Home Improvement → Water spots on ceiling

So I finally decided to investigate 2 small water spots I've had on my ceiling for 8-9 months. When I got up on the ladder and pushed gently on the spot my finger went through the drywall. The hole enlarged to about 1" diameter before I found solid drywall again. The second spot was smaller and a bit stronger but i still was able to push through it with my finger. These spots are in the ceiling cavity between the 2nd and 3rd floor, with no bathroom above. Immediately above the spots I found a flexible duct line which supplies the ducts in the 3rd floor.
I'm guessing I have a bit of condensation that built up on the duct line (which is uninsulated) and it dripped down onto the ceiling. It obviously wasn't that much water or else the spot would've grown but it was enough to weaken the drywall.
What are your thoughts on how to deal with this? The duct line runs the length of the house but I only found water spots in this one area.

if the duct is uninsulated, then the whole duct needs to either be replaced with a proper insulated duct, or the entire length of it needs to be insulated. you could have mold growing near other parts of the duct from condensation. it's also possible the AC is not removing enough humidity, caused by the AC not running long enough. it could build up in the duct. does it cycle often??

I thought duct lines were usually uninsulated when they're run in an interior space. The lines I have in my attic are insulated but when the duct runs in the ceiling cavity between 2 floors I wouldn't have expected it to be insulated. I have a new (1 yr old) a/c system, 2 stage. It usually runs in dehumidify mode and does a decent job at dehumidifying. I don't notice it short cycling at all.

When the ducts are inside the building's conditioned envelope, the ducts can be uninsulated. As you found out, that doesn't necessarily keep them from condensating. It's highly possible, since you have a new 2-stage system, the air velocity in that duct, especially in dehumidify mode, may be too low in that section of duct work. Also, it may be that un-conditioned air is infiltrating into the cavity between the ceiling and floor.

Update: I opened up the ceiling a bit more to get an idea of what's going on and found the ducts are actively sweating/condensing and dripping onto the ceiling. It appears the area that I opened up is the low point in the ceiling so that's where the most water is, but i've found a few more "puddles" that haven't made themselves known on the underside yet. When I looked towards the back of the house I saw there is some fiberglass insulation there, but I can also see sunlight so I'm guessing I'm getting a fair amount of air infiltration from outside into that cavity which is then condensing on the ductwork.
The wet drywall area has some black residue, guessing mold, but the rest of the cavity is clean so I don't think I have a big mold issue. That drywall is going to have to be replaced anyway due to the water saturation. I've covered the hole with plastic for now, both to prevent free air movement into the living space and to stop it from raining condensation in my living room.
I've contacted a spray foam installer to give me a quote to do the front and back of the house at the 2nd floor cavity. They have a $1k minimum and that job might not meet that in which case I'll likely have them do the same in the first floor. If the builders poorly insulated on the 2nd floor, they probably did the same on the first. I do have my HVAC guy coming out on Wednesday to take a look. Not sure if we want to rely on spray foam only or if the real solution is to do both spray foam and insulated duct work.

The bandboard between the 2nd and 3rd floor is probably poorly insulated (if at all). A lot of builders "forget" to do that spot.
There's also probably some outside (humid) air infiltration.