Way O/T but need help with rental house

Doing some work at my sisters rental house between tenants and found a problem I do not know what to do.

Let me see if I can explain this.....

This is a house that is 50 to 60 years old.
The rear of the house is 2 bedrooms; 1 on each side. The house has a cathedral ceiling in this area and the wall separating the 2 rooms is right in the center of the house so it goes all the way up to the very peak of the cathedral ceiling.

I found damp and soft sheetrock about 18 inches by 18 inches on the exterior wall right in the very peak of the cathedral. The wall between the 2 rooms is sound with no moisture. The ceiling is sound with no moisture. It is only damp on the exterior wall and only a 18x18 inch spot right in the very peak. Both rooms show the very same condition.

Thinking we have a leak from the outside in I pulled down about a 2ft square piece of sheetrock in each room. While my nose is not very good sister says she smells mold as I was tearing this sheetrock down.
Found the paper (vapor barrier) was also damp to the touch but the insulation was dry.

Now the problem...........
I was expecting a water leak from the outside BUT the 2x4's are dry. You can see up to the under side of the roof and it is clean and dry. The exterior wall has (from inside out) black tar paper; shiplap wood siding; and aluminum siding over that. All of that looks and feels dry. I can not see where any water is getting in from the outside anywhere.
Striking the 2x4's with a hammer shows they are very sound so I do not suspect bugs such as termites.

Only thing I can figure is the problem is condensation. About ready to put up new insulation and a clean piece of sheetrock and let it go.
Just can not figure out why the only damp moldy spot I can find in the whole house is this wall.

Your Thoughts............

I wouldn't put anything back together until you get a good rain.

Condensation is a possibility with all the layers of material on the outside. I believe it's no longer acceptable to sheath a house with tar paper; Tyvek is used now which is permeable to water. Maybe some vents should be strategically located in the siding so moisture isn't trapped between the insulation and siding. Something like these:
Siding vents

I would say this also, if you suspect mold in the area you need to hit it really good with bleach to kill it. Also does this house have eve vents? If not i would suggest you put some in.

Instead of waiting for a rain you need to go up on the roof with a water hose and try to duplicate the leak (If there is one?). Go lower than the area and work you way up until you have water coming in. It takes two people to accomplish this, one inside looking through the hole you cut and one on the roof with the water hose. Good luck, these can be very hard to find sometimes.

Is there a ridge vent in the roof?
Has anything changed in the last few years? New roof, insulation, eave venting? I am inclined to agree with you it is condensation, but would not be in a hurry to close up.

If metal roof, maybe condesation leaking in at the area mentioned. How about a vent stack/chimney? May be a ridge vent. The wind blows just right, and the metal ridge vents have been known to leak. Is wind driven rain following the soffit panel grooves beyond the siding, to the structure. When wind hits a house, it tends to go up, and will take moisture with it. Fill the upper soffit grooves. If the aluminum siding stops below a freize board, maybe wind driven water is comming in over the J-chanel?

Make sure you use the green, "bathroom" water resistant sheetrock there when you replace it.

Gordo

An area manufactured home builder who primarily sells homes to Colorado placements drills a half inch hole in the osb sheathing in each stud cavity. They alternate between top and bottom of the cavities going along the wall.

Apparently with all the state of the art vapor barriers we have now, condensation is a problem and a problem they have probably dealt with on recalls.

I would sure want to find the source of the condensation or water vapor. Wonder if it is close to a bathroom vent or close to a place where moist air from the bathroom is channeling to it. Dryer vents carry lots of moisture. May be a leak somewhere.

I think you need to keep looking. I can not see condensation happening in that situation. My guess you've got water following a nail or a pin point leak at just the right spot

You have a leak!
I would look at the roof in the overhang, maybe at the top of the facia. The water is wicking to the wall.
Go up the outside of the house pull the facia or the uh, uh, Shoot the underside of the overhang.

Thanks for the ideas guys.

This is a 3 tab shingle roof. No ridge vent. Just has the 1 tab of a shingle nailed over the ridge area type of roof. Shingles were installed by professional roofers about 10 yrs ago.
House has soffit vents but suspect they do not work very good in this area because the ceiling is cathedral style in this end of the house.

Got up on the roof but did not see any problems. Soaked the area for about a hour with the garden hose but could not find a leak. Then took off a few pieces of the soffit siding and felt the wood under the fascia siding and all was still dry.

The bath room door is just outside of both these bedroom doors. That puts this exterior wall about 12 ft from a water source. Dryer is in a detached laundry room so the only water source in the house is the kitchen and bath. Did notice when they put in the add on central air and heat the vents were installed in the wall just above the doors. This puts them about 12 ft from this wall but they do blow right at this south facing exterior wall.

Really think this might be a combo condensation problem and the more I think about it the central A/C installer may have caused some of the problem.
This end of the house faces south.
A/C vents are across the room but blow at this wall.
The problem is right at the highest part of the cathedral ceiling and the air flow is restricted in this area by some cabinets that were installed very close to this corner.
A/C air inlet filter is in hall just across from the bath room door.

I am going to leave this area open as long as I can and repair it last to make sure there is no leaks from the outside.
Move the cabinets in the corner to a different spot to open up better air flow in this area.
Going to install a exhaust fan vented to the outside in the bathroom and wire it to where the fan comes on when ever the light is on to reduce moisture in the house.
And my sister is calling a roofer Monday morning to get a second opinion on the exterior condition of the house just in case I missed something.

Thanks again.