Mold, Moisture, and Your Basement

On a regular basis I find moisture staining and mold in basements that was caused by either condensation or foundation wall seepage.  Many basements in the summer months have a heavy mustiness that is caused by increased humidity and mold gas. There are simple steps every homeowner can take to prevent mold growth and compromised indoor air quality.

One of the first steps when it comes to preventing seepage is with the grading at the exterior and roof runoff control.  So the first item I usually look at when I do a foundation inspection is to check the perimeter of the home for positive slope away from the foundation wall.  It doesn’t have to slope much, an inch or two in 5-6 feet, but it must not slope back toward the home, this promotes pooling and seepage.  The funny thing about water is that if you let it sit against the foundation wall it will find a way inside.  

Have you ever gone into an unfinished basement and maybe saw a crack in the middle of the wall but all of the seepage stains were in the corner?  It is almost guaranteed that the downspout doesn’t have a leader directing water away from the home or that the leader is too short.  There’s nothing like collecting all of the water from the roof and depositing it in one place.  A person may be better off with no gutters in this situation.  So if you have seepage issues in your basement, it is best to take a walk around your home and check your gutters, downspouts, and leaders.  Spring is also a great time to seal any foundation wall cracking that may have occurred in a parge coating at the exterior.  And don’t forget to keep those gutters free from debris.


When you walk down the basement stairs do you notice a difference in humidity levels, maybe even smell a mustiness in the air?  Humidity levels that exceed 60% are not healthy.  This can also lead to mold growth on organic materials such as wood, drywall, paint, and other common building materials.  It is of the utmost importance to control basement humidity with a dehumidifier.  I actually run (2) 50 pint units in my own basement.  It is also recommended to try to keep the relative humidity at 50% or lower.

As we walk around the basement it is generally easy to spot mold like substances, often they show up at the base of the wall material.  Wood, paneling, drywall and other materials should not come into contact with the concrete floor.  They will wick moisture from the concrete through capillary action.  I have seen instances where drywall has wicked moisture more than 3 feet up from the floor.  Guess what was all over that wall surface?  We had to remove the drywall and re-install that section.  A gap at the base of the wall is necessary to create a barrier to prevent the moisture transfer from the concrete into our wall.

Another mistake I see people regularly make is storing possessions in cardboard boxes in their basement.  If you store cardboard on the floor or even on a shelf but allow it to contact a concrete wall, it will likely grow mold.  Plastic totes or other non-organic materials are a much better option.  I was recently in a home that was very damp and there were old board games and other cardboard boxes stored on the damp floor, extensive mold was present, in this basement a mask was necessary, no, not to prevent the transfer of Covid-19 but to prevent possible illness and other side effects from the mold gas.


In most cases when we find these damaged areas a homeowner can perform their own cleanup, removing damaged materials and drying the area quickly is the best defense in eliminating or preventing ongoing mold growth.  Keep in mind if you don’t repair the cause of the moisture, it will return in short order so your first step after cleanup of the damaged materials should be to repair the seepage or other causes of moisture.  Condensation forming on the wall or in the box sill can also cause mold growth.  Air sealing as well as better insulating in these areas will help prevent the formation of condensation.  The water supply line at either the pressure tank or water meter is also a common cause for condensation to form.  Insulating the water line for 6 to 8 feet will  likely reduce or eliminate the formation of condensation on the plumbing lines and will aid in keeping the relative humidity low. This is another common cause to elevated humidity levels. The pressure tank and water softener tanks are another common area to find mold growth due to condensation. Regular cleaning of these surfaces is recommended.

So as you can see even if you don’t have seepage in the basement it is possible and probably likely that mold may be present due to condensation or just elevated humidity levels.  Responsible storage practices can also prevent mold growth and protect indoor air quality. Regularly inspecting your basement area for signs of moisture would be a great addition to your regular maintenance program.

I hope you found some value in this post and as always, if you have a home related question, please submit them at:



Aaron M. Zuehlke is the owner and inspector at Zuehlke Inspection Service, LLC, a full-service home inspection company serving Southern Wisconsin. Specializing in Home inspection, Radon Testing, Mold Testing/inspection, Residential Thermal Imaging, and Manufactured Home Foundation Certifications. He also manages several rental properties through Zuehlke Properties, LLC. He can be reached by email at or by calling the office at 608-931-7485.

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