Flashing a Window: Simple, Standard and Secure

Windows and doors are unique elements of a building’s envelope, as they penetrate the entire wall assembly, connecting the interior and exterior. Unlike small penetrations such as hose bibs or electrical outlets, windows are subject to both interior and exterior design pressures. To understand these pressures, consider how a slightly open back door can slam shut due to the differential pressure or vacuum created between indoor and outdoor spaces. In tall buildings, this effect—known as the “stack effect”—can be even more pronounced. Wind speed, local terrain, and the size and placement of window openings all influence these forces. To withstand them, windows must be tested and rated for design pressure (DP) to ensure durability and performance.

While energy efficiency is crucial, the primary concern for subcontractors is water intrusion. Selecting a properly rated window is only part of the equation; the transition from the window frame to the cladding must also be correctly sealed to prevent leaks.