The building envelope is the primary defense against water intrusion, air leakage, and heat transfer. During construction, the architect's observation role focuses on three interconnected systems: the air barrier, the water-resistive barrier (WRB), and the flashing system. These systems must work together as a continuous, shingled assembly to manage moisture and air movement.

The air barrier controls air leakage into and out of the conditioned space. It must be continuous throughout the entire building envelope, structurally supported to resist wind loads, airtight at all connections and penetrations, and durable for the service life of the assembly. Common air barrier materials include self-adhered membranes, fluid-applied coatings, and rigid materials like sealed plywood sheathing. The architect should observe that the air barrier is continuous at all transitions, including wall-to-roof, wall-to-foundation, and wall-to-window connections.

The WRB (water-resistive barrier) is the drainage plane that sheds water penetrating the cladding. It must be lapped shingle-fashion so that each upper piece overlaps the lower piece, directing water downward and outward. Flashing integrates with the WRB at all penetrations and transitions. The critical principle is that flashing must always be shingled: upper elements overlap lower elements, jamb elements overlap sill elements, and head elements overlap jamb elements.

Through-wall flashing collects moisture that penetrates the wall assembly and directs it to weep holes in the exterior cladding. It is required at shelf angles, window heads and sills, roof-wall intersections, and the base of wall assemblies. The architect should verify that through-wall flashing extends to the exterior face of the cladding and terminates with a drip edge.

The observation timing is critical: these systems are concealed by cladding, interior finishes, or both. Once covered, defects are invisible until water damage appears inside the building.