Ceiling Water Damage – Unventilated, Unconditioned Flat Roof. – GreenBuildingAdvisor

I was asked to investigate water damage at a new apartment building in climate zone 6B occurring near the kitchen ceiling exhaust on the top level apartments. The exhausts go through the roof, and are not insulated above the roof. I assumed condensation in the vent was pooling in the duct and finding its way to the drywall and causing the issue. Their maintenance crew cut out the drywall and removed the insulation so we could see the situation clearly. I cut the vent insulation foil and was surprised to see no signs of water even though the drywall wall still wet. I noticed the roof penetration wasn’t sealed at the deck either and suspected they could be getting moisture forming under the roof penetration boot but doubted that much air was getting up there to cause the amount of water damage we were seeing. The next day after getting on the roof, I could see signs of condensation inside the vent duct, and underneath the boot, but no water or wet surfaces. The previous night the maintenance crew put a sheet of plastic over the ceiling opening they cut. That plastic had significant water on it the next day.

After seeing no active and significant signs of water in the boot or vent while there being significant water on the plastic I am thinking there is condensation forming at the drywall/plastic.

Attached is a diagram of the roof assembly and some observations. Personally, I wasn’t expecting to see this assembly in our region. I would have expected some type of vapor barrier between the drywall/truss even though I know this isn’t a great solution and is tough to seal. My assumption at this point is that moist conditioned air is getting into the cavity of the roof and that the heat/moisture is building up there. I am having a hard time imaging that enough moisture would be accumulating in the space that it would condense on something like a room temperature ceiling. I am waiting to get access again to take some temperature/humidity measurements to confirm so this is all speculation at this point.

Has anyone encountered something like this before?