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Mold in the basement

I am finishing a basement project. The basement is dry carpeted living space. Ripped out the old sheet rock walls, insulation and just finished the second coat of Baer Concrete and Masonry Waterproof Searler on the cinder block wall. The sheet rock was installed in essentially three sections. The knee wall about three feet high had sheet rock supported by ferring strips on the cinder block. The framing  continued horizontally along an 8" wide wood shelf on the top of the concrete block also coverd by sheet rock and then continued up the studs along the upper half of the wall to the ceiling.


There was mold on the underside of the wood that was used to frame the shelf of the lower wall. How do I prevent the mold that is a result of the moisture coming, not through the cinder block, but up through the open caveties of the block? Can I put a vapor barrier along the top of the wall? Can I fill the top with foam of some kind?


Please advise... thanks

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Posted 2011-03-14T14:34:56+0000  by Rob13 Rob13

Hello Rob13 and welcome to the community.


In order to be able to give you a real and sensible answer i would need to know a little bit more about the elevation that is on the outside of the knee wall.


In other words does  the dirt level on the outside of the home goes over the point where the  bottom plate meets the cinderblock knee wall?

Is there a foam sill gasket under the bottom plate?

Can you post a picture of the wall?


Posted 2011-03-14T14:56:38+0000  by George_HD_CHI

Thank you for the quick reply.  Please define the bottom plate. The dirt level on the outside of the home does go over the point where the wood frame meets the cinderblock knee wall. There does not appear to be a foam sill gasket at that point. The house was built in 1962. The cinder block foundation goes down about 2' below the concrete floor. I stuck a stick down through a cavetti in the cinderblock wall and there is about 1" of water at the bottom of the wall (2' below the concrete floor)

Posted 2011-03-14T15:10:59+0000  by Rob13


Bottom plate is the horizontal piece of lumber that is pinned down with vertical framing members and rests on the top of the cinder block wall.


The dirt level on the outside of the home does go over the point where the wood frame meets the cinderblock knee wall.



That’s where the problem is; water accumulates and penetrates in between bottom plate and the top of the cinder block filling the cavities of the block.

With a change of temperature ( conditioned indoor space in oppose to outdoor soil  temperature)  water  evaporates to a certain extent creating mold on the underside of the bottom plate and the 1x lumber  that is used on  the  horizontal 8" section of the block wall.


Placing vapor barrier will only work on a section 8" section being that is now too late to place one under the bottom plate section.


In order to address this issue you would need to bring the soil level below the point where the bottom plate meets the cinder block wall.

Posted 2011-03-14T15:28:04+0000  by George_HD_CHI

Additional question;


Is there a membrane on the outside of the cinder block wall?


There is a simple test you can perform to determine placement of water penetration.

 Bring the soil level below the bottom plate cinder block section and water the area with garden hose.

If the level on the inside of the wall rises than you could have a drain tile or membrane problem as well.


Posted 2011-03-14T15:46:51+0000  by George_HD_CHI

So the mold is caused by the moisture at the bottom of the cinder block wall evaporating.  This water is not ground water  coming up from underneath (as I assumed) but a collection of water coming through the exterior wall of the cinder block and entering from the top at the bottom plate where the cinder block meets the wood and the wall. 


We know there is a problem with the grade of the soil along the front of the house and side wall. The house is on a hill. The house next door is elevated above our property with a retaining wall between the homes. There is significant run off from their driveway between the homes. Our neighbor told us and his his neighbor (both on the down hill side from our house) both installed a catch basins and a storm drain that flows across both properties and into the sewer. He said that both their properties had lakes in their back yards from heavy rains.


With the slope of the property, it would be very hard to lower the soil line around the house. It was in fact built up to angle the slope away from the house.   


So interior...

Can I add a vapor barrier to the top horizontal edge of the cinder blocks to "cap" the moisture?


What to do to prevent additional moisture from getting into the cinder block wall?

1. lower the height of the soil to below the bottom plate

2. slope the grade of the soil away from the house

3. add a french drain and catch basin and tie into the neighbors storm drain


Can I add anything to the exterior below the soil line or at the point of the bottom plate?

We are planning to reside the house some point in the next 6 months.


Posted 2011-03-14T17:24:30+0000  by Rob13






Yes, mold it's caused by a high moisture levels that are present in the cinder block wall.


Once the water get's saturated around the foundation wall and reaches the bottom plate level it seeps in between bottom plate and top of the foundation wall.


Now, this may not be the only source of the water penetration, you could also have a leakage in the foundation wall itself but lowering the grade will definitely result in a significant improvement.


That should be your first step, bottom plate or sill plate should be at least 6" inches above the soil line and in addition it is suppose to be sloped away from that point.


Your home was most likely not build with bottom plate bellow grade, soil was probably added along the way attempting to redirect water from the home.


Soil line need to be sloped AT LEAST 6" within the first 10 feet around the perimeter of the house.


If grading soil it’s not an option there are still couple preventive methods that you can implement.


One of them is to try to control a flow of water from your neighbor’s property.

This can be done by building a swale in between your home in neighbor’s retaining wall.


In addition to swale you can also place perforated drain pipes around the perimeter of your foundation walls and direct the water  away from the house or to the above mentioned swale.

drain pipe.jpegswale.jpeg





Another preventive method is to extend downspouts and direct water away from the foundation line. You would be surprised how much water comes from the roof line.




Adding the catch basin and sum pump will also help but only if the extracted water is directed away from the house.

In other words if you keep pumping water on the improperly graded soil significant percentage of the same water will flow back in to the house.

Same applies to rain water as well.


To simplify you need to find a way to direct rain water away from your foundation line to minimize the penetration with either building a swale, lowering the grade or all of the above.


There are many products and many more methods that are advertised when it comes to adding on the exterior of the foundation, but honestly the only ones I would recommend would be to build a new stem wall with membrane and drain or to  lower the grade like mentioned.


If you find this to be a solution to your question please click on the "Accepted solution"



Posted 2011-03-14T19:16:00+0000  by George_HD_CHI
Hi Rob13,

Once you get your water source re-directed, allow a week for the moisture to evaporate and then go back and cover the entire area with a fine mist of Concrobium Mold Control.

This product lays down a barrier that prevents mold from growing back.

Your Tool Rental Associate can show you a misting pod that creates a fine mist over the entire surface.

When the surfaces appear damp, stop and allow the coating to dry without any additional wiping or treatment.

Concrobium will prevent the mold from germinating.

Pat InPaint
Posted 2015-11-19T21:05:34+0000  by Pat_HD_ATL
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