Condensation at the Slab Edge in a Cold-Climate Residence Hall
A new six-story, 180,000 sq ft residence hall in a cold climate (winter design temperature −10°F) has brick veneer on 6-in. steel studs with R-13 batt in the stud cavity and 1 in. of foil-faced polyisocyanurate outboard of the exterior gypsum sheathing. The insulation is interrupted at every floor by an uninsulated steel shelf angle, and the air barrier at the slab edge is lapped but not sealed to the shelf angle. Aluminum storefront windows have thermally broken frames. In January, with interior conditions of 70°F and 40 percent RH, residents report condensation and frost on window frames and sills and dark staining on the gypsum board along the perimeter floor line on floors 2 through 6. The contractor's proposed fix is to add portable dehumidifiers and to raise the perimeter heating setpoints.
1. Evaluate the contractor's proposal using the dew point of the interior air. What will dehumidification achieve and what will it not?
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Do the psychrometrics before accepting any proposal. At 70°F and 40 percent RH, the dew point is about 45°F. Any interior surface below 45°F will condense. Dropping the space to 30 percent RH moves the dew point to roughly 39°F - a margin of only about 6°F.
What that means at the perimeter. With an outdoor temperature of −10°F, an uninsulated steel shelf angle penetrating the insulated plane and an unsealed air barrier at the slab edge, the interior surface temperature at the floor line will be far below 39°F, let alone 45°F. Cold outdoor air leaking through the unsealed lap drops it further. Dehumidification to 30 percent RH reduces the frequency of condensation but does not eliminate it, and it will fail outright in a colder snap.
Raising the perimeter setpoint makes it worse, not better. Warming the room raises the moisture-holding capacity of the air, but it also raises the interior vapor pressure driving moisture into the assembly, and it does nothing to raise the temperature of the shelf angle, which is thermally tied to the exterior.
Conclusion. Dehumidification is symptom management with an energy cost and a resident-comfort cost. It is not a repair, and it should not be accepted as one. The window frost is a separate but related thermal-bridge and air-leakage symptom at the frame.
2. What is the most likely physical cause, and what would you do to confirm it before anyone spends money on a repair?
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Most likely cause: a thermal bridge and an air barrier discontinuity at the slab edge, made worse by a low-perm membrane on the cold side of the cavity. The uninsulated steel shelf angle at every floor is a direct heat path from the interior to the exterior; its interior flange will run well below the dew point whenever it is cold. The lapped-but-unsealed air barrier admits cold outdoor air and, in winter, carries moisture to a cold surface. Separately, foil-faced polyiso directly outboard of the sheathing makes that sheathing the coldest surface in the wall and effectively closes off outward drying - a wet cavity will not dry.
Confirm before spending:
- Thermographic scan from the interior on a cold night, comparing a typical bay to the slab-edge condition. Look for the linear cold band at each floor.
- Air leakage testing - a whole-building blower door or a targeted ASTM E783 test on a representative assembly.
- Moisture content readings in the sheathing and framing, plus a couple of small openings to observe the air barrier and insulation continuity directly.
- Verify whether the cavity is wet; if it is, the fix is larger than the interior finishes.
Only after those correlate should you commit to a repair scope. The pattern - floors 2 through 6 but not the roof or ground floor - points to the repeated floor-line detail, not to a random material defect.
3. Specify the repair for this building and the changes for the next one.
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Repair, in sequence:
- Remove the perimeter gypsum board and any wet insulation in the affected band and allow the assembly to dry; do not close it up wet.
- Install a structural thermal break at each shelf angle, or otherwise separate the interior flange from the interior environment with continuous insulation and a cladding that keeps the interior surface above the dew point under design conditions.
- Make the air barrier a continuous plane - a flexible, vapor-permeable membrane sealed to the shelf angle and to the slab edge, with the lap fully sealed rather than lapped.
- Restore the interior finish and re-test with thermography and a blower door to prove the fix.
- Keep dehumidification only as a temporary measure during the repair.
For the next building:
- Continuous exterior insulation that is not interrupted at floor lines, with the R-value chosen so the interior surface of every element stays above the interior dew point at design conditions.
- A structural thermal break at every shelf angle and balcony penetration, detailed and specified, not left to the fabricator.
- An air barrier drawn as a single continuous plane with the same sealant and membrane at every slab, angle, and penetration.
- A vapor-open strategy for the sheathing so the wall can dry, and a mechanical design that holds winter interior RH at a level the envelope can actually tolerate. Housing authorities frequently target 40 percent, which many cold-climate wall assemblies cannot support.