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  • Vittorio Gianni

Proper Window Installation

Updated: Jul 22, 2020

Accepting That Windows Will Leak

The second half was our dawning realization that windows are, by nature, failure-prone. It’s not a matter of if windows will leak, but when. Windows are complex, full of junctions, corners and seams — rife with opportunity to leak moisture.


Easily 20 percent of windows are already leaky when delivered to job sites, based on sill dam tests. A certain percentage of windows will leak coming off the assembly line. More will leak after being packaged and shipped, yet more after unloading at the warehouse. Still more will leak once the windows arrive at the job site and are moved around, and even more after they’ve been handled and installed. Then come the forces the windows will be subjected to in situ: structural shifts as the building settles, years of wind load and weathering, aging materials and sealants.


After sanding the rough edges, Leak Doctor applies a joint-filling to seams and penetrations to reinforce against potential movement at joints.


At some point in a window’s life, it will leak. To ignore this reality is to play Russian roulette with your building envelope, yet that has been the traditional approach. When a wall rots under a window, the traditional response is “that window is defective,” not “that opening wasn’t properly flashed.”


We realized we had to detail rough openings knowing that windows will leak sooner or later. Rough openings need to be completely water-resistant to keep water from penetrating the wall assembly, but also vapor-permeable to let construction moisture, seasonal moisture fluctuation and exterior moisture evaporate from the wall. That is true for conventional installations with windows at the exterior edge of rough openings as well as high-performance installations with windows centered in the wall.


In our climate, the simplest way we found to achieve water-resistance was with vapor-permeable liquid-applied flashing. Any rain that hits the opening or leaks through the window drips down to the liquid-applied membrane and out to the exterior moisture-management layer.



Leak Doctor installs an expanding foam tape into a flashed doorway rough opening. Under compression, the tape expands to create an airtight, insulated seal that sheds water.

Keeping Water Out in Five Steps

The beauty of this approach is that it is easy to install: simplicity over complexity.

Gone are the layers of sheet goods and traditional tape membranes building layer upon layer. No “tents” of tapes in the corners. No fish mouthing to funnel moisture where it shouldn’t go. No inadvertent reverse-lapping.


A flashed rough opening awaits water-resistive barrier and window installation.

As an installation method, liquid-applied flashing is as close to foolproof as anything we know. It is much harder for a carpenter having a bad day to make mistakes with a liquid-applied membrane than with tapes, peel and stick, and papers. And errors, like inadequate coverage, are easy to identify because they aren’t hidden by layers of tapes and paper. A little spot-touching fixes the problem.


Installation involves five steps. (Note: for a detailed, illustrated description of our approach to window flashing, see the “New Window Installation” of our Best Practices Manual.)

1. Prepare the Rough Opening:


Start by ensuring that the rough opening is ready for the install. It should be dead level and dimensioned precisely with a consistent installation gap — we shoot for three-eighths inch (0.95 cm) around the perimeter of the window unit. Double-check the measurements of the rough opening and modify as necessary. If the opening is too large, for example, you can add filler pieces coated with liquid-applied flashing.


We recommend budgeting time into projects for corrective action to make sure openings are ready to accept a window. The opening should be rough-sanded and dust-free. A few minutes extra prep time makes flashing application smoother.


2. Reinforce Structural Seams:


The traditional way to do that is with fabric mesh tapes, but they go on slowly and are prone to ripples and fish mouths. We’ve seen testing that shows that a thread poking through the flashing material can serve as a wick to bring water through the membrane.