My home’s wooden windows are about 40 years old. Like most wood windows of that era, these Weather Shield windows lacked any exterior cladding. The double-glazed windows had no nailing fins: instead, they were sold with wood exterior casing already attached, so that the installer could fasten the windows in place by nailing the exterior casing to the wall sheathing.

The good news is that my walls never experienced any water entry problems, in spite of the fact that I neglected to flash the window rough openings when I installed the windows back in the early 1980s. The bad news is that Weather Shield decided to make the windows’ exterior sills from finger-jointed pine. Even though the sills were painted, they eventually started to rot—most noticeably on the east side of my house, which faces downhill and receives more wind-driven rain than the other exposures.

Assessing the condition of my window sills, I recently decided to replace seven windows.

Measuring new windows

You don’t have to remove your old window frames if you don’t want to. It’s possible to replace old window sash with new sash and new vinyl jamb liners; these components can be installed inside your existing window jambs. If you take this approach, you don’t need to remove the exterior trim, interior trim, or the window frame.

Window manufacturers (especially manufacturers of vinyl windows) also make framed replacement windows without nailing flanges; these units, consisting of a window frame and sash, are designed to slide inside your existing window frame after your sash are removed. This replacement window strategy leaves the existing exterior trim and interior trim in place, simplifying installation; the downside…