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Condensation inside wall cavity

I'm getting condensation inside the west facing wall cavity of my 9 year old ranch house. I first noticed the issue when I smelled mold next to the bottom of the wall. I removed the baseboard and the bottom few inches of drywall and I had moisture in the cavity. At first I thought the moisture was due to a leak but the moisture came back when we didn't have any rain. The condensation is collecting on the insulation side of the vapor barrier. The vapor barrier is between the drywall and the studs. The wall has fiberglass batt insulation in it and the exterior wall surface is brick over OSB sheathing. Please help.

Re: Condensation inside wall cavity

What else is along that wall?

Are there any water pipes going through it to an outdoor hydrant?

It could be that air leakage through electrical outlets and switch plates is accumulating on the cold brick all winter, and then melting in the spring. You SHOULD have weep holes at the base of the brick wall to allow any condensation that forms between the brick and the OSB to drain out.

In fact, it's a real good idea to drill some holes through vertical mortar joints both at the base and at the top of your brick wall. Angle the holes so that rain water drains out of them. Do you see a white powder (called "efflorescence") form on any parts of your exterior brick walls in the spring?

The whole idea here is to use the unavoidable heat loss from your house to drive a convective air current between the brick and the OSB. The idea is that air between the brick and the OSB is heated by heat loss from the house and rises. As it warms up, it's relative humidity decreases, and any moisture present evaporates into that warmer air. As that warmer, moister air escapes through the holes at the top of the brick wall (under the eves), it draws in colder dryer air from the holes at the bottom of the brick wall. In this way, you use the heat loss from your house to drive a convective air current that continually works to keep your exterior walls dry. It's exactly the same principle as attic ventilation applied to exterior walls.

Re: Condensation inside wall cavity

First of all I should have mentioned in my original post that I live near St. Louis and its been very hot and humid here (like usual). I do not have any water or drain pipes running in that wall. The brick wall does have weep holes every 24" or so on the bottom. I have't checked to see if there is any type of ventilation for the brick at the eaves. Actually, the wall only has brick on about 3/4" of the surface. Half of the wall the brick goes all the way up to the eaves and the other half the brick only goes 1/2 way up and stops at the bottom of the window. There is flashing on both sides of the window where the vinyl siding meets the top of the 1/2 brick wall. I'm getting the moisture behind both the full brick area and 1/2 brick area. I haven't noticed any efflorescence on the brick but my brick is a light color that would make it hard to spot. I seem to be getting more moisture not directly under the windows but just to the sides of them. Actually I noticed moisture on the inside surfaces of the studs that support the window framing. I'm beginning to wonder if the builder did not insulate very well between the window framing and the window. If that was the case, wouldn't it make it more condusive for moisture to build up in those areas?

Re: Condensation inside wall cavity

The same mechanism that causes moisture to collect inside a wall in winter, works just the opposite in summer.

In winter, warm moist inside air enters the wall cavity from inside the house and is cooled by the cold outside air. The moisture from the inside air condenses in the wall cavity. A vapor barrier helps keep the inside air inside the house and not get into the wall cavity. The exterior is intentionally ventilated so that the cold outside entering the wall cavity and warming up will have its relative humidity go down. This air will absorb any moisture in the wall cavity and remove it as it is expelled through the upper vents.

In summer, warm moist outside air enters the wall cavity via the vents where it hits the cooler inside surfaces of the wall cavity. The inside surfaces are kept cool by the homes AC system. The moisture in this outside air condenses on the inside surfaces of the wall cavity.

There is not much you can do about it unless you want to strip off the sheetrock every six months and reverse the insulation.

Re: Condensation inside wall cavity


If Kieth is correct, and the condensation is being caused by a convective current working in reverse, then that exterior wall ventilation that I was talking about would be about the worst thing you could do to your wall.

The reason why is that in summer, with your A/C operating, the air current would flow backwards, drawing in warm moist air from outdoors and cooling it and drying it so that it deposited it's moisture inside your wall cavity.

Re: Condensation inside wall cavity

It's difficult to tell sight unseen.

It may very well be from a water leak from the exterior. If liquid water is able to enter the wall cavity it will eventually heat up during warm summer temperatures and turn into vapour.

Otherwise if it is condensation occuring during warm summer temperatures -- it may be from running the air conditioner and having a poor seal between the floor and wall joint. This would allow cooled conditioned air to enter the wall cavity meeting warmer moist air from outside -- this has the potential for condensation to occur.
In this case it would be a matter of removing the base borads and sealing the floor/wall joint with either a caulk or foam in a can.

In some cases if mould is forming along the bottom portion of the wall --- a section of the drywall should be removed ( maybe to 4 ft. ) --- the vapour barrier cut --- insulation removed --- all this to provide a clearer view of what's happening inside the wall cavity.
Make the appropriate repair and put everything back together.

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