Condensation Dripping from Fasteners in a Heated Shop with Draped Insulation
A 150- by 60-ft heated manufacturing building has a 3:12 gable roof on cold-formed purlins at 5 ft on center. The roof is 26-gauge through-fastened panels with exposed fasteners over a single layer of R-19 vinyl-faced fiberglass insulation draped across the purlins. Facing laps are taped and the facing is pulled tight, but the insulation is visibly compressed at every purlin. Winter design conditions are 5 F outdoors and 68 F indoors at 45 percent relative humidity; the process generates moisture and two exhaust fans run continuously. Since the first cold snap, the owner reports water dripping from fastener heads and along purlin lines, rust staining on the panel underside, and wet insulation near the ridge. The roof is four years old, and from above the panel finish is intact with no obvious punctures or open laps. The contractor's first suggestion is to add another layer of insulation on top of the existing and reinstall the panels.
1. Is this a leak or condensation? What evidence in the description drives the diagnosis?
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The pattern points to condensation, not a roof leak. Drips appear at fastener heads and along purlin lines, which are exactly the cold thermal bridges; the paint is intact on top, so there is no obvious path for rainwater; wet insulation is concentrated near the ridge; and the problem began with the first cold snap rather than with a rain event. A leak would more likely show up at laps, fasteners, or penetrations after rain and would not track purlin lines so neatly.
The mechanism is vapor drive from a warm, moist interior to a cold surface. At 68 F and 45 percent relative humidity, the dew point is roughly 45 F. The panel underside and the purlin line are much closer to the 5 F outdoor temperature, partly because the R-19 is compressed at every purlin and partly because each fastener penetrates the facing. The vinyl facing is a vapor retarder only where it is continuous, and it is compromised at compressed purlins, laps, fasteners, ridge, eave, and every penetration, so humid interior air reaches the cavity and condenses on the cold metal.
2. What corrected assembly would actually control the condensation, short of replacing the roof?
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The fix has to address air and vapor movement, not just R-value. Establish a continuous air and vapor retarder on the warm side, with all laps sealed, penetrations and fasteners detailed, and the perimeter, ridge, and eave tied in so interior air cannot enter the cavity. Break the thermal bridge at the purlins with thermal spacer blocks or a comparable detail so the interior surface of the assembly stays above the dew point and the insulation is not crushed to a fraction of its rated thickness. If the cavity above the retarder is vented, vent it to the exterior at eave and ridge, never to the occupied space, and block the ridge so interior air cannot spill into the pan cavity.
Where the existing facing is damaged or cannot be made continuous, install a new sealed vapor retarder below the existing insulation or replace the system with a liner system designed for the building's interior humidity. Confirm the assembly with the panel and insulation manufacturers in writing, since the required permeance depends on the climate and the interior moisture load, and reduce the interior moisture load where the process allows.
3. Evaluate the contractor's proposal to add another layer of insulation on top of the existing and reinstall the panels.
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Adding insulation above the existing layer would likely make the condensation worse. More R-value between the warm interior and the panel lowers the panel temperature toward the outdoor temperature, so any interior air that still reaches the cavity condenses on an even colder surface and in greater volume. It does nothing about the air leakage paths at laps, fasteners, ridge, and eave, which are the actual transport mechanism. Compressed insulation at the purlins also means the added layer may not deliver its nominal R-value, and reinstalling the existing panels will reopen the same holes and facing penetrations.
There is a role for added insulation, but only after the air barrier is continuous and the thermal bridging is corrected, and the added R-value should be placed where it keeps the condensing surface warm rather than colder. If the added insulation is on the cold side, it must be paired with a sealed vapor retarder on the warm side. The proposal should be rejected as written, with the recommendation that the contractor first document the air leakage paths and the dew point calculation for the corrected assembly.