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not insulated correctly or blow by condensation, or condensation leak.
..or high humidity in the crawl. My house is on high ground, the crawl is dry and the duct (1" board and wrap) doesn't sweat. But the same ductwork can sweat in other crawls.:001_unsure: Bad situation to deal with. Does the crawl have plastic vapor barrier laid down? That can make a big difference.
 

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The duct surface temperature is lower than the dewpoint under your house. Humidity is too high for the duct temperature. Reduce the humidity (plastic on teh ground, dehumidifier) or increase the duct surface temperature (increase air flow). Most codes call for sealed crawlspaces in new construction now to avoid this (condensation can cause mold).
 
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