In this video, This Old House general contractor Tom Silva shows the proper way to install a new window into an old—and oversized—doorway opening.
1. Remove the existing door and doorframe. Then, build a short wall to span opening.
2. If necessary, cut a rabbet in the 2x6 bottom plate to fit over the finished floor in the room. Cut the rabbet on a table saw.
3. Fasten the bottom plate across the bottom of the opening with 3½-inch nails.
4. Cut four 2x6 cripple studs to length and nail them to the jack studs and bottom plate.
5. Fasten a double-2x6 sill plate on top of the cripple studs.
6. Sheathe the outside of the wall frame with ½-inch moisture-resistant plywood.
7. Cover the seams around the perimeter of the plywood sheathing with 4-inch-wide self-adhesive flashing.
8. Install tarpaper to the vertical sides of the window opening. Be sure the tarpaper overlaps the flashing on the plywood sheathing.
9. Apply a thick bead of adhesive caulk along the front of the window's upper nailing flange. Also, run caulk along the exterior sides and bottom of the window opening.
10. Set the window into the opening, and slip the upper nailing flange underneath the siding.
11. Center the window in the opening and check it for plumb using a level. If the window is not plumb, slip a pry bar underneath the window from inside and raise the lower corner until the level reads plumb.
12. Secure the window to the house by hammering 1½-inch-long roofing nails through the slots in the nailing flange. Space the nails 6 inches apart.
13. Insulate around the inside of the window by spraying minimal-expanding foam into the gap between the window and wall framing.
14. Install fiberglass insulation between the cripple studs. Then, staple up a vapor barrier and install drywall.
15. Cut and install casing around the inside of the window.
16. On the outside, install new siding over the exposed nailing flanges and plywood sheathing.