- 1). Measure the opening with the old windows still in place, from the interior of the home. Take the measurements from two or three points along the width and height of the window opening, moving the sashes as necessary to access the edges of the opening. Order the replacement windows using the smallest span you find in both directions.
- 2). Remove the existing windows after you receive your replacement windows. Remove them by prying off the perimeter of trim around the inner edges of the opening, from the inside, so you can pull them into the room. Don't break the trim as you take it off. Set it aside. (Note: There will be an exterior trim border as well that should be left intact.)
- 3). Set the new window unit into the opening. Push it until it comes to rest against the exterior trim border. Set a level on one side of it to check for square. Insert shims around it until it's right.
- 4). Mount the window unit using 2 1/2-inch mounting screws, shooting them outward through the screw holes in the edges of the unit and into the surrounding frame.
- 5). Spray insulation into the spaces around the perimeter of the window unit.
- 6). Set the trim back in place around the window unit using a finishing nailer and covering the insulated space.
- 7). Run caulk all around the trim inside and out. Re-caulk the lines where the exterior trim meets the aluminum siding so the caulk looks the same as it does around the rest of the area.
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