Moisture issue in attic

Inspected an attic yesterday with a blackened sheathing on the underside of the roof sheathing. It is only on the northern exposure side of the roof and I am assuming it is related to moisture and inadequate ventilation ( no soffit or ridge vents but there is an attic fan). Also, there are two layers of shingles on the roof but I’m not sure if this is relevant. I intend to suggest improving attice ventilation and mold expert. Can anyone definitevely tell me if my assumption is correct or is there something else that I might be missing here. Thanks as always.

Its a high moisture issue in the attic. Mildew/ mold is growing on the North side roof decking because it dosnt typically heat up as much like the southern exposure side does. But it all comes back to a lack of ventilation. Your on the right track.

Sounds reasonable to me.
Also, suggest the possibility of Ice damming as a cause . . .

Also check out if there are can lights and whether they have been sealed. Many times, around here, I see liquid moisture in the rafters and underside of the roof decking in the winter. Humidity from the house going up through the can lights and condensing on the rafters.

People tend to set their humidifiers too high and dumb a** builders don’t seal the can lights.

Hope this helps;

Not all cans are ‘Air-Tite’. People still like paying for the least expensive types and sealing them won’t make any difference. I would recommend having soffit vents installed and to make sure that the attic fan has a humidistat as well as a thermostat.


Was it black or dark brown? FRT plywood will turn dark brown and delaminate.

You can easily seal vented can lights. Wrap with unfaced fiberglass and spray foam over that. I know and insulation company that does it all the time.

As long as they know to check if they’re IC rated or not.


Correct me if I’m wrong but sealing the vents n a can litght would change it’s ability to shed heat and probably violate the manufacturer’s instructions.

At the very least I would think the fixture could no longer be used at it’s maximum lamp capacity.

I would think so too. In fact, I’d expect to see the heat sensor shutting off the light intermittantly.