This is just a thought and IâÂÂm guessing here but how well is your attic ventilated? Do you have soffits that breathe and adequate roof vents that remain above any snow accumulation in the winter? The attic should be at the same temperature as outdoors to not accumulate moisture.

SR

Thank you for commenting on this but how does the moisture from the attic transfer inside of the ductwork when it is tightly sealed up with mastic, aluminum tape and the thick blankets of insulation? I do have ridge venting and make every attempt to get the snow off when it comes. The addition which has the most problems is vented very well with each soffit vented. The older attic area that doesn't have any condensation issues needs imrovements for better soffit venting. I improved it somewhat when I had the siding and soffits replaced using a sledgehammer to remove the blocking just so I could HAVE vents because someone must have thought that it might get cold up there if it open to the outside.(sarcasm)

Now that I have had time to think about it a while. The only way condensation can form inside the ductwork is when there is humidity entering that particular duct, correct? Would a likely suspect then be if there is any type of air leaking even a tiny amount into those ducts that have a 3 foot or so lift into the attic before they turn horizontal to go into the trunk line?
Justa thought but I have other vents in other parts of the house that way and they do have not ever had condensation even after the 1st winter when I didn't know I had to seal them up.

I donâÂÂt have all the answers, maybe I donâÂÂt have any. I was just trying to provoke some useful analysis of the problem. The attic and everything up there should be as cold as possible.

You mentioned that the new addition has most of the problems and the older attic has none. Might this be because the newer section is too tight and the older one âÂÂbreathesâÂÂ?

You should not have to clear snow off the roof with proper roof vents. See link below for roof vents designed for a cold climate. TheyâÂÂre very common here where snow can be quite deep on the roof, I donâÂÂt know if theyâÂÂre common in your region. They always extend above the snow.

SR

Here is a link that might be useful: 'Maxie Vent'

Only R10 above the ducts? That's a BIG problem. If you have ductwork in the attic it should be below the insulation level, which should be R-40 something in your climate. You shouldn't be stuffing anything in to the ducts at all. You should have proper vapor barrier at the ceiling level, then the R-40 over that and the ducting, and then the attic itself should be properly vented. You'll never seal the ducts well enough to account for air infiltration, and you will always get condensation when the interior moist air encounters a poorly insulated space above.

I should clarify that I have at least R-40 insulation levels in my attic if not more. I have R-10 insulation wrap for my duct work only. I have vapor barrier on all the ceilings. I find it very odd that some ducts I have in the older part of my attic are running above the insulation but do not have the problem like the ones in my addition that run above the insulation level. I was thinking that I should rerun the duct work for the 2 that are creating such an issue. Thanks for the reinforcement. I will look at getting some pictures of these ducts and ones that are run the same way but not posing any issues.

How cold is the air coming from the registers? You may have low airflow causing your ductwork to be too cold.

I need to use insulation bags to seal the ducts during the winter since they are only used for A/C so I do not what the air temp is. If you are asking the question for when the A/C is running in the summer, the temp is the same coming out of all the registers and the airflow is very good.