Mold on the Corridor Side of a Two-Year-Old Brick Veneer Wall
A four-story, 120-room hotel in Climate Zone 2A was completed two years ago. The exterior wall is brick veneer on a 1 in. air space, 6 in. steel studs at 16 in. on center with fiberglass batt, exterior glassmat gypsum sheathing with a fluid-applied air and water barrier, and interior gypsum board finished with vinyl wall covering in the corridors. Weep holes are at 24 in. on center at the base and at shelf angles; several are mortar-bridged, and the shelf angles have no through-wall flashing. Corridors are held at 72 F and 50 percent RH. Facilities reports efflorescence on the north and west elevations, mold at the base of the corridor wall on the north side, and pin-meter readings of 24–30 percent in the affected gypsum. Sleeves for condensing units were field-cut through the sheathing and left unsealed.
1. Rank the moisture sources and explain why the mold appears on the corridor side rather than the exterior.
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Rank by driving force and by evidence:
- Rain penetration - dominant. No through-wall flashing at the shelf angles and mortar-bridged weep holes mean water that enters the brick has no path out. It accumulates in the cavity, saturates the sheathing, and migrates through the stud cavity. Efflorescence on the weather elevations is the signature: soluble salts carried through the brick and deposited as water evaporates at the surface.
- Air leakage - significant and localized. The unsealed condensing-unit sleeves puncture the air barrier, letting hot, humid outdoor air into the stud cavity. That air contacts interior gypsum cooled to 72 F, drops below dew point, and condenses on the back face.
- Vapor diffusion - minor. Diffusion is a small load compared with these two.
Why the corridor side: the vinyl wall covering is a Class I vapor retarder on the interior face. Whatever moisture enters the assembly can dry to the interior only through that vinyl, and it cannot. The wall has effectively one exit and it is blocked, so the mold blooms at the base where water accumulates and drying is slowest. In this climate the wall should be able to dry inward.
2. Write a corrective scope that does not require removing all of the brick.
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Target the water, not the wall.
- Rebuild the brick only where flashing is missing: the base course and a 3 ft lift at each shelf angle. Install continuous stainless through-wall flashing with end dams and upturned back leg, lapped and sealed to the fluid-applied barrier, then reinstate weep holes at 24 in. on center with open-weave mesh and drip edges.
- Clear the existing weeps by routing out bridged mortar at every elevation where the flashing condition already complies.
- Seal every penetration through the sheathing - sleeves, brackets, and any other field cut - with a compatible fluid-applied membrane lapped onto the existing barrier.
- Remove the vinyl wall covering in affected corridors and replace it with a vapor-permeable finish, permeance of at least 5 perms. Do not install a new interior vapor retarder or a vinyl-covered gypsum board.
- Replace mold-contaminated gypsum and treat framing; allow readings to fall to within a few percentage points of unaffected control samples before closing up.
Hold points: inspect flashing before brick is reinstated. This solves the source for a fraction of a full re-clad.
3. What field testing and quality assurance would you require before accepting the repair?
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Three things need independent proof: water stays out, air stays out, and the wall dries.
- Water penetration. AAMA 501.2 hose testing at agreed checkpoints - base of wall, a shelf angle, and around a sleeved penetration - on each elevation after brick is reinstated but before interior finishes go back. Where a test chamber is practical, ASTM E1105 provides a more controlled result. Document spray duration and observed leakage at the sheathing, not just at the interior finish.
- Air leakage. ASTM E783 chamber test at a repaired penetration to confirm the air barrier is continuous. A smoke pencil or tracer test is a cheap supplement for finding remaining paths.
- Drying verification. Pin-meter readings at marked grid locations and a control area in an unaffected wall, weekly for three weeks, and document that the trend is downward and stable before finishes are restored. An infrared scan in late afternoon will show whether wet patches persist.
- Submittals and observation. Flashing product data, membrane compatibility with the existing barrier, and full-time observation of the flashing installation. Also require a maintenance note to the owner about keeping weeps clear - this failure will recur otherwise.