6 Household Mold Hot Spots and How to Clean Them

Concerned about household mold? Check out these six common mold hot spots and learn how to deal with each unique situation.

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According to the 2015 documentary MOLDY, half of all U.S. homes have active mold growth, and 28 percent of us are genetically susceptible to mold-related health issues. That’s why you should have a zero-tolerance policy on mold inside your home.

The first step toward keeping mold at bay is to inspect the most common household mold hot spots, which vary by climate. Regardless, you need to kill existing mold and eliminate excess moisture — ultimately, the cause of most mold problems. Though bleach has been used to kill mold for decades, it gives off harsh fumes and doesn’t work as well on porous surfaces. Instead, use a non-toxic fungicide approved by the Environmental Protection Agency (EPA) to attack mold in the following hot spots.

Bathrooms

Cause

Lots of moisture and insufficient ventilation are the common causes of mold problems in bathrooms, especially showers stalls and tub surrounds. Besides nooks and crannies that stay wet, look for mold growth underneath or behind toilet tanks that sweat during warm weather. Most bathroom exhaust fans are too small to ventilate the room properly.

Solution

Choose a fan that moves 10 to 12 times the air volume of the entire bathroom per hour, and leave that fan running for at least 20 minutes after each shower. Kill mold with a registered fungicide, then clean off mold stains with an oxygen-based mold stain remover.

Window Frames

Cause

This is mostly a problem in cold climates, where windows develop condensation when temperatures plummet — usually because indoor humidity levels are too high.

Solution

Boost ventilation. Kill the mold with a registered fungicide. If mold staining is present, attack it with an oxygen-based mold stain remover, an old toothbrush and rags. To prevent future mold outbreaks, increase indoor ventilation so window condensation never runs down the glass, even during the coldest weather.

Behind Furniture

Cause

Mold can grow on walls behind furniture and drapes in hot and cold climates. Although the dynamic behind the growth is different, the root cause is the same: too much moisture and too little ventilation. In hot climates, the relative humidity of air can be high everywhere, but is often higher where the air is stagnant, such as behind furniture, drapes and beds. In cold climates, condensation sometimes forms on the the walls behind furniture for the same reason — stagnant air. Soon enough, these tiny droplets of water kickstart surface mold that spreads.

Solution

In hot and cold regions, it’s important to use an EPA-registered fungicide to kill mold and mold spores below the surface on porous materials like drywall, drapes and furniture. Moving furniture away from walls and opening drapes can help, but that’s not enough. You also need to lower indoor humidity using air conditioning in hot weather, or boost household ventilation with a heat recovery ventilator (HRV) in winter.

Basements

Cause

This is the biggest indoor mold hot spot of all because basements are large, they attract excess water and they’re prone to condensation, especially during hot weather.

Solution

Besides killing visible mold where you see it, ventilation during warm weather worsens mold growth because the outdoor air itself is the source of damaging moisture. Rule of thumb: Open basement windows only when outdoor temperatures are cooler than indoor temperatures. Keep basement windows shut when it’s warmer outside than in the basement and use a dehumidifier or air conditioner to reduce basement humidity. Relative humidity of 65 percent is as high as your basement should get. Discard moldy items that can be removed (furniture, carpet, drapes), treating other mold outbreaks with a fungicide and oxygen-based stain remover.

Rim Joists

Cause

This part of a basement needs special attention because it’s hidden. Rim joists are the visible wood around the perimeter of unfinished basements, where the wood frame of the ceiling meets the outside walls. Rim joists are usually insulated with fiberglass batts, with a plastic vapor barrier over the top. The problem is, it’s impossible to seal the rim joist area properly this way. That leads to condensation within the fiberglass.

Solution

Put on a dust mask and gloves, pull out the insulation and look for gray or black staining. Remove the batts, bag them and discard them with your regular trash. Then treat the area with a fungicide. Next, apply at least three inches of closed-cell spray foam against the rim joists to seal and insulate them.

Ventilation Systems

Cause

Air conditioning and ventilation systems can grow harmful molds internally. This is a big problem because the air movement will spread mold and its spores quickly throughout your whole house.

Solution

An airborne test kit is the best way to discover if your home has this problem. Take air samples near vents. If you do have a mold-infested ventilation system, you’ll need to clean the inside of the air conditioner and any ducts you can reach with a registered fungicide.

Professional duct cleaning will take care of the rest if the technicians do a good job. Fumigation of ducts with a non-toxic fungicide can kill mold throughout a duct system, but fumigation is only appropriate after duct dust has been removed. Always maintain system filters to reduce dust buildup within ducts. With sufficient moisture, dust is a food source for mold.

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Steve Maxwell
Steve Maxwell is an award-winning content creator who has published more than 5,000 articles, shot countless photos and produced video since 1988. Using his experience as a carpenter, builder, stone mason and cabinetmaker, he has created content for Mother Earth News, Reader's Digest, Family Handyman, Cottage Life, Canadian Contractor, Canadian Home Workshop, and many more. Steve lives on Manitoulin Island, Canada with his wife and children in a stone house he built himself. His website gets 180,000+ views each month, his YouTube channel has 58,000+ subscribers and his weekly newsletter is received by 31,000 subscribers each Saturday morning.