Indoor Mold

Mold and mildew, lovers of water, will grow almost anywhere where damp or humid environments are present. Once established, mold releases spores into the environment in an attempt to spread to new locations. Some molds may be more toxic than others, but all of them are hazardous to human health, especially in large quantities. Their toxic spores cause and worsen asthma, allergies, and numerous other ailments. If you suspect a mold problem, it needs to be dealt with!

Ceilings in poorly ventilated bathrooms will accumulate the black smudges of mold growth. Likewise, damp windowsills will rot and sport discolored black spots. These are just some of the obvious visible locations mildew can occur. Mold can also lurk unseen behind walls, under carpets, up in attics, and down in crawlspaces.

Even if you can’t see mold there may be signs of its presence. Water damage and damp spots are prime suspects for harboring mold. A musty smell is another give-away that mildew lurks somewhere nearby. Sometimes, the water has long since dried up, though often there is visible damage, but the dead mold will continue to contaminate the surrounding air when disturbed.

To eliminate mold from an area, the first and most important step is to remove its water source. If the water remains, the mold will return no matter what you do. Leaks and condensation are common causes for consistently damp spots. Leaks needs to be repaired. Condensation can be remedied by improving ventilation and/or using a dehumidifier. Once the source of moisture is removed, the moldy spot should be scrubbed clean and optionally disinfected with something, bleach for example.

If you suspect mold, but can’t find it, you should hire a professional mold inspector. Home mold test kits are not always reliable and won’t help much in finding the source of mold. For more in-depth discussions on locating and eliminating mold problems, please see the EPA guide to household mold and

Common locations for mold and mildew include:


Steam and water from showers and baths create an ideal habitat for mold. Good ventilation and drying the shower/bath after use help prevent moisture buildup. Always run your bathroom fan during and after bathing. You can check to see if your exhaust fan has adequate airflow by holding a piece of toilet paper up to it and releasing it. If it has good suction, the toilet paper will stay in place, bad suction and it will fall down. A poorly operating fan should be cleaned, repaired, and/or replaced as needed.

Mold is most often found on shower curtains, ceilings, and under the floors if carpeted or wood based. Flooring in bathrooms should always be tile, vinyl, or another water resistant substance. Even a bathroom with no shower/bathtub will occasionally experience an overflowing toilet!

Carpet should never be used in places that regularly get wet, especially bathrooms. Special care must be taken when using wood, laminate or other wood-based floors in bathrooms. These floors must be well sealed to prevent water from ever getting underneath their protective top coatings.


The unavoidable fact is that water flows down. If there’s a leak anywhere in a house, chances are the water will find its way into the basement. Rainwater that isn’t adequately routed away from the house will also seep down into the cellar. Even well-designed rain drainage can fail in the onslaught of a major storm. Despite all of these more significant sources of water, there’s the additional fact that soil retains moisture and concrete is semi-permeable. Sealing in the area with drywall and water susceptible flooring like carpet, wood, or laminate is a recipe for mold growth. There’s a reason so many basements smell musty!

Fortunately, provided nobody lives there, the fact that finished basements will experience some amount of mold growth is generally not a huge deal.

Window Condensation

A bane of moist climates, when the air inside a house is humid and the temperature outside is cold, condensation will form on the windows. This in turn leads to water dripping down the glass, building up on the window sills and surrounding frame. Not only does this result in mold, but visible water damage as well.

In the short term, if you notice water build up on and around the windows, wipe it up with a towel. To permanently solve the problem, excess moisture will need to be removed and kept out of the house.

Activities that release steam should be vented near the source while ongoing. For example, run bathroom fans during and after bathing, run the kitchen exhaust fan while running the dishwasher or washing dishes, and make sure your clothes dryer is properly venting outside. Additionally, dehumidifiers can be used to draw extra moisture out of the air. In humid areas, houses with central air can benefit from a dehumidifier being added to the HVAC system.


Leaky pipes and roofs will drip water into the walls, attics, crawlspaces, and under floors. Small leaks often go undetected until enough deterioration from mold and rot has occurred that water damage is visible or a musty smell develops. Hidden leaks can be hard to find and are often hard to fix as well since they can reside in walls, ceilings, and under floors. Once discovered, extensive damage repair may need to be done to shore up structural degradation and clean up extensive mold growth.

A common source of plumbing failure is in the pipes underneath sinks. Use a water detector to be quickly alerted of these leaks. They’re like smoke alarms, but sit on the cabinet bottom under the sink plumbing. When one detects water, it blares the alarm. It’s also useful for rescuing items from under sinks before they’ve become too waterlogged.

Leaks can also occur in water heaters, refrigerator ice-making lines, and around the bottom of toilets. Any discovered leak should have its water source shut off and be repaired as soon as possible to prevent future mold growth and damage.

