How do we stop basement walls from sweating/molding?

Rhonda Craft
by Rhonda Craft

We have a walk out basement. The walls that are exposed outside are painted concrete. Inside is also painted but even with a dehumidifier, they sweat and mildew, especially inside the cabinets. What can be done OUTSIDE to stop this from happening?

  6 answers
  • 2dogal 2dogal on Jan 19, 2019

    Redirect the water from the foundation. Gutters. Slope the ground away from the foundation. Paint the walls with a waterproof mastic and caulk every little crack.

  • Seth Seth on Jan 19, 2019


    This could be happening for several reason, but we need more info to know which direction to point you in. You did not mention if this is a finished space with walls, flooring, and a ceiling or if it is conditioned (heated or cooled by your HVAC system) and if this is happening anywhere else in your basement. What is the condition of the paint and do you see any efflorescence? The paint may actually be part of the problem. How long has it been on the walls?

    • Rhonda Craft Rhonda Craft on Jan 19, 2019

      Mostly finished. Interior walls are reclaimed wood, floors are painted concrete. 1 ventless lp heater. 25 yrs ago, we painted 1 bedroom with a masonry paint. The wall with the window has mildew. The other concrete walls are fine where they have either dirt or are protected by the new sun porch. Outside has no drainage issues.

  • Lifestyles Homes Lifestyles Homes on Jan 19, 2019

    Yes, make sure all the downspouts extend at least 4’ from the house, that the dirt on all sides of the house slopes at least 4% in that 4’ zone.

    Water will wick from 50’ feet away to find the lowest spot, which is your basement walls’ foundations.

    Buy a good dehumidifier & get a drain hose for it and run it 24/7. Mold will contaminate the entire house and is dangerous.

  • Oliva Oliva on Jan 19, 2019

    Lower your dehumidifier's reading to 35%, letting it run, continuously. If the problem continues (until you've redirected outside water at least 10' from your foundation), lower the dehumidifier to 30%.

    Keep the dehumidifier closer to the affected walls, adding a hose extension to your dehumidifier hose, if needed, and letting the water drain into your floor drain.

    I'd further avoid drying clothes on a line, or leaving water in your laundry tub.

    If your soil is heavy clay, you'll need to dig up the ground near the affected walls and add gypsum (west of the Mississippi) or glauconated greensand (eastern half of U.S.), in addition to perlite and sphagnum peat.

    If you're lower than your neighbors, you'll need a swale, French drain, or Hydro Blox system. If rains are excessive, you may need wider downspouts.

    Your gutters should be clear of obstructions, year round. Install Gutter Guard on gutters, if you don't want to climb a ladder twice a year to check gutters.

  • Karen Karen on Jan 19, 2019

    In addition to all the great advice given above you could also use a couple of fans in your basement. My mother did this and set them on low and never turning them off. No mold or mildew. In addition, you can kill the mold but what about the spores. Some time ago I read about the spores but do not recall how to do this. Please google for this information. Good luck.

  • Seth Seth on Jan 19, 2019


    The problem may be your heater. Ventless heaters create a large amount of water vapor as the byproduct of the LP gas combustion. It will produce more in colder weather, especially if you are running it more. If the heater is oversized for the space, the issue will be worse. The tighter the room, the more water vapor is being retained. The temperature difference between the outer walkout wall and the interior of that wall will result in the sweating you are seeing. The warm, moist air is providing the perfect conditions for mold growth. Mold will grow on any organic substance like the wood in the cabinets or on your walls. You might want to contact an HVAC person regarding the heater. In the meantime follow the suggestion of others as to providing air flow and dehumidification. Below is a picture of the set up I use in my basement. I do not have a floor drain, so I have my dehumidifier drain into a Little Giant condensate pump and it pumps the water from the dehumidifier up along the ceiling to my washer drain through clear 3/8 inch PVC tubing.