Installation Practices and Field Workmanship Verification
Field workmanship decides whether designed performance is achieved. Verify substrate condition, surface preparation, primer, application rate and thickness, lap width and direction, terminations, and treatment of every penetration and transition. Laps on vertical surfaces should shed water and follow the manufacturer's shingle order. Terminations need mechanical fastening and sealant where specified, not tape alone. Sealant joints need the correct width-to-depth ratio, backer rod, and tooling; joints squeezed thinner than the sealant's movement capability fail in shear.
Sequencing is the practical issue. The air barrier must be inspected and documented before cladding, insulation, or finishes conceal it. Establish hold points at rough-in completion and before enclosure, and require notice before concealment. Typical defects: unsealed top-of-wall and floor-line transitions, penetrations for conduit and pipe, ducts and dampers crossing the boundary, unsealed rough openings at windows and doors, damaged membrane at corners, and rim joist gaps.
Trap: accepting a visually neat installation without checking thickness, adhesion, or continuity at the boundary the drawings actually specify. A second trap is confusing water-resistive barrier lapping with air barrier continuity - the same membrane may serve both, but the criteria differ.
Worked example (rule of thumb): a sealant joint should be roughly twice as wide as it is deep, with minimum depth near 6 mm (1/4 in.) set by backer rod and never thinner than the manufacturer's minimum. A 12 mm wide joint should be about 6 mm deep; a joint squeezed to a thin film has almost no movement capacity and will tear rather than stretch.