There are three methods of installation:
New Construction Style-
As the name indicates, this style is used mostly when a home is being built or extensively remodeled. This installation style eliminates the existing frame and the new window is fastened directly to the 2x4 wall framing members. Once secured into the framed opening, the window is then flashed, lathed, and stucco patched. The interior is finished with drywall. Joint compound is skimmed over the drywall to blend the interior patch in with the rest off the wall.
Advantages: Very few, if any. In certain instances, this method may be required as the size of the original opening will be enlarged or otherwise modified. In the event a window needed to be lowered or raised, or if a door is to be installed in the place of a window this is the appropriate method of installation since the original frame cannot be reused.
Disadvantages: The biggest disadvantage is the extra cost. Because of the additional work involved in breaking back stucco, removing the old frame and then patching the exterior and interior walls to blend the installation back in, the cost is more than double the cost of other installation styles. The additional cost of this type of install is magnified when you factor in the cost of replacing the interior window treatments that are affected when the original frame is removed. In most cases, mini-blinds, drapes, shutters, etc., can't be re-used when the original frame has been removed, not to mention the extra cost in replacing tile, wallpaper, moldings, or anything else that comes in contact with your existing window. And don't forget you'll need to repaint the area around the new window, inside and out.
"Insert" Style Replacement-
This method uses the frame of the original window. After removing the sash (the moveable panels that hold the glass), the replacement window is "inserted" into the old frame, and trimmed out with molding.
There are two basic types of "Insert" style installations depending upon your existing windows.
Block Frame- The block frame replacement window is designed to fit into an existing wood window frame. The wood frame of the typical wood window is deep enough to allow the replacement window to fit completely inside.
Flange Frame- The flange style frame replacement window is designed to fit into an existing aluminum frame window. The window is inserted into the opening from the exterior up to where the flange makes contact with the old frame. Referred to as 'retrofit', 'Z-Bar', 'flush fin', etc., by different window manufacturers, the "flange" is designed to cover up the aluminum frame that remains in the wall after the sashes have been removed.
Advantages: Because the old frame is not removed, you'll enjoy cost savings over the new construction install method. Reusing the existing frame saves costly and unsightly stucco and plaster patching. The job is also completed quicker and with less mess.
Disadvantages: In the case of an existing wood window, the wood sill and exterior trim is still exposed and subject to continued maintenance. The overall appearance of the new window is often diminished when surrounded by a weathered and worn sill or one with peeling paint.
In the case of existing aluminum windows, the "flange" frames are produced by a window manufacturer and the flange is designed as a "one size fits all" application. Unfortunately, with so many brands of old aluminum and steel windows used in the past fifty years, the flange style frame doesn't always work. The typical flange has about 1 1/2" of coverage, but this assumes that the existing window has a smaller frame than 1 1/2" to be concealed. The flange frame also requires a smooth stucco surface to mount over. A heavy textured stucco, or a window surrounded by brick, rock or even siding can pose problems for the flange style method of installation. Even something as simple as wood trim around the old window frame requires that the flange be cut back or otherwise modified to allow the flange to fit the window. Many times, when the flange is modified in the field, the result is a rough edge, extra unsightly caulking and a potential for leaking. In addition, many homeowners have objected to the flange frame as appearing too "bulky" and heavy. With the extra 3" of flange around the replacement window frame, the window appears to have more frame than glass.
Finished or "Wrapped" Replacement Style-
This method starts out like an "insert" style replacement, but continues on and addresses the issues unresolved by the "insert" style.
For existing wood windows, this type of install uses the block frame and fits the window into the wood frame just like an insert style installation. What makes this different is, while the frame is left untouched with an insert style installation, the "wrapped" style wraps or clads over the exposed sill and parts of the remaining frame with a capsheet that matches the new window.
For existing aluminum windows, again, a block frame is used and inserted into the old aluminum frame. However, instead of relying on a factory designed 'flange', the installer creates the exterior trim specific to the existing wall conditions. If a wider trim piece is required to get adequate coverage over the old frame, the installer makes the adjustment. If the trim needs to be smaller to accommodate decorative wood trim around the window, or if siding or brick border the window, the installer creates a narrower trim to fit precisely. This big difference between this install style and the "flange" style in this type of installation allows for varying field conditions, where the factory flange requires a flat stucco surface for mounting.
Advantages: The window trim is "site specific" and created on site after the installers have had a chance to inspect field conditions. The homeowner is assured of getting the proper installation job, and not a compromised installation due to a factory flange frame. Since the original frame is being used, there are big cost savings over the new construction style of installation.
Many homeowners with existing aluminum windows prefer the look of the 'wrapped' window as opposed to the bulky flange frame. In some cases, a contrasting color wrap can be used on the exterior to create a dramatic effect. In addition, the field applied wrap can be painted (if desired) or if required by a homeowners association. In certain cases a homeowner's association will reject the "flange" style windows in favor of a block frame window with a wrap color of their choosing.
In the case of wood windows, no part of the wood frame or sill is left unprotected as would be with a straight 'insert' replacement. The maintenance of the remaining wood is eliminated. And with a matching capsheet around the frame, the appearance is much improved over a window with the worn, peeling and weathered wood frame still exposed.
Disadvantages: As with anything that is custom created, there is additional time and expense involved. However, the additional cost is generally less than 10% of the job, and is far outweighed by the numerous benefits provided by a style of installation that fits the window to your home, rather than relying on a factory created system that can't anticipate field conditions.