Air Conditioners

Humid air quickly condenses on the cold coils and grills inside air conditioners. Window and portable air conditioners are designed to release or store this excess water to prevent it leaking into the house. Window air conditioners typically shunt it outside. A leaky air conditioner will build up moisture on the window, causing damage and forming mildew.

Conversely, portable air conditioners generally store the collected water in a reservoir that periodically needs to be dumped. If these leak, the water will go onto the floor or whatever surface the appliance is resting on. Of course, sometimes the act of hauling off the filled water tank will result in spilled water, which should always be dried up immediately.

The duct work in central air systems can also generate water while air conditioned air moves through. The cold air can cause humid air to condense on the pipes which will then drip into the walls. It is also possible for an HVAC system to become contaminated with mold, if there’s a moisture problem near the air intake, for example. If you suspect central air of harboring mold, it should be turned off to prevent spreading mold spores throughout the house until it can be evaluated by a professional.

Washing machines

Mold can reside in washing machines, especially older model front loaders. If your laundry smells like mildew after it’s been washed, your washer contains mold. Not only is the smell unappealing, the “clean” clothes are a health hazard! All front loader washing machines, regardless of age, should have the rubber gasket drum ring, door, and detergent dispenser wiped down following wash cycles. If possible, remove the detergent dispenser and dump out the water, allowing it to air-dry. The dryer should then be left open until next used. Prevention of mold is best.

To remove mold from a washing machine, the gasket on front loaders should be thoroughly cleaned with bleach all around and underneath. The detergent tray should also be removed and washed. Next, the washing machine should be run through a cleaning cycle by adding a cup of bleach and setting it to run on its hottest, fullest, setting, or a clean cycle if it has one. If this doesn’t fix the problem, the drum can be removed to clean behind it or you may need to buy a new washer. To help prevent future mold buildup, don’t let wet clothes sit in the wash and always be sure to leave the washer door open between loads to release the trapped moisture.

Clothes dryers

Dryers that aren’t properly venting outside can contribute to mold growth. If the vent hose is poorly attached or leaks, it will vent warm moist air into the house. Alternatively, if the vent hose is kinked and/or clogged with lint, the lint in the tube will be regularly bathed in this warm moist air, a perfect breeding ground for mold. You may not think this is a huge deal, but the air that escapes the hose and makes its way into the house will be filled with mold spores!

To remedy a poorly functioning dryer, first check that the rear vent hosing is not kinked and is properly attached. To remove excess lint and check the airflow of your dryer, pull the front lint filter tray, and use a vacuum with a skinny flexible attachment (I use a clear piece of hosing that is inserted into the end), to vacuum out as much lint as you can from inside the dryer using the lint filter access slot. Replace the lint filter and turn on the dryer. Go outside to where the dryer vent cover is located to check the air flow. If your outside dryer cover has louvers, the air should be pushing them straight out.

Dish washers

Standing water in a dishwasher food trap

If water doesn’t completely drain from your dishwasher, it may be harboring mold. Make sure you check the food trap for standing water as well. Stagnant water and leftover food scraps make a welcoming home for mold and this is not something you want to be washing your dishes with!

First, check that the food trap isn’t clogged. If cleaning the trap doesn’t fix the issue, another cause for poor water drainage is an improperly installed dishwasher drainage line. This is a very common problem, unfortunately. The drainage hose, connected to your garbage disposal or sink waste pipe, should either be installed connected to an air gap or have a high loop. Failure to install the dishwasher drainage hose correctly leads to waste water backsliding into the dishwasher which can lead to mold buildup.

House plants

If you put your potted house plants straight onto windowsills, wood floors, carpet, or any other water permeable material, you will get mold and water damage. Even if you never spill water and the pot never overflows from over-watering, the area underneath pots stays moist. Pots should either be placed on water resistant surfaces like tile or, better yet, they should be raised up on a wire or metal platform to give adequate space for the bottom of the pot to air out. Soil that is kept constantly moist will also grow mold, so it’s best to let the top of the soil dry out before watering again.

Flood damage

Any time your house experiences flooding, either from a natural disaster or a major water leak, improper or delayed cleanup can result in mold and mildew. It takes just two days of standing water for mold to grow. Rampant mold-growth after flooding results in large quantities of toxic mold spores in the air, so it’s important to wear a mask while cleaning inside the house.

If there was standing water, there’s a good chance your flooring will need to be replaced, especially carpet which can never be completely cleaned of mold. The most important thing is to get everything dry as quickly as possible and replace/repair what can’t be salvaged.

It can be difficult to find and dry all lingering sources of moisture. Water gets behind walls, under floors, and into crawlspaces. It’s generally best to hire a professional cleanup crew and a mold inspector for large-scale flood damage. For more information on recovering from flooding, check out the EPA guide.

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