Cannot Identify Source of Wall Moisture

Home owner removed drywall to install outlets and found moisture in wall.
Home is approximately 20 years old, vinyl siding and tar paper exterior wrap. There are no penetrations of exterior wall or interior wall in area where moisture was discovered.
Moisture is located about a foot above floor sill plate on siding and no signs of moisture or staining above affected area.

Property is located in Orillia ON.

Any ideas on what could cause this one area of wall to be wet ?

Is there a type of inspection home owner could get that would correctly identify where moisture is coming from or caused by.

Any information would be appreciated.

Exterior pic on the other side of that wall could possibly help.


It could be “wicking up” from the ground. Sometimes water from the roof hits the ground just right and dampens the sheathing behind the vinyl siding. This repeated over time can cause moisture issues.


Was the grade below the siding?
Was there a high moisture reading?

A possibility is that someone pressure washed it using high pressure and forced water behind the siding.


As you’re in Canada, I’m assuming this is a basement wall?
How was this wall insulated?
Vapor barrier? Installed how and where?
More photos, inside and out.

With all due respect, you’ve been an inspector for a long time.
This is very basic stuff. What’s up with that?

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Melting snow‽


Possibly a dryer exhaust or other exhaust vent below, exiting through the foundation wall or rim joist?

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I would reach out to a qualified home inspector.


A gap in the sheathing between the sill and bottom plate of the wall allows cold air to enter the warm wall cavity creating condensation.


It is a poured concrete foundation and had about 3 feet of exposed foundation prior to siding starting.

I agree, it has all the signs of air movement. I see those often along bottom plates of unfinished basements. Typically not concentrated like this however.

Also, to further your point. It’s barely damp with no deterioration. Which to me means it is slow and dries out intermittently.


Yup, OSB is considered dry at 19% or less.


There was moisture that you could feel on aspenite sheathing. No signs of rotting. But no visible gap at sill plate and siding so not sure there would be a gap there. I will suggest that he caulk this areas prior to replacing insulation and drywall. Just would hate to see him replace it all and have problem reoccur.

Go outside and see. Pull back the vinyl a bit and see if the paper is torn or missing. See if there’s a small air gap between the sheathing and the framing etc.


Gaps as small as 1/32’’ could allow air to move up into the wall cavity.


I don’t know if you are still there, but if you have a chance to do more testing you could turn on all the bathroom fans, range hood and even run the clothes dryer to create some negative pressure inside the home and then go inspect that area some more. With thermal imaging or even you bare hands you may be able to find that there is a significant air leak there. Or not. Either way, you will learn something and be a little closer to your answer.
The guys before me already offered some good things to check, some of which you already have done.

Please do let us know if you figure this out, as we can all learn from your investigation.


JJ mentioned vapor barrier… My observation/speculation from 1400 km away, that looks like an interior air leak. That’s my first focus of investigation.

I see no sign the original air/vapor barrier was caulked at the sill plate. If that’s the case, warm, moist, interior air can enter at the base of the stud cavity and make its way to the back side of the sheathing. In this stud cavity, the dew point of the moist air is located on the OSB. Why only in one stud cavity??? Further exploration.

As a retired building contractor, I’ve seen this condition during remodeling. In winter, frost is present.

Was the air/vapor barrier poly? Look at adjacent walls. Is it caulked at the sill and top plate? Is it taped at the joints? Is the vapor barrier Kraft paper? Was the wall covered with drywall, boards, or sheet material?

Get a building contractor to look at this, one who is trained in energy conservation construction methods, and who has extensive experience with building tight homes.

Could also be a rain barrier/flashing issue on the exterior… or ???

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Advise your client to use “smart vapor barrier” on the interior so the insulation can dry out if it accumulates moisture.


It allso could be before the house was dryed in, and the water puddled in that area, draining under the bottom plate of the wall, I see that the builder drilled drain holes in the subfloor.