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.
Prepare to spend about 4 hours removing old sashes and installing a replacement window. Before you begin, read and follow manufacturer's instructions for measuring and installing the window.
To check a frame for twist, tape two tightly stretched strings at each corner so they cross in the middle, as shown. If there is a gap greater than 1/4 inch where they meet, the replacement window will not fit well.
Also check the sides for straightness. Hold a level or other straightedge against each side. If at any point you see a gap between the straightedge and the jamb that is greater than 1/4 inch, the bend will be noticeable when you install the replacement window. You may be able to remove some trim and adjust the jamb so it is straight.
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.
Gently remove the inside stops. Pull the bottom sash out of the frame, cut the cords, and remove the sash. Remove the parting stop and the upper sash.
Remove the pulleys and the weights, remove the casing, and install insulation in the cavities where the weights were. Replace the casing, and caulk to seal it to the wall.
Apply a bead of caulk at the joint between the jamb and the sill. Cut a piece of flashing tape (sometimes supplied with the window) 12 inches longer than the opening width and apply it to the sill and partway up the jambs on each side.
Check the sill for level. If it is not level, place a shim 1/2 inch from either of the jambs to achieve level. Attach the shim with a finishing nail.
Remove the outer packing from the replacement window, but follow manufacturer's instructions for leaving some of the packing bands and plastic spacers.
Pull the sill adapter all the way out. Follow manufacturer's instructions for any other needed adjustments.
Set the window in the frame for a test fitting. Center the window so it fits with about 1/8 inch gap at the sides and the top. Make sure you can drive screws into the frame.
Apply a bead of caulk along the outside (blind) stops at the side and head jambs, and where the interior stool meets the exterior sill.
Insert the window into the opening. See that it is centered in the frame and press it into the bead of caulk. Near the top on each side, slip in shims and drive mounting screws only partway. (You may have to temporarily remove covers in order to drive mounting screws.)
Slip in shims near the bottom and the middle of each side. Check the frame for square by using a framing square or measuring the diagonals. Slide the sashes up and down, checking that they operate smoothly and align accurately.
Cut and remove any remaining packing bands. Check the sides for straightness. You may need to tighten or loosen one or more adjustment screws.
Finish driving the mounting screws near the top. Raise the lower sash, temporarily remove covers if necessary, and drive mounting screws near the bottom.
Cut off the shims. Apply a bead of caulk between the window and the side and header jamb. Reinstall the interior stops that you removed in Step 3; the stops will cover the bead of caulk.
Install the casing and set the nail heads. Apply the finish of your choice to the interior of the window. You may need to touch up the caulk and the exterior paint.
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