Gas and Oil Home Heating Furnaces - condensation dripping down wall

i am about to pain my living room and have noticed a few lines of moisture have dripped down from my air returns on the wall - i'm guessing this is from condensation. of course, i want to correct the problem before painting.

i have fairly new (less than 5 year old) central gas heat and air. the coils and heater are in the attic. we live in mississippi - which is generally humid and warm - but have a central humidifier on the heating unit for use in the winter (water to it is shut off in the summer). all our vents come out of the ceiling except the 3 in our living room which is a very high cathedral ceiling - and thus come out of the wall - that is where the drip lines have been noticed.

the ductwork is all wrapped with insulation - but that is a bit old - don't know if any gaps have developed.

also - don't know if the drips have been occuring in summer or winter - so don't really know if it's coming from AC or heat.

any help? thanks - fred

It's happening during the cooling season. The sheet metal piece that the duct is connected to at the outlet is called a boot. Then the register is attached on the finished wall side. You'll probably have to reseal the area where the duct attaches to the boot. It's important to get a vapor tight seal at the juncture. Some people think I'm nuts, but I use expanding foam all around the connection and even make sure some gets into the area where the boot abuts the drywall. I then wrap some insulation around the duct and into the foam while it's fresh. After the foam sets, I paint the whole thing with mastic. The key thing is to form a vapor barrier, otherwise, the duct/boot will sweat.

Footnote: some people thik I'm nuts and it has nothing to do with sealing ductwork.