Best Practices Manual

3. Windows & Doors
Rough openings are inherently dangerous spots on a building, like big holes in the hull of a ship. So it is vital to flash them well and install windows and doors in an airtight manner that also manages moisture and thermal transfer. Our preferred method outlined here – fluid applied flashing – is guided by three truths:

1. Windows will leak. Not all of them, but over a whole building, it’s a matter of when, not if. We therefore need to detail our rough openings so that when a window leaks, the water can drain out harmlessly. (Note: this is why we place the air seal at the inside edge of the window, or door, assembly, to allow water to drain outboard of that seal.)

2. Origami is hard. The conventional way of flashing a window with papers and tapes depends on dozens of steps being performed perfectly every time: careful folds and precise manipulation of less-than-forgiving materials in the field. Simple mistakes, like reverse lapping, can be catastrophic to the assembly but can be covered up by subsequent layers of material, so checking work thoroughly can be impossible. Now multiply these risks by the number of windows on a building and you will understand why window installations can be anxiety inducing.

Fluid applied flashing, by contrast, is more simple to apply in a few steps, and quality control is easy; if the applied layer is thick enough to be opaque, then it is thick enough to do its job. As long as proper materials are used, one fluid applied layer integrates seamlessly with the next, eliminating the risk of reverse lapping (except where the fluid applied system integrates with building paper). And unlike tapes that often require dry conditions for proper adhesion, many fluid applied flashing products are actually easier to apply on wet materials, a very common situation on Pacific Northwest construction sites. We can all sleep well at night.

3. Flashings should be vapor permeable. If flashings are not vapor permeable then moisture can build up behind them and cause rot. The high vapor permeability of fluid applied flashing ensures that construction moisture and seasonal water vapor migrating through the wall assembly does not accumulate behind the flashing and can readily dry.

Continue to Windows & Doors 3.1: New Window Installation >>>