Finally figured out why my sub-roof was stained

Had almost the same wake-up call two winters ago in our 1964 ranch. Brown stains and fuzzy patches showed up on the north side of the sheathing, right around where the chimney passes thru, so I was sure the chimney flashing had let go. Rented a moisture meter, hosed the roof down with my son watching from the attic with a flashlight, never saw a drip. The real story was the bathroom fan. Previous owner had just let it dump straight into the attic, and all that warm shower air was condensing on the cold nails and decking every time my wife ran the fan in January. The nails were the giveaway, they had little frost rings around them on cold mornings. Rerouted the duct out through the gable wall, pulled the old batts back and added baffles at the soffits so air coud actually move, and laid down more insulation over the bedroom ceiling where it had thinned out. Took most of a season, but the stains stopped spreading and haven't come back since. If anyone reading this has mold near plumbing vents or a chimney, check what's venting into that attic before you blame the roof.

Chasing the roof side of things makes sense until you've been burned by the ceiling below, which is what happened to us. Our cape cod had black streaks all over the decking in a band about three feet wide along the eaves, and I tore apart the ridge vent twice thinking it was clogged. Turns out the insulation guys had blown cellulose into the kneewall spaces but left the vaulted ceiling bays over the bedrooms almost empty. Warm air from the house was rolling right up through those thin spots and frosting the cold plywood every January, then melting and dripping when the sun came around. The give away was that the mold pattern followed the framing bays, not any roof penetration. If your damp spots line up with joists or rafters, that's a pretty good hint the problem is heat getting through, not water coming down.

My fix was less elegant than yours. I pulled a few boards off the ceiling in the closet, dense-packed those bays, and air sealed the can lights that were basically chimneys into the attic. Took two weekends and a lot of crawling, but the frost lines never came back.

One thing I'd add is to check the attic on a cold morning before you commit to any repair. If you see frost or heavy condensation on the nails poking through the decking, that's your answer right there.

My situation is a little diferent, but I've been seeing dampness in my crawlspace, not the attic itself. I figured it was just moisture from the ground, but after some poking around, I realized the problem might be air infiltration. I had someone check the perimeter and they found that the spray foam around the outside wall was actually leaking air into the structure. It seems like even those tiny gaps are letting in the wrong kind of air. It's confusing because the humidity inside the house should be controlled.