Curtain Wall Leaks Only Under Wind-Driven Rain
A 12-story office tower has a unitized curtain wall on an exposed corner. During a 35 mph wind-driven rain event, occupants report water at the sill on floors 7 through 9 on the windward elevation. A field hose test at low pressure does not reproduce the leak. Investigation finds that maintenance has sealed several sill weeps to stop drafts, the sill has no visible end dams, and the interior air barrier is discontinuous at the floor slab. The project specification requires compliance with AAMA 501.1 and a field water test before closeout. The contractor wants to apply exterior sealant along the entire sill and move on. The owner is concerned about concealed damage and wants a durable repair, not a cosmetic fix.
1. What is the most likely leakage mechanism, and why did the hose test miss it?
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The curtain wall likely relies on a drained and pressure-equalized sill chamber. Water enters through gasket joints or the sill, collects in the chamber, and should drain through baffled weeps. If the weeps are sealed and end dams are missing, the chamber becomes a reservoir; wind pressure then pushes water inward past the inner seal. The discontinuous air barrier at the slab can also allow a pressure path that drives leakage. A low-pressure hose test does not reproduce the wind-induced pressure difference across the assembly, so it can miss a leak that only appears when pressure and water are present together.
2. Outline a diagnostic field test program that will locate the path without removing large areas of cladding.
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Isolate one leaking bay and one non-leaking control bay. Use a chamber test such as AAMA 501.1 or ASTM E1105 to apply uniform pressure while spraying the exterior, with observers inside. Start at a low test pressure and increase in stages while watching the sill, mullion joints, and slab edge. Confirm weep condition and end dams before testing. If the leak appears only at a specific joint, isolate that joint with localized spray and pressure. Record the pressure and location at first leakage so the repair can be verified by a repeat test.
3. Describe the repair sequence and explain why exterior caulking at the sill is likely to fail.
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Remove the improper exterior caulk from the weeps and restore baffled weeps, end dams, and sill flashing. Replace damaged gaskets and seal the interior air barrier at the slab edge with a compatible membrane. Re-establish positive drainage and then retest the repaired bay under pressure. Exterior caulking along the entire sill is likely to fail because it traps water in the sill chamber, relies on adhesion at a moving joint, and does not restore pressure equalization or drainage. It may hide the leak briefly while allowing concealed water damage to continue.