Another Duct Condensation Question
New construction (built May-Sept last year, moved in Oct)
Central NC (very hot & humid May-Aug)
Block crawl space
Trane electric heat pump that switches to LP when temp reaches the threshold set for the heat strips to come on
All ductwork insulated
Crawl space vents open
It all started with a moldy musty smell in our master bath near our whirlpool tub. Removed the front panels and realized the cutouts in the subfloor for the plumbing were never filled in around the pipes--just wide open space between the crawlspace and tub deck. Warm air was blowing up from the crawlspace and bringing that musty smell with it. DH used spray foam insulation to fill in the open spaces the next day and smell is better but not fully gone. Then the HVAC guy we've used for years in previous houses came over to see about installing system for upstairs expansion (builder used a different contractor). He went under the house and the exterior of the ductwork insulation was literally dripping wet and puddles were forming on the vapor barrier. The main part of the system was also drenched in condensation. His recommendation was to close the crawlspace vents and use a couple dehumidifiers under there for now to try and dry things out. We closed the vents immediately and 24 hrs later it seemed to slow things down some.
We understand how condensation works, what I don't understand is if the ductwork is insulated, would it really get cold enough on the outside of the insulation to form condensation when it comes in contact with the moist hot air of outside? We don't understand closing the vents--we always had the vents open in spring/summer/fall to keep the air moving under the house and closed in winter to try and retain heat for the plumbing. I searched online and saw differing opinions on closing the vents in this type of situation. Is there anything else that could be causing this issue that we need to look for? We have lived in this area for 30+ years, always had open vents spring/summer/fall & closed in winter and have never had this problem in any other house (including a new build 12 years ago)--what has changed? Any other advice or suggestions (other than retrofitting the crawlspace to a cleanspace type area)?
Thanks for any advice you can give!