No matter how well a window is made, it will not perform properly if it is not installed properly. I recently encountered some poorly installed windows on a remodel project. The exterior was traditional stucco, so at first glance we had no idea the extent of the damage. Once we started to remove the layers, we uncovered a slew of mistakes. There were no head flashings, the flanges were not back-caulked, there was no drainage plane behind the stucco, they had taped the bottom flanges of the windows, they had not integrated their sill pan flashing into their weather resistive barrier (WRB), layering of tapes and WRB’s were incorrect, and they caulked any sign of a leaking after the fact which only exacerbated the issue. Let’s dig into why these mistakes were so costly and go over some keys to proper installation.

If the goal is to seal a new construction window to the exterior of a home, why would you not want to tape the bottom flange? Imagine an exterior wall assembly with a window installed tightly to the sheathing. If rain were beating against that wall from directly above, from an angle, or even from the side, would it be possible for that water to travel upwards and under the bottom window flange? No! Water will enter the flange at the top and sides, but not the bottom. This is why it is not critical to tape that flange. Now, what happens if water compromises the window or wall and gets behind those flanges? It will travel down the wall and hit the rough sill, and at that point, it needs somewhere to go. By leaving the bottom of the window untaped, we are allowing any water that might otherwise be trapped to instead drain down the wall, which is why it’s called the drainage method of installation.

This entire area and detail is where I experience and witness most issues with window installations. If you are using traditional sheathing and a separate WRB, you must make sure that your sill-pan flashing laps over top of your WRB. If the home has yet to have WRB installed, you must install a small piece of that material under the window for your siding contractor to tie into. I often see the sill-pan flashing or tape applied directly to the sheathing and then the WRB is installed later on top of that. Where will all of the water that is traveling down the window go in that scenario? Directly behind the WRB. Understand your finishes: Account for later trades by integrating your WRB into your sill-pan flashing. 

The sill-pan flashing was installed first on our project, which led to the tar paper being installed after and on top of the sill-pan flashing. The water simply ran down the window and sill and then directly behind the WRB.