Called in to address an attic mold problem, the author was confronted with more mold than just what was growing on the roof sheathing. The owners liked to keep indoor temps high (around 78° F) and ran six humidifiers all night, creating a jungle-like environment rampant with interior mold.
The bay window had been freshly painted less than a month before this photo was taken, yet mold was already popping through.
The main cold air return ran under the bay window, an area that was also used for storage. Note the moldy pressboard, which was soaked and falling apart.
The temperature of the wood beneath the bay window was cooler than the dew point, so moisture was condensing on it and allowing mold to grow.
The rim joists were air-sealed well. But see the mold on the concrete-block wall that had been painted only months before the photo was taken.
Even without a thermal camera it would have been easy to find the spots where the dense-pack wall insulation was missing or had settled. Just look for the mold, which was growing wherever the surface temperature was lower than the dew point.
The attic floor above this area of ceiling was missed when the attic was insulated. The house was so humid that water condensed on the cool part of the ceiling.
Condensation formed on the plastic covering the windows. Leaks in the plastic allowed condensation to form on the windows as well.
When the attic was insulated, its temperature dropped, so condensation now formed on the underside of the roof, followed quickly by mold.