Measure Your Window
- To order your new window, you need a height-by-width measurement. To get the measurements of the opening without taking out the old window, measure from the inside vertical channel behind the front piece of trim, in the trough where the bottom window sash sits. Measure the width in three places, top, middle and bottom, moving the sash up or down to get at the channel. Take the smallest of three measurements and subtract 1/4 inch; this is the width measurement to order your window. Do the same with the height measurement, taking three readings and subtracting 1/4 inch from the smallest. Order your window.
Remove the Window
- After your new window arrives, double check its measurements to make sure it will fit in the opening. Then, with a flatbar and hammer, carefully remove the thin strip of trim going all around the window on the inside toward the room, which holds in the sashes. (Don't break the trim; you'll need to reinstall it later.) Pull out the bottom sash. Remove the vertical divider behind it, and take out the top sash. Leave the exterior stop-trim in place.
Install the Window
- Set the bottom of your new window unit onto the sill, and push the top in. The exterior stop-trim on the outside will prevent it from falling through. Shim it all around the window casing to get it level and square, using a small level on the sides and middle. Once it's square, screw it in from inside the vertical spans of the new unit, into the side-jamb, going through the provided screw holes. (Generally, there are just three screws on each side, and none on the top or bottom.) Stuff loose insulation all around the edges. Put the face trim back around the front, trapping the window into place. Caulk it well all between the window and the trim, inside and out.