Q: How can I kill mold in a house that has wood slats on the walls and is 100 years old? Why does the mold come back after I have applied a bleach solution?
A: “Killing” mold is a never-ending task unless you change the conditions that allow the fungus to thrive. Plenty of fresh spores are always floating in the air, ready to set up new colonies wherever they land on surfaces that are damp for long periods. The first step in dealing with a mold problem is always to figure out why it is occurring. Fix that, clean away the mold, and it should stay gone.
You don’t say whether the mold is showing up on wood indoors, or on the outside walls of your house. If the mold is on indoor paneling, is it a cold outside wall? Thin or missing insulation could be allowing condensation to form where warm, humid indoor air hits cold wood. Try moving furniture out from the wall a bit so there’s better air circulation to dry the wood.
Another possible solution is improving insulation. A home inspector or insulation contractor with a thermal imaging camera can scan the wall and pinpoint areas where insulation is thin or missing — or maybe you already know that there is no insulation, given the age of your house. A thermal scan can also help locate plumbing leaks, which are often the cause of mold in houses. And better ventilation of steamy rooms, such as the kitchen, laundry room or bathrooms, might help. Exhaust fans and dryer vents must send the moist air outside. That may seem obvious, but previous owners might have taken shortcuts, so it’s worth checking. An oversize air-conditioning system can also leave air too humid; have a heating and air conditioning company look at it.
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If the mold is growing on wood siding on the outside of your house, try to figure out ways to keep the walls from staying damp for long periods. Mold is especially likely to grow on walls in the shade. You can’t change the direction of the sun, but you can cut back trees, shrubs or vines growing close to the house; clean gutters; divert water from downspouts so it flows away from the walls; and make sure sprinklers don’t spray the house. Sometimes, especially on north-facing walls, mold appears in vertical sections with no mold or little mold where wall framing is hidden. This happens because the wall cavities between studs are insulated but the framing is not, and the wood transmits heat. Those strips of siding stay warmer and therefore drier, preventing mold.
Solving the underlying problem or problems is key, but if the mold is indoors, it’s also important to remove it. Mold spores, which make up most of what you see on surfaces, cause allergic reactions. Those can range from a stuffy nose and itching eyes to intense breathing problems, especially among people who have asthma or mold allergies. In people with chronic breathing problems, mold can cause lung infections. Because cleaning spores inevitably causes some of them to go airborne, and because there is no way to know which kind of mold will cause health problems for specific individuals, the Environmental Protection Agency recommends professional cleaning of any indoor mold that covers more than 10 square feet. If you do the cleanup yourself, wear protective goggles and gloves.
To remove mold, many people assume bleach is the best option. Chlorine bleach does remove the dark stains that mold can leave, so it can be useful on surfaces such as tile. It also kills mold spores, but that’s not as useful as it might seem. “In most cases, it is not possible or desirable to sterilize an area,” the EPA says in a section of its website devoted to helping people understand mold issues. “A background level of mold spores will remain — these spores will not grow if the moisture problem has been resolved.”
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Dead spores can still cause allergic reactions, though, and using bleach can trigger its own set of health problems if you inhale the fumes or get it in your eyes or on your skin. It can also damage your clothes. The EPA recommends scrubbing mold from hard surfaces with water and detergent, such as the kind you’d use for washing dishes by hand. Wood with an intact finish — paint or clear — can be cleaned the same way. But if the wood is bare, or has finish that has worn off in places, it’s a different story. Then you’re cleaning the wood fibers.
Indoors, the best solution for bare wood, is to scrub off the mildew as well as you can, using water and a little dishwashing soap. Wipe with clear water a few times, then let the wood dry. If the air is not humid and the wood stays dry, the mildew should not come back. If the stains come back, try painting the wood, which will make it easier to wipe clean in the future. For the primer, consider using Kilz mold-killing primer ($16.98 a gallon at Home Depot). Besides preparing the surface for paint and blocking stains, as many other primers do, it is registered with the EPA as effective at killing mold and mold spores on the wood. After the primer dries, paint with a water-based paint, possibly one labeled as resistant to mold.
Outside, you might want to follow advice from the U.S. Department of Agriculture’s Forest Products Laboratory about the care of cedar shakes and shingles used for wood siding. It notes that mold does not damage wood and feeds mostly on contaminants that settle out from the air. So outdoors, mold is mostly a cosmetic issue: It looks ugly. They recommend avoiding chlorine bleach, which can turn wood fuzzy. Cleaners that contain sodium hydroxide or potassium hydroxide are even worse. Instead, they suggest something that contains sodium percarbonate or another oxygen bleach, which will be gentler to wood fibers. But even then, they advise starting with a more diluted solution than the manufacturer recommends. If it works, continue to use that. If it isn’t cleaning well, increase the concentration up to the recommended amount. (Scotts outdoor cleaner, which has hydrogen peroxide as its active ingredient, is $12.97 a gallon at Home Depot.)
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If the mold doesn’t come off with an oxygen bleach, the laboratory’s report says that it may have grown into the finish, which could be blocking the cleaner from reaching the mold. If that’s your situation, you might need to strip the finish and redo it. If you paint, the mold-killing primer is also suitable for exterior use. Most exterior paints are formulated to resist mold growth on the surface.
Have a problem in your home? Send questions to localliving@washpost.com. Put “How To” in the subject line, tell us where you live and try to include a photo.
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