Question 7 of 7: Window/Wall Air Barrier Continuity and Flashing Failure Case Study

Question 7: Window/Wall Air Barrier Continuity and Flashing Failure Case Study (20 marks: Part A 10, Part B 10)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part A.1) Tracing the four control layers (part of 10 marks). In this exterior-air-barrier assembly, the SAME sealed sheathing membrane does double duty: as the air barrier (red in Fig. 5) — the taped/sealed gypsum sheathing board (legend item 2) with its membrane (items 10/11), wrapped continuously into the rough opening and sealed to the window's nailing flange/mullion, since ANY gap here breaks air-barrier continuity right where wind pressure is highest — and as the water-resistive barrier (green, dashed) — the same membrane's drainage plane (item 5) shingle-lapped so every course overlaps the one below it, carrying incidental water down and out rather than into the wall. Because the membrane's own permeance is high enough (per Q1 item 10's 10–20× rule), it does not also need to act as a distinct vapour barrier in this configuration; where a Class I vapour barrier IS required by climate, it would sit further inboard, at or near the warm face of the insulation (not shown at this exterior sheathing plane). The rainwater-shedding surface (orange, dashed) is the exterior cladding together with the head flashing (item 7, metal flashing) above the window and the sloped sill-pan flashing below it, backed by the rod-and-sealant joints (item 8) at the window perimeter.

Part A.2) How the window manages rainwater (part of 10 marks). The window is treated as a "leaky" element by design, not a perfectly sealed one: a sloped sill-pan flashing beneath the window frame collects any water that does get past the frame joints, glazing seals, or a failed perimeter sealant, and directs it back out to the exterior face via end dams at each jamb (so it cannot run sideways into the wall) before it can reach the rough-opening framing. A head flashing above the window sheds water around the top of the opening, and jamb flashings at each side tie the window into the surrounding WRB. The critical sequencing is shingle-lap order from top to bottom — head flashing laps OVER the jamb flashing, which laps OVER the sill-pan flashing — so gravity always carries any water it intercepts outward and downward, onto the face of the wall below, never into the assembly.

Part B.1) Flashing failure mechanism (part of 10 marks). In the case study, the through-wall flashing in the brick veneer cavity terminates roughly 13 mm (½ in.) short of the exterior face of the brick, instead of projecting past it with a turned-down drip edge. Water entering the drainage cavity above (an expected, designed-for occurrence in any brick veneer rainscreen) runs down the back of the veneer and along the top of the flashing exactly as intended — but on reaching the flashing's recessed leading edge, surface tension pulls the water sideways and back UNDER the flashing rather than letting it drip clear of the wall, delivering it directly back into the wall cavity below. The flashing is otherwise well-lapped and adhered, so the failure is entirely due to this one geometric detail — a flashing that does not extend to (and turn down past) the face of the veneer cannot do its job no matter how well it is installed elsewhere.

Part B.2) Remedy (part of 10 marks). Extend the through-wall flashing fully across the cavity so it projects a minimum of about 6–13 mm past the exterior face of the brick veneer, and turn that projecting edge down at roughly 90° to form a positive drip edge — this breaks the surface-tension path that let water track back underneath, ensuring collected water is thrown clear of the wall face instead. At the same time, verify (and correct if needed) that flashing end laps are a minimum 100 mm with end dams turned up at any step or termination, and that weep holes are provided immediately above the flashing at a maximum 610 mm (24 in.) spacing, as open tubes (which drain more reliably than weep rope) and to allow the cavity to dry — matching the source article's own cited best practice.