This story shows you how to install an insert replacement window.
A replacement window is made to fit into an existing frame so you don't have to remove the jamb, casing, sill, or exterior trim. Installing one is nearly as simple as installing replacement sashes. It is, however, important to get the measurement exactly right; err on the side of ordering a replacement smaller than the opening.
These units have for years been called "vinyl replacement windows," but now you can also buy wood and clad-wood models. You'll also find plenty of options for muntins and grilles that offer you the chance to upgrade your windows' style.
Because the insert fits inside the existing window jamb, interior casing and exterior trim may need to be wider than the trim for the old window.
About 4 hours to remove old sashes and install a replacement window
Flat pry bar, chisel, framing square, screwdriver, drill, saw, tape measure, hammer, string, level
Measuring accurately, removing trim pieces, stuffing insulation, caulking
Read and follow manufacturer's instructions for measuring and installing
Replacement window to fit, caulk, shims, finishing nails
Check that the window frame (made up of the side and header jambs plus the sill) is square. Hold a framing square against each corner or measure the diagonals. If there is more than a 3/8-inch difference between the two measurements, check with your window dealer to see if the window will fit. Also check for straightness and twist.
Measure the distance between the insides of the right and left jambs. Take this measurement in three places: top, middle, and bottom. Also measure from the top of the jamb to the point where the front of the bottom sash touches the sill. Use these measurements to order a replacement window, which should be about 1/4 inch narrower and shorter than the frame.