Mysterious moisture in attic at firewall

[amn0270]

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 to sheath the area directly over the top of the cinderblock wall with it essentially resting directly on the top of the wall. 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 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 neigbors side. After that I had not noticed any new moisture stains 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 what can what is the best way to handle this problem?

Thanks in advance, Adam

[DanG]

I have not heard of your magnitude of problem before.

A few ideas:

Assume the block wall is an 8" wall. If the wall is not grout filled, perhaps you can insulate the interior of the block cells with expansive foam

What is your climate? Cold and humidity hitting a hot roof? Perhaps adding a power vent or several in the area of the problem to reduce the heat and humidity would help.

You might also consider sealing the "blue stuff" (I don't recognize the description) with a shellac type sealer to reduce the damage to the material.

I hope someone has a more direct experience.

Your construction should be listed or detailed as a party wall or area separation firewall.

[mm]

That's pretty funny. Around here in Baltimore we have semidetached homes.

I wonder what would happen if you put a semiattached home next to a semidetached home.

[amn0270]

The wall is grouted. The only access to its interior is through the tiny gap where it meets the blueboard which he already sprayed the foam into originally. Right now we are in winter here in NY so a hot roof isn't the cause. We do have a power vent in the attic but it does not cycle in the cold weather. Sealing the blueboard would not help since its surface is not accessible where it lays over the top of the wall. Thats what the foam was sort supposed to do. The only way to seal it would would be to rip the roof off, remove the damaged blueboard, get new pieces and shellac them and then lay them in place and reroof. At that point it would make more sense to seal the top of the firewall once it was exposed. Hoping someone here has a better solution though.

Thanks anyway, Adam

[Norminn]

It is pretty difficult to envision the structure and damage you describe. It is also difficult to imagine enough condensation travelling up within a concrete block wall and causing serious damage. Are there rafters or beams resting on the wall? Is there a solid footing beneath the entire wall? What is on either side of the wall .. exterior? rooms from each residence adjoining? finish on exterior portion of the wall? Single story on slab? Crawl space? Attic? Photos might help.

Have you had a termite inspection?

[trader4]

My guess would be that it is condensation, but it has nothing to do with moisture traveling up a block wall. If that was possible, there would be all kinds of problems with cinder block construction. My first suspicion would be that you have inadequate attic ventilation and/or some source of moisture getting into the attic. The water then condenses against the cold block wall. How is the top of the attic vented? Good soffit venting? Any bathroom fans venting into the attic instead of going outside?