Wind-blown rain found its way through flashing and housewrap, damaging the walls and windows.
Reverse-lapped housewrap on the original dormer directed water into the head space above the window, rather than out and down.
Here, flashing was applied over bituminous membrane, creating another leak point. However, the Grace Ice and Water Shield membrane did protect the substrate from most of the water in spite of the reverse lap.
This original dormer window, shown during demolition, bears the tell-tale dark stains of wind-blown rain, followed by mold and decay.