Humidity & Condensation under Flat Roof

Hello, new forum member here!

I live in the top floor of a new construction triple-decker in Boston with a flat rubber roof. Last winter we experienced several water spots on our ceiling in multiple places (including a large one in our kitchen -- see below), which we suspect were due to condensation in the space between our ceiling and the roof. The challenge is that we have no easy way to access that space, and the only glimpse we've gotten so far has been by removing a small light fixture, recording a cell phone video, and planting a bluetooth temperature/humidy sensor to collect some data.

Here is what we know at this point:

• There is ~10" of blown insulation in the space, with ~12" of air gap to the roof (unclear whether or not there is additional insulation under the rubber membrane)
• There does not seem to be ANY roof ventilation (soffit, turbine, or otherwise)
• Sensor readings show very high humidity in the space (60%-80% over the past month), compared to an average of ~40% in the living space, and large temperature variations that mimic outdoor temps (high of ~80 degrees F, low of ~25 degrees F this past month)
• We see an immediate increase in both humidity and temperature in the "attic" space whenever we turn on the heat (maybe duct leakage?)
• The water spots last year occurred after running the AC for the first time after the winter season (in March, I believe)
• Our Carrier HVAC unit sits on the roof (near where the large water mark had appeared in the kitchen). An inspector came by last Spring and installed a condensate trap, which was missing

Given the clues above, my instinct is that roof ventilation may be the core problem (although I have no idea how the building could have passed inspection in 2016 without it). That said, the spike in temp and humidity when the heat is turned on seems to indicate a potential HVAC issue as well.

Any theories as to what might be going on here, or at least recommendations for where to start?

 

This looks like a really tough one. Sounds like the issue started when built and with no access the solution could be a major expense.

Your local building department should have the building plans that were submitted and it should tell you what the design requirements were. They can also provide a record of all inspections, start collecting.


All ducts should have been insulated and sealed but confirming that or fixing that will require some ceiling removal and you should not have to pay for it.


Attic ventilation becomes ineffective when there is no height difference between intake and exhaust. You or someone needs to determine what is there and what is needed.


As for the insulation you see, an attic space in Boston looks like R-49. 10 inches of fiberglass insulation is in the range of R-35. Even high density fiberglass doesn't make it. If they were allowed to install less that should be noted on the building plans.


I can do more speculating but will wait until you get to see the original plans and talk to the building department. Somewhere along the line you need to get someone in authority on your side.


Bud

 

Thanks Bud, I've looked through all of the digital records but have plans to head to the ISD tomorrow to (hopefully) dig up the building plans.

To clarify, would you expect that turbine fans on the roof would be ineffective, assuming my notes are accurate?

 

Turban fans spin due to air flow and air flow occurs due to intake and exhaust being separated by some distance, like many feet. So, turbines will not move a lot of air, plus any high vent needs a low intake.

A common flat roof solution is a cupola of sorts that provides an exhaust well above the roof surface. Intake can be from the sides or even short pipes through the roof.

Bud