The number one housing defect is water intrusion. The number one entry path for water is around things that penetrate the exterior walls. Let’s talk about windows.
Most houses are built today with vinyl windows. Don’t get me wrong – vinyl windows are great products. They are energy efficient, relatively inexpensive, recyclable, and require little maintenance. But vinyl windows have some inherent problems that must be correctly addressed during installation.
20% of new windows may leak because of damage that occurs when the window is handled and installed. Unfortunately, you can’t see this damage. Vinyl windows are especially vulnerable because the corners of the window frame are “fused” together on many brands. During shipping, handling, or installation, the seal at the corner can break.
We used to slam new windows into houses with little regard for water intrusion other than squirting some caulking around it. Have you ever looked closely at a typical vinyl window? A fin extends from the edge of the frame all around the perimeter. This fin extends over the edge of the opening in the wall. But it’s not very wide. On most windows the fin is less than 1-1/2 inches wide, with holes in it for fastening. So it is really a mounting flange. It is not really sealing out much moisture.
Water can travel horizontally where building wrap, siding, and flashing overlap vertically on a wall. Therefore, you need wide flashing to prevent leaks.
The window industry has come up with the solution: a more stringent installation standard for the person installing the window. The new standard says you must flash around the window when it is installed, using specific steps.
The flashing must be at least nine inches wide and sealed to the window fin on the top, bottom and sides. The idea is to create a nine-inch-wide waterproof flashing around the window.
In addition, the flashing at the bottom of the window extends into the rough opening to protect the framing and finish materials inside, should the window frame leak. Ideally, a sill pan should be installed at the bottom.