Black stains on ceiling .
Hi. I have a weird situation happening on my clients upstairs ceilings. There are black stains appearing on the drywall ceiling. Up close it looks like moisture was collecting on the surface because the stucco ceiling was crumbling in certain areas. (See pictures) the stains range from right against the ceiling/wall junction and about 1 foot away from wall. I went up in the attic to check above and found no signs of water stains on top of the drywall. The vapour barrier was in tacked. Insulation is blown- in fibreglass (roughly 18-20” in thickness) but less as the roof slopes towards the soffits. Any help would be appreciated.
House located in Toronto, Ontario. Canada.
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Looks something like 'ghosting' although that's usually more prevalent at thermal bridges like framing members. Image 1 & 3 seem confined to single truss bays.. I assume soffit and ridge vents are present; make sure there is a clear path for at least 2" of airflow along the roof sheathing at every truss bay. Install baffles if there are none currently, then stuff as much insulation as you can at the low eaves in the attic being careful not to block the soffit vents.
My suspicion is that excessive heat loss is occurring at the eaves in absence of raised heel trusses, which gives the ceiling cold spots where condensation can occur. Excess humidity can contribute to the problem.