Mysterious moisture problem in attic

We live in a semiattached house with a cinderblock firewall separating the homes. The roof is sheathed normally except for about 2 feet on either side of the wall. There they used what looks like deep blue papered sheetrock, not cementboard, to sheath the area directly over the top of the cinderblock wall with it essentially resting directly on the top of the wall and then extended about 2 feet on either side of it. They did a double layer of this material and then topped that with sheathing and then of course roofing paper and shingles. As I understand it this blue material is a firebreak to prevent a fire in one side from traveling to the other.
When we had the roof replaced about 5 years ago they replaced the original material which was destroyed by moisture and literally caving in and it was assumed that was caused by a leaky roof. Not long after the new roof went on we noticed water stains appearing along the edge of the new material where it first meets the cinderblock wall along practically the entire length of the wall. We called the roofer back in to look it and he said it was caused by condensation traveling up the cinderblock wall from the ground. I do see what appear to be mineral deposits on side of quite a bit of the wall. He said little could be done about it but sprayed expandable foam in the seam along the top edge of the wall along the entire length of the wall on both our side and the neighbors side. After that, for quite a while, I had not noticed any new moisture so we assumed it was fine.
Now 4 years later I am again seeing signs of new moisture.along this same area. Not sure what to do from here. He claims the foam was really the only thing he could do. My feeling is that if it is condensation coming up the wall then to block it, it would mean tearing the roof off the entire length of the firewall including the blue board material and capping the cinderblock wall with some sort of barrier before relaying new blue board and reroofing the area. Any ideas if the condensation scenario is the plausible cause and/or what is the best way to handle this problem?
Thanks in advance, Adam
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There are two key things to prevent condensation - prevent the surface from getting below the dew point and remove the source of moisture.
Questions: have you noticed if this is a winter or summer situation? is there any exposure from the house up to the attic that would allow air to travel to where you're seeing the damage? This could be small cracks or a space between the block wall and the sheet rock in your walls. Are your bathroom fans properly vented outside?
I had a situation in a rental property where the pipes kept freezing. They ran through interior walls and seemed well protected from the elements. We found that in the attic, there was an air gap between the firewall between units and the inside walls. This allowed the cold air to flow down the interior walls, into floor cavities and freeze pipes. Something similar could be happening to you. The warm, moist air from the house could be escaping through cracks and travelling up the firewall and condensing on the cold underside of the roof and the fire wall.
Another real possibility is the bathroom fans. Seems every unit I look at has the fans vented into the attic spaces. Obviously, this will just pump warm, moist air up into the attic, which will condense on any cold surface. This is a big problem.
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