Unlike full-frame windows, which are designed for new construction, replacement windows are made to fit into existing window openings. They’re available in dozens of standard sizes, from as narrow as 11 ½ inches to as wide as 68 inches, and come in wood, vinyl, fiberglass, vinyl-clad wood, and aluminum-clad wood.

There are three basic types of replacement windows: sash kits, insert replacements, and full-frame units. Sash-replacement kits—what Tom found on the Newton house—give an old window frame new movable parts, including jamb liners and sash. The liners are fastened to the side jambs of the window opening, then the sash are slipped in between.

For these to work, the existing window frame must be level and square. An insert replacement window consists of a fully assembled window in a ready-to-install secondary frame. Sometimes called a pocket window, an insert replacement slips into the existing opening and is then fastened to the old side jambs. Because you’re adding new jambs and liners, the glass area will be slightly smaller than it was before.

Full-frame replacement windows are similar to inserts, except that they have a complete frame that includes head jamb, side jambs, and sill. These are the only option when the old window frame, sill, or jambs are rotted. To install these, you must strip the window opening down to its rough framing, inside and out.