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Crawl space ductwork and condensation on adjacent wood

williamfinch | Posted in General Questions on

Life in warm humid climates (zones 2 and 3, Gulf Coast with 70 inches of rain a year) creates many problems, of course, particularly when old houses get burdened with air conditioning. So there’s an ongoing issue with ductwork in 19th and early 20th century houses in our historic district, and home owners, buyers, sellers and realtors make it worse. Increasingly ductwork is going underneath houses with very high and open crawl spaces (often 3 to 6 feet high, typical of early construction). Besides the usual problems with strong temperature gradients between house and basement air, there’s also the problem of the temperature gradient around ductwork. Of course, that means the ductwork insulation gets soaked, but we’re hearing from folks who are seeing mold on their floors and cupping immediately above the ductwork (but not elsewhere) and even when the ductwork does not actually come in contact with wood. Short of substantially rebuilding these houses to make them safe for air conditioning, or completely replacing HVAC with something like a ductless system, are there cost-effective ways to at least reduce the condensation issues on the wood immediately around the ducts? Would lowering the duct work (and increasing the distance between joist and ductwork) be at least a good experiment?

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  1. Jon_R | | #1

    Consider closed cell spray foam to insulate ducts - it blocks air, water and water vapor.

    While more suitable to indoor ducts, consider mixing interior air into the supply duct to make supply air temperature closer to the dew point of the surrounding air.

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