If you are replacing old windows, you need to choose one of three basic types of installation:

Sash Pack

If your existing frames and trim are in good shape, and you mainly want to get better energy efficiency or reduce drafts, consider a sash pack—the least expensive option. In a typical installation, old sash and tracks are removed, and jamb liners are installed against the sides of the window frame. They seal and secure the new sash.

Some manufacturers carry over 100 stock sizes for sash packs, fabricate custom sizes, and offer many colors, cladding, tilt-in hardware, divided-light grills, and other features. Some sash-pack sellers provide very good instructions for homeowners who want to save money by doing their own installation work.

Frame and Sash

This more common and more expensive replacement option consists of a fully framed sash unit that slips into the existing window frame after the old sashes and tracks are removed. In this case, again, the existing frame and trim are left in place, so they must be sound; framed replacements can’t compensate for major leak damage.

The key to this option is the amount of space between the old and new frames. A good match will produce a close fit, with no two-by-fours added to pack out and significantly downsize the opening. Glass area may be reduced by an inch or so, but not by several inches. Small gaps between the old and new frames are fine; they can be insulated and existing trim built up with narrow strips that blend into the overall façade. Installers should tuck in loose-fill insulation or spray in low-expanding foam. (Standard foam has enough pressure to bow the jambs.) On the other hand, if the fit leaves large gaps, they typically require wide boards or aluminum panels to bridge the openings; the result looks out of scale and doesn’t fit in with the façade.

Full Window

This start-from-scratch option is the most expensive approach. The old unit is pulled out, any damaged framing repaired or replaced, and then a new window installed as if it were new construction. This is the only option if the framing needs to be significantly altered. It also avoids the reduction in glass area produced by the frame-and-sash approach.

If your old windows are stock sizes (most are), you shouldn’t have to spend extra for custom construction. Window manufacturers’ catalogs often list 75 or more sizes just for double-hungs. For the rough opening (the distance between framing members that allows for shimming space), typical widths start at 24 inches and increase at four-inch intervals up to 48 inches. Typical heights start at 36 inches and range up to 72 inches.