How to Replace a Window Unit Frame

Measure carefully to assess the correct size of the new window unit. Measure the distance from the top of the sill to the bottom of the head jamb near the left jamb, the middle of the window and at the right jamb. Write down the smallest measurement.

Measure the distance from one side jamb to the other on the top, middle and bottom of the window unit. Write down the smallest measurement.

Measure the diagonal of the unit in both directions. If the measurements are equal, the window is straight. If they are off by about 1/4 inch, the new frame can be wedged into place.

Consult your window retailer for the correct unit size to purchase based on your measurements.

Pry off the window stop around the sash, then carefully pry off the lower sash, taking care not to break them.

Remove any counterweights located behind the side jambs.

Remove nails from any flange holding the window on to the exterior. Remove trim and sheathing or siding, if necessary, to remove the window.

Carefully pop the entire window out from the outside, gently pushing and prying it free.

Repair any holes with putty, and scrape off chipping or peeling paint. Sand the jamb, prime and paint it. Let it dry completely.

Remove the current frame, if necessary. Replace the header with a new one cut to appropriate size per the recommendation of the new window's manufacturer. Nail it in place at the top of the rough opening. Cut and install side joints and a new sill, if necessary. Cut new cripple studs to sit in the space above the new header, using the previous framing as a guide for design.

Drill 3/8-inch holes along the sill and into the bottom of the header. Fill the holes with expanding foam per label instructions, letting the substance seep out. Also spray the sash weight pockets located in the side jambs. Let the foam dry per instructions.

Caulk the exterior facing along its inner face or on the top and side's blind stops as well as the windowsill. Let the caulk dry.

Set the window in place, bottom first, pushing it up into its space carefully so it sits flush against the exterior casings.

Install a screw on the upper jamb into one of the side joints. Test the window sash by opening and closing it. Use shims to stabilize the frame underneath the jambs, if necessary. Screw down the shims. Screw the window in place along the perimeter once it's functioning and stable.