Hi! I noticed that during last hot days we had in Chicago, the ceiling on the top floor bathroom had condensation and moisture is running down the walls but not dripping. I have fans in both rooms upstairs and they constantly running but the condensation does not go away. Does this have anything to do with ventilation problem of the temperature difference inside and outside? Does anyone have the same problem with cathedral ceilings?

Condensation forms when the humidity is too high in the house. I had to open the attic door for an hour to get some wood up there (inland NC) and there was condensation on the vent in the room with the attic door. New construction always has more moisture from paint, drywall mud, concrete etc. During construction, the doors tend to get left open. Depending on where you are, the humidity should be falling significantly right now. But make sure the house is closed up. I have a house on the NC coast, I had to replace all the metal registers after 2 years from rust. Mostly that is renters leaving windows open and running the a/c. We also got 2 mold spots over 6 years that were just covered with Kilz and never returned. After Florence, I learned what real mold is..... Conclusion - don't stress. Mostly a side effect of ongoing construction. Around me - multiple jurisdictions - we need to have sod (or suitable alternative) to get a CO. Cities don't like mud either....

I thought we agreed that we were not going to limit the discussion to the climate of Katy Texas? Well the OP who started this thread made it sound like he lived near me... which would mean using my climate numbers as opposed to somewhere else. I don't service areas outside my climate so why on earth would I entertain this idea of yours? (I offer advice on occasion but that is all it is --- professional advice --- as this is what I do for a living, I don't sit all day in forum board --- I do have a life outside of HVAC, what little it is right now it's still a life...) You again trying to fit HVAC in a confined specific box in which it does not fit. What are the odds that someone will find that information useful? Probably close to 0% if I had to guess. That sounds like a waste of time... I know this seems productive to you so knock yourself out if you want to do that. Exact numbers are not necessary to educate everyone the source of the condensate. Then why not use the 'about 0.8 gallons estimate' and run with it? An air conditioner in my climate can produce upwards of 5 gallons of water an hour. A condensing furnace will not produce that much. An 80% non condensing furnace will produce 0 condensate water. Oh so let's use the AC condensate line for the furnace condensate. Problem solved right? Most likely not. Drain systems are complex especially when you throw in the variable that a condensing furnace imposes. See I've led your hand to this point --- where the concern right now up to this point is how much more condensate a condensing furnace will produce in my climate Katy, Texas versus other less humid climates in which those furnaces are installed typically in a basement. You don't have the over head flood threat you do here in my climate. All of this --- except: there are safeties on that condensing furnace that if you don't do it properly, the furnace will shut off on nuisance trips. So the furnace is not likely to be the cave in threat due to condensation production. They tend to leak water in drips around the furnace while having very erratic operation most often. You don't get these kinds of problems with a 80% non condensing furnace. What a home owner knows versus that of a pro... you think condensate is the only problem you have to contend with. What is your definition of making this actually work? How does the condensing furnace then become the ceiling cave in threat? Changes to the condensate drain system by people who don't know what they are doing. It may be a home owner, land lord or even a technician who doesn't understand the implications of what they are doing. How much water one produces whether here in this climate in Katy, Texas or a less humid climate (in winter) like Chicago / Wisconsin --- the different in condensate production doesn't matter... *IF* the system to get rid of condensate is adequate. This thread was about a young guy presumably in his 20's attempting to install a condensing furnace in a probable high humid climate. That condensing furnace was bought used. Who in their right mind sells a condensing furnace (used) in a probable high humid climate? One that tried to install said furnace and couldn't get it to work. I have an innate ability to read between the lines. I've seen quite a bit of 'Tom Foolery' in my nearly 26 years of doing this for a living. We can't get it to work so we'll take it out and sell it to someone else. So what makes the condensate drain system of a condensing furnace adequate? The right way. What is the right way? The opposite of wrong. If you want to learn how to do it: There are trade schools for that. Class dismissed......

We framed around the base of the shaft and inserted a pane of plexiglass. This stopped the condensation in the shaft. In your case, I’m guessing warm moist air is coming down the shaft and condensing on the cooler surface of the ceiling. ETA: closing off the shaft may help with your problem as well.