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Zillow Real Estate

Damp basement? How to prevent mold in your home

By Bob Vila

If your basement has a damp, musty odor, it is probably a breeding ground for mold. Mold spores are notorious for causing reactions such as wheezing, stuffy or runny noses, itchy and watery eyes, rashes or hives, and even asthma attacks. Because these invisible spores, or seeds, can catch convective drafts and begin new colonies elsewhere in the home, remediation and prevention should be your first order of business.

Get organized

If your basement is unfinished, begin by hauling out any junk and rubbish. Get rid of what you don’t need, especially organic materials such as old piles of newspaper, clothing, scrap wood and wood furniture. Put what you need to keep in plastic tubs that have tight-fitting lids.

Kill existing mold

Next give all surfaces a thorough scrubbing. Begin with a solution of TSP and water; this will remove the dirt that mold often dines upon. Then spray all surfaces with a 50-50 vinegar solution to kill existing mold. A garden sprayer will help speed up the process.

Allow the vinegar solution to stay on surfaces for 5 or 10 minutes to give it time to penetrate all of the mold. Rinse and reapply occasionally to deter mold from returning. Avoid using bleach (its active ingredient doesn’t reach the roots of the mold on porous surfaces) or ammonia (the cure may be worse than the illness). Eco-friendly commercial mold cleaners, without bleach, ammonia or volatile organic compounds (VOCs) are available as well.

Seal surfaces

Once you’ve cleaned the interior foundation and slab surfaces, it’s a good time to seal all surfaces with paint. Begin with a primer formulated for masonry. Then apply a semi-gloss latex top coat to the walls, and floor enamel or epoxy to the slab. Paint helps seal out water vapor and makes the surface easier to clean in the future.

If you live over a crawl space, you can do the same: Empty out the space as much as possible and clean it. If the floor is dirty, seal it with a heavy, tear-resistant vinyl liner. Consider encapsulating and insulating the entire crawl space by running a vapor barrier up the walls as well as across the floor.

Control water & moisture

All of the above will be futile if you do not take steps to control excess moisture in your basement and around your home. Mold needs about 70-percent relative humidity to be active and produce spores. Reducing moisture begins with preventing water infiltration. If, for example, puddles form along basement walls after a heavy rain, examine your gutter and downspouts for clogs and disconnects. Install splash blocks and downspout extenders, if necessary. Take a level to concrete walkways that run along the perimeter of your home. If they have settled so that they now slope toward your foundation instead of away from it, call a contractor to make repairs. Slab-jacking companies can use polyurethane-injection methods to lift concrete structures, including patios and pool decks. If your walkways are built with pavers or bricks, regrading and rebuilding the path may be necessary.

Relative humidity (RH) rises above 65 percent in basements during the summer in much of the United States and Canada. If this is your situation, run a dehumidifier and set the relative humidity at 65 percent or less. Choose a model that is adequately sized for your space — small room-size dehumidifiers will not survive in a basement. Ideally your dehumidifier should include a high-performance air filter. Find a way to set up the dehumidifier to drain automatically, so you don’t have to remember to empty the reservoir every day or two. You can accomplish this in several ways. For example, you can run a gravity drain to a sump basin or install an add-on or built-in pump to drain into a utility sink. A unit with continuous fan operation, which runs even when not dehumidifying, helps distribute heat evenly around the basement and lessens the chance that condensation will form on cool surfaces.

Other ways to reduce mold

  1. Fix plumbing leaks immediately, even the minor ones.
  2. Seal crawl-space vents, install weather stripping around basement windows and doors and air-seal gaps and cracks at the sill plate and rim joists. Without these precautions, humid summer air can infiltrate and cause the relative humidity in your crawl space and home to spike.
  3. Install and use exhaust vents in the kitchen, bathrooms and clothes dryer.
  4. Insulate pipes, ducts and other cool surfaces upon which water vapor can condense.
  5. Monitor areas that are likely to produce mold, such as closets, the insides of cabinets near sinks, the floor around toilet bases, areas under appliances such as washing machines and dishwashers, and areas around plumbing pipes.
  6. Keep walls, floors and ceilings clean. Wiping surfaces with a solution of vinegar is a safe and effective mold deterrent.
  7. Clean whole-house humidifiers regularly and run them only when necessary.
  8. Seal cracks in basement slabs and fit your sump pit with a lid.
  9. Have your air-conditioning equipment serviced regularly. An evaporative coil, which is often damp, can become a source of mold if your AC is not maintained properly.


Bob Vila is the home improvement expert widely known as host of TV’s This Old House, Bob Vila’s Home Again, and Bob Vila. Today, Bob continues his mission to help people upgrade their homes and improve their lives with advice online at His video-rich site offers a full range of fresh, authoritative content – practical tips, inspirational ideas, and more than 1,000 videos from Bob Vila television.

Note: The views and opinions expressed in this article are those of the author and do not necessarily reflect the opinion or position of Zillow.

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