Wet Sheathing in Attic Above Concrete Partition Wall

Hello all,

I have found some wet sheathing in the attic above a concrete separating wall for townhouse units. The concrete wall you see is not an exterior wall.

Roof has been checked by 2 separate roofers and the roof “is fine.” Second roofer mentioned a couple nails were tapped down implying they were out of place. Insulation appears to be fine and the attic hatch doesn’t appear to be a source of heat loss ether.

Could too much fire insulation being stuffed between the truss and and concrete wall be trapping moisture and causing the condensation on the sheathing?

Any help is appreciated. Everyone other place in the attic is fine. All exhaust fans are vented properly. Please note the weather has been mild and dry the last little while and most of the moisture has receded. I pulled out some of the fire insulationstuck up there some of it was a bit wet and some of the sheathing directly over the concrete is still damp… Wonder if the extra ventilation would help. Also note fire stop is on both sides of the wall at the sheathing hence why I’m wondering is moisture is getting trapped.

During very cold day:

Mild weather:


Moisture is above bathroom and ensuite closet:


From today after a few days of mild, sunny, and windy weather:


Update showing today’s status after several days of mild, sunny, windy weather…



Appears to be efflorescence on the surface of the CMU indicating that moisture is moving through the CMU. Efflorescence forms from moisture moving though CMU, not condensation forming on its surface.
The source of the problem may be located on the other side of that wall, or if the CMU wall extends up through the roof, I’d say the source is inadequate method of sealing the top of the wall.

The CMU extends to the sheathing but does not go above the roof line. There is a flat sloped roof over this entire section. I have squeezed my hand in a few spots and the top of the CMU does not appear to be sealed. I noticed the CMU is pinching the sheathing in a few places but the roofers said that is of no concern. The builder dismissed the concern that perhaps heat loos thru the CMU is rising up to the sheathing. I have asked the neighbours on the other side of the CMU to take some photos so I can see what it looks like on their end. I would have to assume they have the same thing as the CMU is only so thick.

If they don’t then the only thing I can think of is heat loss where the attic meets the CMU- but if that were true the wetness would wetness would probably show on he insulation. If appears the efflorescence is coming from the top. The next possibility is heat loss thru, say the bathroom wall, and the warm air is rising up the middle of the CMU cinindernblocks and getting trapped at the sheathing. The only thing I could think of here is the walls of the house don’t have alenough insulation in that section if that were the case.

If th efflorescence only appears on “my” side then perhaps

It’s very bizarre… household activity does not seem to have any bearing on the amount of wetness on that sheathing. Dishwasher 2 floors below, no impact. Range exhaust fan, no impact. Shower, no impact. Dryer, no impact. Ironing, no impact, lights in the bathroom, no impact. The neighbour on the other side has been home this whole time, no impact.

It seems to be the same amount of dampness regardless of what’s occurring inside the home and it hasn’t got worse despite all these isolated “tests.”

Perhaps it’s general heat loss through the home walls into the crevices of the CMU… Or there actually is something going on with the roof, despite 2 roofers saying otherwise.

Are there any spot tests you can do to determine if there is seapage through the shingles into the sheathing?

As Kenton said CMUs are porous and therefore suck moisture like a sponge.

This maybe originating from below and traveling up. I won’t get into the what/why and how of what water/moisture does or can do, and I have no idea where you live or an idea of what your place is like or what kind of weather you have been experiencing. There are just too many variables.

You have a problem and the roofer and builder obviously don’t understand building science.

Yes you could do a thermal scan of the roof but I see that as just one part of the entire picture of what needs looked at. You really need someone who understands building science and can think like moisture.

I had the builder in the attic last Thursday. He played around with the bathroom exhaust fan in the attic. He said it was leaking air (bough I could never tell and you could never see steam in the coldest of days… maybe it was running underneath the insulation.). He said that and the frame around the attic hatch could have been causing this. Long story short, he re-sealed the exhaust fan and foamed it down and also foamed the 1" gap in the framing around the hatch. The area in question appears to be dry. We also found a few small wire holes by the wall that may not have been sealed from underneath so we foamed those. Lastly, the vent pipes and hoses are not buried by the insulation deep enough- so we are going I boo that up.

Lastly, I should mention I had a contractor perform a 1 hour water test with a hose on the roof and there were zero signs of any leaking.

In all, it seems as though a combination of a few small things has led to the problem. Absolutely bizarre.

@Kenneth A. Ram and @Kenton Shepard, CMI

In the absence of moisture, could the concrete block party wall contacting the wood sheathing on the roof cause any condensation issues, ie Cold contacting not so cold surface?

My WAG would be hollow blocks are wet or water filled at ground level causing water vapor to rise up through the hollow voids until it condenses in the cold attic.

Good one I like this info Thanks … Roy

Good one I like this info Thanks … Roy

Water will go up a tree 400 ft. It will travel through concrete for 6 miles. Readily passes through and is probably adding moisture to the area. Needs more venting for certain and a mold remediation.