or if it is available, may be difficult to locate. These windows should be checked to ensure that they operate properly.
  • Painted shut or painted open windows – Quite often, windows in older houses are painted shut or painted open.
  • Failed thermal seals – Failed insulated glass seals will allow condensation to form between the two panes of glass. Sometimes, you can see the actual condensation, or other times you can only see the residue. In the early stages of failure, it may be difficult, if not impossible, to see evidence of the failure. The location of the sun, glare, reflections, time of day, angle and whether you are looking from the inside or outside may impact the visibility of stains from failed seals. Significant staining is easy to recognize, whereas early failures and modest staining may not be easy to see.
  • Deteriorated or missing weather stripping – This should be checked, particularly on wood and aluminum windows. Older steel casement windows quite often do not have weather stripping, and there is not a way of easily correcting this situation, short of replacement.
  • Condensation – Condensation may form on the inside of glass or metal frame windows when the relative humidity inside the house is high and temperature difference between the inside and outside is large enough to cause the moisture vapors to change to liquid on the colder surfaces. Look for water or water stains on the stool or windowsill, or at the drywall adjacent to the windows