A 1974 Curtain Wall That Weeps
The owner of a 1974 twelve-story office building in ASHRAE Climate Zone 5A reports that every winter the inside face of the single-glazed aluminum curtain wall runs wet, carpet edges along the slab perimeter are stained, and the humidification system cannot hold 30% RH. The wall alternates 1.2 m spandrel and 2.4 m vision glass, is thermally broken only at the mullions, has no continuous air barrier, and the slab-edge condition shows a linear thermal transmittance near 1.1 W/m·K. The owner's engineer proposes adding a steam humidifier and raising the setpoint to 40% RH to "fix tenant comfort." The owner also wants to cut heating energy 35% and is considering interior storm windows at roughly 25% of the cost of a new curtain wall. Envelope work is capped at $2.4 million, and the building has 14,000 m² of wall.
1. Evaluate the engineer's humidification proposal using psychrometrics and envelope physics.
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Reject it - the proposal makes the problem worse.
Condensation occurs when an interior surface falls below the dew point of the interior air. At 21 °C and 30% RH, the dew point is about 8.5 °C; at 40% RH it is about 12.5 °C. Raising the setpoint moves the threshold up by roughly 4 K, so every square metre of uninsulated spandrel and every slab edge that previously stayed just above dew point now condenses. On a wall with a linear thermal transmittance near 1.1 W/m·K at the slab edge, that surface will sit close to outdoor temperature in a cold snap regardless of humidification.
Adding steam also raises the interior vapor pressure, driving more vapor into the wall assembly and increasing the risk of concealed condensation, corrosion of the mullion anchors, and mold on the concealed gypsum. The humidifier will run constantly, increasing energy use against the owner's 35% reduction target.
The correct sequence: first reduce air leakage and thermal bridging so interior surfaces stay above dew point, then set humidity to what the envelope can tolerate. Recommend a blower-door test and infrared survey to locate the worst bridging and leakage before spending on mechanical capacity.
2. Compare interior storm windows with full curtain-wall replacement against the energy target. What is your recommendation and what verification do you require?
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Interior storm windows are a legitimate first phase but they do not solve this building's problem alone.
What they do: adding a second pane and a still air cavity raises the interior surface temperature of the glass, cuts convective losses, and reduces infiltration at operable joints. On the vision glass, that can be a real improvement for a fraction of replacement cost.
What they do not do: they leave the slab-edge bridge and the spandrel untouched. The linear transmittance of 1.1 W/m·K means the coldest surface simply moves to the frame and slab junction, where condensation will continue and may be less visible. Sealing a cold cavity on the interior also creates a double-vapor-barrier risk if the cavity is not vented to the interior; specify the system with a documented drying path.
Recommendation: a hybrid - replace the spandrel with insulated panels, add continuous exterior insulation at the slab edge, install a continuous air barrier, and use interior storm windows on the vision glass where the frames are sound. Test it against the energy model, not against a rule of thumb.
Verification required: a mockup with a guarded hot box or calibrated field test, a whole-building energy model showing the 35% target, a blower-door test before and after, and a winter infrared survey to confirm interior surfaces stay above dew point at design conditions. Full replacement remains the durable answer if the frames are corroded.