Great windows are only great if installed well
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.
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The toughest details are those that have to match up with someone else's work, or those done — even done well — when energy was really cheap. These details are a collection of some common — and tough — dovetails of existing work with retrofits or additions. Bear in mind that green remodeling means creating a new operating regime that is better, not worse, than the one that may well have been working just fine before. Integration of energy efficiency, moisture management, and indoor air quality is much more important and challenging in remodeling than in new construction.
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:
Self-adhered membrane flashing at window head
Double coverage of membrane flashing or tape around the metal window cap
Note that either tape or membrane flashing covers the metal window cap flashing. The interior flashing or tape covers the window flange so that wind-blown rain that gets up underneath the metal window cap cannot get behind the window flange. The weatherlapped exterior membrane flashing or tape handles water running down the weather-resistive barrier (WRB) to the window.