A. Corresponding editor Henri deMarne responds: The water is probably condensate forming on the underside of the metal roof on cold nights when night radiation causes the metal to become several degrees colder than the ambient air. The problem may start as frost that melts when the outside temperature rises above freezing or the sun shining on the roof heats the metal. It is also possible that moisture from the interior space is driven through the T&G ceiling boards by convective currents, finds its way through the joints between the rigid insulation panels (which will shrink somewhat as they age), and condenses on the underside of the metal roof. Then, when enough water accumulates, it drips onto the rigid insulation, is blocked by the 2x4 strapping and finds its way through the joints in the rigid insulation and the ceiling boards. I have seen both of these situations a number of times over the last 20 years.
Had you installed the recommended 6-mil plastic vapor retarder on top of the T&G ceiling, you would have accomplished two things. First, you would have prevented air leakage into the ceiling cavities — air that takes moisture with it. Second, any condensate generated from outside would have been prevented from leaking through the joints...
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