A good window installation converts a hole in the wall into an integrated part of all three important barriers that make up a wall assembly: the air, thermal, and water barriers.

Because all windows leak at some point, rough openings need to be designed to handle water entry. The rough sill should be flashed with either a pre-formed manufactured sill pan or a site-built pan.

This detail shows a site-built sill pan installed in a wall sheathed with rigid foam. The outside face of the foam is the wall’s drainage plane.

For more on windows:

GBA Encyclopedia topic page

Video Series: Flashing a Window in a Foam-Sheathed Wall

Water is both the most useful and the most damaging liquid. It needs to be moved off of or out of our buildings almost religiously. These details cover the most common flashings for homes, especially the weatherlap — shingle-style shedding of water from one overlapping component to the next. There are very few sealants or caulks in these details (especially compared to the air-sealing details). When caulks or sealants are exposed to liquid water and the sun, they will eventually fail. And when they do, they can trap as much water as they were intended to keep out. There is seldom just one way to flash any one penetration, and never just one flashing material; these details often represent just one of many best practice approaches.

Even the best window details can be hard to follow because so much depends on the sequence of installation. Use the GreenBuildingAdvisor window installation series along with these window details:

Flashing Details