I thought the same thing. But during the winter the indoor humidity usually hangs aroung 16%. I have oil hot air heat which is very dry. It seems moisture penetrates the indoor wood paneling, then the 2 inches of insulation , then condenses on the inside of the exterior aluminum siding and runs down...

My wife and I live in a 1976 Marlette on the eastern shore of Maryland. This winter I noticed mold growing on the bottom of interior paneling on a bump out facing east. Today I removed a 4 ' section of the paneling. The original fiberglass insulation was in good condition and filled the area tightly...