Cover both the supply & returns. Here's my story. New house. Northeast US. Baseboard heat, A/C in attic. First winter condensation formed in the attic vents. So much that it ruined my ceilings. Installer came back & replaced entire system after 2nd year. Installed all insulated ducts and installed supply registers that close and have a rubber gasket that seals them shut. For the returns, I made up some covers out of White board that i put a self stick foam gasket and then glued some quarter size magnets to. I then put these on the returns. The magnets (4 corners) hold with no problem. This fixed the condensation problem 100% and also keeps the heat from escaping into the a/c system.

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'

jb I have had a similar issue and finally fixed it. I have a 3 year old home that is well insulated and tightly sealed. It is a 2 story home with a basement with geothermal heat/cooling. The second story of the home contains only guest bedrooms that are rarely used. The second floor has its own dedicated hvac and I set the stats to cover extreme temps (set temp = 60 in winter, 82 in summer). These settings cover the rare extreme temperature cases but also enure that the hvac unit doesn't normally need to run to save on heating/cooling costs. It turns out that in the winter the warm air rising from the first floor actually keeps the rooms on the second floor warm enough that I don't normally need to run the hvac unit upstairs. However, not running the hvac upstairs is what caused the condensation problem like you have. The key to the issue is that the insulted ducts for the second floor rooms run throughout the attic. Even though the ducts are insulated and my attic is over insulated, the air and surfaces inside the ducts will get very cold if the hvac is left off. When the warm air rising from the first floor gets into the duct inlets a lot of condensation forms. But I notcied that the condensation only formed when the outside temps fell below about 20 degrees. The first time this happened, I soaked up the water on the carpet and then turned on the heat and everything dried up. I also noticed that as long as I ran the heat up there I did not get condensation even when the temps outside dropped to zero. I figured out that as long as I kept warm air moving through the ducts that the condensation would not form. But I did not want to run the hvac up there all winter because it is a waste of money. I had an idea that whenever the temps dropped below 25, I went upstairs and turned on the hvac fan but left the heat off. The idea was that continuously moving the warm air from the second floor rooms (which naturally get heat from the first floor) through the attic ducts would be enough to keep the inside of those ducts warm enough to not create the condition for condensation to form. This process proved to work for me but was a bit of a pain. I had to always remember to go up and turn the fan on at night and then shut the fan off in the morning. But it worked. AFter one season on manually turning the hvac fan upstairs on and off I decided to automate the system. Since I have a zoning up there, I checked the manual for my zone contoller (EWC Controls Ultra Zone 4) and found out that is has a dry contact input for a carbon monoxcide sensor. This feature allows the automatic detection of CO (with a seperate CO sensor) and allows the hvac fan and a freash air inlet to be automatically turned on to circulate fresh air through the system. I used that CO dry contact input to hook up a battery operated progammable timer with a dry contact closure so that every night at 6pm the hvac fan turns on, and every morning at 6am the fan turns off. I only turn this timer on during the months when nightime temps drop below 25 (generally Jan & Feb). This fixed my problem and saved me the hastle of doing it manually. When this problem first surfaced I thought about sealing up the registers upstairs but decided against that approach becuase there are times when I do run the hvac (when we have guests or when it is really cold outside) and so I would have to run up there and take the seals off. There are quite a few regsiters and it just wasn't a practical solution even though it probably would have worked. The challenge is making sure that the seals are air tight.

We took a similar approach years ago to help screen/distract your eyes from the air conditioner. To keep airflow as open as possible, we used a wire screening material with a fairly fine mesh of small squares that is about 80% 'transparent' to airflow. You can still see the air conditioner behind it but the green-painted frame, some copper pipes for ornament - to reference a nearby fence and gate constructed with copper pipes, the metal screen , and nearby plantings help distract from the presence of the air conditioner. By the time the peak season for ac use rolls around, some of the nearby plants are ready to be cut down due to sawfly damage. We did replace the air conditioner in 2016 - 17 years after the original one. The new ac was taller so we needed to modify the screen to make it taller too. Below are two pictures - the first showing the modified screen being painted - you can see the structure of it and how transparent it is. The second shows it in place in June when all the plantings nearby are in vigorous growth. Perhaps you could incorporate some of the ideas we did to improve the airflow for yours....? We used brackets to support the side wings so there is nothing across the back. It is easy to lift the screen away for servicing of the unit. None of the service technicians have ever raised concerns about the screen.