Rain on the Inside and Water on the Floor
The 28-storey office tower you designed at Tseung Kwan O, 40 m from the sea, was completed in March. The envelope is a unitised curtain wall: 8 mm low-e / 12 mm argon / 8 mm laminated insulating glass, 1.4 m spandrel panels with 50 mm mineral wool insulation, and openable vents. The first typhoon-force winds in August came from the south-west. Since then, water staining has appeared on carpet tiles along the slab edge on floors 12 to 18, all on the south-west elevation. Separately, in June and July, occupants reported condensation on the room-side face of the spandrel glass and on the aluminium frames in corner offices where fan coil units run at 22 degrees Celsius. Site records show the stack-joint sealant was applied during rain in February, and the insulation was installed with its foil facing the interior. The contractor says the design is defective and has asked you to approve a remedial scheme within four weeks.
1. Distinguish, with reasoning from the facts, the likely causes of (a) the water staining and (b) the condensation, and state what further information you need to confirm each.
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(a) Water staining at the slab edge, south-west, floors 12 to 18. The pattern - a single elevation, a band of floors, first appearing after the first south-west typhoon - points to wind-driven rain entering the system rather than a general failure. Candidate mechanisms:
- Stack-joint sealant applied in rain. Sealant will not bond to a wet or damp substrate. Adhesion failure is progressive and becomes visible months later under wind pressure.
- Loss of pressure equalisation. If the cavity is not compartmentalised, with missing or discontinuous air seals at the transom and stack, rain is driven across the line of the inner gasket.
- Blocked or absent drainage. Weep holes and sill flashing must take any water that enters back to the exterior. Staining at the slab edge suggests water tracking along the slab-edge detail instead of draining out.
- Missing end dams at the perimeter, a common omission at the junction with the structural slab. The band of floors suggests a particular installation gang or batch of units rather than the whole elevation.
(b) Condensation on the spandrel and frames, June to July. This is a different mechanism. In Hong Kong's summer the vapour drive runs from the hot, humid exterior to the cooler, drier interior, so the vapour retarder belongs on the warm side - the outer face of the insulation. The record says the foil faced the interior, putting the barrier on the cold side, where it traps moisture in the insulation and at the inner skin. The corner-office frames and the slab edge are thermal bridges; where surface temperature falls below the dew point, condensation forms on the room-side surface. Consistent with this, complaints come from air-conditioned corners in humid months, not after rain.
Distinguishing test. Water penetration correlates with wind and rain and appears at joints. Condensation correlates with internal temperature and external humidity, appears on cold surfaces, and is worse in the early morning. A summer thermographic survey and a field water test will separate the two.
Further information needed. As-built details and approved shop drawings; sealant application records; the cavity drainage path; whether staining lies inside the line of the inner gasket; internal relative humidity and set-point records; and whether affected floors share a supplier or installation team.
2. Specify the investigation and testing regime you would instruct before any remedial scheme is approved, with reasons.
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1. Record review. Obtain approved shop drawings, structural movement joints, sealant specification and application records, cavity drainage and venting details, and the insulation specification with the as-installed orientation.
2. Intrusive opening-up. Remove one unit per affected floor zone - say three units - to expose the stack joint, end dam, sill flashing, weep holes and insulation. Photograph and log everything. This is the only reliable way to confirm the vapour-barrier orientation and the drainage path.
3. Field water test. Test a representative unit to a recognised field standard, using a calibrated hose test and, where the system is designed for it, a dynamic pressure test at the specified pressure and duration. Testing should be witnessed and recorded, and should cover both an unrepaired and a repaired unit so that the before-and-after comparison is meaningful.
4. Thermographic survey. Conduct an infrared survey in the early morning in summer with a known internal/external temperature difference, to map cold bridges at the slab edge and frame. Supplement with spot surface temperatures and a dew-point calculation.
5. Sealant adhesion testing. Test the stack joints on the affected elevation and check whether sealant was applied to a dry substrate.
6. Internal conditions. Log temperature and relative humidity in the affected corner offices over a representative period.
Why this order. The tests must be capable of separating design from workmanship before anyone commits to a scheme. Opening up is decisive. If the foil faces the interior, that is a design and detailing issue. If the stack joints are unbonded, that is workmanship. Most real cases are a mixture, and the apportionment should follow the evidence.
Procedural points. Require the contractor to cooperate during the defects liability period. Record that the investigation is without prejudice to rights. Notify insurers early. Keep the employer informed. Do not approve the contractor's four-week remedial scheme until the diagnosis is complete, because a scheme that treats the wrong mechanism will fail and will be paid for twice.
3. Recommend remedial detailing for the spandrel and slab edge, and explain the liability position between the contractor, the specialist and your practice.
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Remedial principles.
- Vapour barrier on the warm side. Rebuild the spandrel so that the continuous air and vapour barrier sits on the external face of the insulation, with joints lapped and sealed. Wet mineral wool will not dry out inside a sealed spandrel; remove and replace it.
- Continuity at the slab edge. Insulation should run continuously across the slab edge and the frame should incorporate a thermal break. A 50 mm pack that stops at the slab leaves a cold bridge and a condensation line exactly where the complaints are.
- Drained and vented cavity. Provide a clear drainage path to the exterior at the sill, with weep holes and end dams. The cavity should be ventilated and compartmentalised so that pressure equalisation works and any water that enters can escape.
- Sealant and gaskets. Replace unbonded sealant with a specified sealant applied to a dry, primed substrate, with backer rod and correct joint geometry. Check the gaskets at the stack joints and corner details.
- Verification. Retest after repair on the same basis as the investigation.
Liability. The starting point is the contract. Water penetration caused by defective workmanship or materials during the defects liability period is the contractor's responsibility. A design deficiency - the insulation orientation, if that is what the approved drawings show - is the architect's. In practice the contractor will say it built to approved shop drawings. Whether approval of shop drawings transfers design responsibility depends on the contract: approval usually confirms compliance with design intent and does not make the architect the author of the specialist's detail, but it does not insulate the architect either. If the practice specified the spandrel build-up, expect a contribution claim.
What to do now. Notify insurers. Avoid a blanket approval of the contractor's scheme. Require a joint inspection and a written apportionment before work starts. The cost of opening up and replacing insulation across the affected elevations will be substantial, and a documented apportionment now is far cheaper than an arbitration later.