Condensation from kitchen vent duct
No leaks from the roof but when I went to inspect the ducting today, I noticed that there's moisture in the ceiling drywall under the vapour barrier. Not too much so I don't think it is due to rain but would be consistent with condensation from the very recent "cold" spell - hard frost and overnight temperature dropping below freezing. I don't want any ongoing problems with wet drywall.
When I installed the ducting, I had foil taped the duct joints hoping any water would drip down into the kitchen rather than anywhere else. I think I also foil taped the elbow swivels. Vapour barrier on the attic side and drywall on the room side were taped to the duct to prevent condensation from air leakage (was up in the attic checking ceiling light boxes and bath vents for air leaks prior to topping up insulation). The duct was wrapped in foiled bubble wrap.
What can I do to seal this? Spray insulation foam around the ducting where it comes up from the ceiling and makes the first ~60 degree turn?
Kitchen hood backdraft flaps close (just checked) but I am sure they're not air tight.