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.

Sealant should never be used as a waterproofing detail. It has a service life, and it is not a robust or durable material. Once you discover a leak, there is no way to “fix” it with sealant. This “fix” oftentimes makes things much worse. Once you start applying sealant to the exterior of a home, you start trapping even more moisture, leading to more rot. Rather than attempt to slap a band-aid on the issue, fix it the right way. 

Most of the exterior trim on our project was sealed on all four sides. This was all done after the fact. If the windows, sheathing, and envelope were properly detailed and waterproofed, there would be no need to apply sealant to these areas other than for aesthetics or paint. To allow water to escape, I would rather see tight joinery and no sealant. In our instance, all of the sealant led to additional trapped moisture and rot.