My Foundation Windows are Rotting. What Should I Do?
Even though my house is only about 17 years old, the sills of the basement windows are beginning to rot where they rest on the concrete foundation. How do I remove these windows and install new ones?
— Maurice Corbeil, Compton, Quebec, Canada
Tom Silva replies: The first thing to do is measure the size of your old windows and order replacements. If you want wood windows again, make sure you prime every surface before installing them. It's a good insurance policy for any window, especially one surrounded on three sides by damp masonry. If you go with solid vinyl or fiberglass windows, there's no priming and you'll never have to worry about rot again.
When the new windows arrive, take out the old ones with a hammer and a flat pry bar. Clear away all the rotted wood and clean the exposed foundation surfaces thoroughly to remove dirt and debris. Slip the new window into place and shim it level and plumb. Working from the outside, fill the gap between the window frame and the foundation with polyurethane caulk. Let it cure a day or so, then squirt expanding foam sealant between the window frame and the foundation, this time working from the basement side of the window. The foam and caulk will seal gaps and hold the window in place permanently; it's amazing how strong the stuff is.
But if security is a concern, you can pound galvanized nails up through the window frame and into the underside of the house's mudsill, the board that rests on top of the foundation and spans the top of your windows. In addition, drive masonry fasteners such as Tapcon screws through the window frame and into
pilot holes drilled into the concrete. One screw on each side of the window, near the bottom of the frame, ought to do the trick.