Question regarding the method of sealing fiberglass duct to the supply register floor

[RJOB]

About two years ago the duct work was replaced due to mold. Reason given for the mold was the fiberglass supply duct exterior coating had failed allowing moisture to saturate the duct over time. The home is located in coastal North Carolina, has a crawl space sized for a very small person. The Trane gas pack is new about three years old. All the work has been done by a local HVAC firm that has been in business over thirty years.

Recently while doing some work in the crawl space I inspected one of the supply duct connections and found air leakage at the termination point of the fiberglass duct to the floor box. The leak was also causing condensation to form, which was running into the duct. The connection is made with a Panduit strap. The metal floor box has insulation wrapped tightly around it. Inspection of several other connections also revealed the same issue, supply air leakage resulting in condensation running into the duct creating mold.
What is the proper sealing method to prevent this problem?

[markwolf]

Have all the connections sealed with mastic.I prefer brush grade for the type of duct & connections you have.

[lbf]

Your discription is a little vague, but one of the things to look at is whether the crawl space has a high humidity level. The air passing through the duct is the same temp as the air leaking from it, so the metal floor box you describe would be cold even if there were no leak. High humidity levels caused be missing or improperly installed vapor barriers or open crawl space vents could be the source of the condensation.

[Swampfox]

Agree on the crawlspace humidity issue, its a big problem in your area (and mine), dewpoints have been in the upper 70's lately, if your crawlspace is ventilated your ductwork is very likely going to condensate, air leaks or not

however to answer your question, the proper sealing method is a UL 181B-FX rated tape, assuming this is a flexible duct, or, preferably mastic which was mentioned above.