How to Replace Very Old Window Sizes

Measure each window's width from jamb to jamb at the top, middle and bottom. Measure each window's height from the top of the sill to the underside of the head jamb in three locations across the width. Record the smallest dimension in each direction for every window. Check the frames for square by measuring on the diagonal in both directions. Take the measurements to a home improvement store and order custom-sized windows if the diagonal measurements are within 1/4 inch of one another. Wait for the new windows before proceeding.

Remove the window sashes by prying the old sash stops from the inside of the window frame, being careful to not damage the trim. Tilt the inner sash toward you, until you can access the window weight cords. Cut the cords and allow the weights to fall inside the wall. Remove the sash from the window frame. Repeat for the upper sash, after removing any additional interior trim that may be holding up the sash.

Pry the old jamb liner out of the window frame carefully, using a pry bar. Remove the window weights from the inside of the wall. Take this opportunity to vacuum the insides of the window frames thoroughly; they may be coated in a lifetime of dust and debris. Use a paint scraper to eliminate damaged paint or ridges left over from multiple paint jobs. Fill holes with wood putty and sand it smooth.

Drill three holes in the header jamb: one near each corner and one in the center. Repeat for the sill. Vacuum the debris. Fill each hole with expanding foam until it begins to ooze out. Fill the sash channels with expanding foam along the inside corners. Allow the foam to expand fully and allow it to dry.

Knock down any excess dried foam using a putty knife -- you will be able to slice through it easily. Replace the side jambs over the foam insulation using trim nails. Caulk the side jambs where they meet the interior and exterior trim.

Tilt the replacement window into the frame with the help of a friend. Check the level of the window while your friend holds it. Insert shims along the top and bottom of the window to level it in the opening. Check the square using a tape measure as described in the first step. Screw the window into the frame near the corners, then screw the window to the frame near the center.

Caulk around the outside of the new window where it meets the old window frame. Reinstall the stops you removed to evacuate the sashes for a more authentic look. Finish the window insert to match the rest of the window, if applicable (do not try to paint or refinish a vinyl replacement).