Did I create condensation inside the return air ducts?

You cannot sweat unless you have the wrong air mix. Dustin in probably correct, its probably their laundry routine creating super humidity in their utility space. The average homeowner creates their own issues that no amount of engineerimg will solve, then they blame anyone and everyone else.

I’ve seen this many times in the humid, not so much a heat load area of the Baltimore area of Maryland to the east and Eastern PA when basement ducting is sealed and insulated before finishing the basement. Without having the air space in the soffits and joist spacings around the duct be able to cool those spaces, humid air leaks into those spaces from outside walls and sweats around any areas of the ducting that couldn’t easily be insulated or did not have a good vapor barrier. Water stains on soffits and around vents are common.

Is it possible that before there was predominantly supply side air leakage and now after the aeroseal there predominant return side leakage? Same effect I suppose. Can the ducts be re-tested but separating the return from the supply perhaps? This would serve a huge purpose from a diagnostics stand point.

At first my mind went to the same place as some of the other comments with the on demand dehumidification airflow reduction that would make the supply air colder. But if the system was replaced by an arguably undersized system then that means that the humidity, at least on the inside of the drywall is likely under control. This leaves us with the longer on cycles as the possible culprit for the duct surface temperature.

Either way, this one reminded me about Hotel Mold from Lstiburek, where you had positively pressurized living space, but negative walls. Here is the link. Good luck, keep us posted please.

https://www.buildingscience.com/documents/building-science-insights/bsi-130-hotel-mold?utm_source=Building+Science+Corporation+List&utm_campaign=ef0f503ba1-BSC+Newsletter+Issue+%23102_COPY_01&utm_medium=email&utm_term=0_194890bc8c-ef0f503ba1-121072954

I’ve seen a number of articles over the years about how duct sealing can cause problems to the extent that it impacts airflow. When working on BPI-3300, I argued that duct sealing should always be accompanied by airflow testing to ensure adequate airflow in cooling mode. Your diagnosis in this case suggests that’s not enough to guarantee “no harm” from duct sealing. Supply duct leakage may have been the only thing preventing interstitial infiltration at the band joist (rim bands are notorious for air leakage).

Another possibility is that the HVAC contractor set the fan to “Continuous” or advised the owner to do so. We know that can raise indoor humidity.

Regardless of the cause, buildings have many complex interactions and while it’s certainly possible to avoid duct condensation with proper design and installation practice, the surest defense is to minimally insulate the ducts in a way that also provides a vapor barrier. I think it’s unfortunate that the IRC/IMC doesn’t require this in the “A” and “C” climate zones.

As an aside… A senior code official with the North Carolina Dept of Insurance (code regulatory authority) once told me that while building codes don’t require interior ducts to be insulated (they should), contractors are nonetheless liable for any damage caused if the ducts sweat. I’m curious how other states address this issue, if at all. Do any jurisdictions require insulated interior ducts?

I would not, however, rule out the new HVAC without at least a quick gander at the system’s indoor temperature split – return vs supply temperature. Tech should check static pressure, as well. If the system has a variable speed air handler check for active dehumidification configuration – this can dial back air flow when humidity is higher than setpoint – increases run time and wrings more moisture out of the air, but may also result in duct condensation.

Lastly, do not neglect cooling setpoints – these sometimes take some sleuthing to suss out since occupants aren’t always truthful or may be unaware of the habits of others in the home – teens etc with their own thermostat / system / zone setting it inappropriately. I advise asking “The Question” in two parts: “What (temperature) do you normally set the system at?” and then, crucially “…any lower at night?”. Life / health / household changes sometimes lead to reduced setpoints. In my area in and around Jax, FL, we try to steer clear of anyone setting any system below 72*F. Those that set below that are best served by one of our competitors.

All of these are situations I have encountered in the course of duct sweat investigations

See it more with people who have sealed ducts but then have attic fans.