When warm, moist interior air hits cold metal, the moisture condenses out of the air and clings to and runs down on the metal. Any untreated wood--like purlins--that is continually wetted will rot, and informed people do not want treated wood inside their homes. Any insulation such as fiberglass becomes soaked and looses its insulation quality. One solution is to leave a dead-air space next to the metal and to have adequate insulation and interior air-tight barrier to keep the warm, moist air inside the structure and away from the metal.
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