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As part of a basement renovation I added a heat pump for heating and cooling the basement. I also added a 10×18 rectangular sheet metal duct to provide fresh, outside air to a 100K BTU gas furnace and 38K BTU gas water heater. The duct runs between two manufactured plywood I-beams that are 12″ deep. I plan to drywall the walls and ceilings. The HVAC mechanic who installed the fresh air duct said I wouldn’t need to insulate it. I’m not sure. Does anyone have experience with such ducts? Do they sweat in winter? Should I insulate? Faced or unfaced insulation? Which side? Any thoughts will be appreciated.
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If the ducts are uninsulated, the metal surface will be between the temperatures inside and outside the duct. But much closer to the air inside the duct because of its velocity. So expect it to sweat a little bit. (Unless you add moisture to the air in the house, then the dew point would remain at or below outside air temperatures. But there's always moisture being added by cooking, cleaning, perspiration, bathing). Insulation with a vapor barrier on the outside would prevent the sweating.