New Window in an Old Opening
A leak-free installation depends on compatible materials and multiple layers of defense.
A flanged new-construction window in an existing opening can be difficult to install. The difficulty is integrating the new window’s nailing flange into the house’s existing drainage plane so that any water leaks are directed outside the building.
A leak-free installation is important because water in the wall can give mold, rot, and insects a foothold in the wall cavity. In my hot, humid Southern climate, I’ve seen poorly installed windows create major problems in only a few months.
Fortunately, you can install a new window in an existing opening without problems if you just remember the four Ds: deflection, drainage, drying, and durability. Deflection means that rain is naturally moved away from the window and the opening. Drainage allows water that does infiltrate to find its way outside the assembly unimpeded. Drying suggests that any material that does get wet shouldn’t stay wet for long. Durability tells us that the products we use to shed water should last at least as long as the window itself.
You’ll also want to follow the window manufacturer’s installation instructions to the letter. If you can’t find the instructions or if your situation is unusual, follow the guidelines in ASTM E E2112-07, which is titled “Standard Practice for Installation of Exterior Windows, Doors, and Skylights.”
Another important consideration: a properly installed window starts with a unit that’s the right size. You may be able to measure the inside of the existing window and guess the jamb thickness and rough opening, but that’s a potentially expensive mistake. A better approach is to remove the interior casing and measure the actual opening.
Remove the old window
The first step is to cut back the siding with a circular saw and oscillating multitool so that you…