Causes of mold

Molds play an important role outdoors, breaking down dead leaves and other organic material. The spores that molds use to reproduce are microscopic and can float in the air. This allows them to get inside buildings in a variety of ways. In fact, mold spores are all around us, but what causes mold to grow from those spores?

Common causes of mold indoors

Mold takes hold in humid or moist conditions. Any circumstances that increase the humidity levels in a home also increase the risk of mold problems. Some common causes of indoor mold are listed below.

causes of mold

Once within a home or other building, mold spores can grow and cause problems inside buildings under the right conditions. Once there are visible signs of mold in a house it’s too late to prevent it and time to focus on mold remediation.


Flooding causes water to permeate materials in and around homes. It can take weeks for these areas to dry out again. During this extended period of dampness, mold can take hold and thrive.

Humid rooms lacking proper ventilation

Controlling moisture in a home is critical. Poorly ventilated areas can harbor moist air that does not circulate. This excess moisture puts the space at risk for mold growth.

Did you know? Air purifiers and air scrubbers can both remove airborne mold. But, only air scrubbers are up to the task of taking on mold remediation.

Improperly sealed windows or doors

Homes in humid regions need to be sealed properly. If this is not the case, humidity from outdoors can seep indoors, creating a favorable habitat for mold.

Lack of proper foundation drainage

Yards should be landscaped in such a way that water flows away from the home. Sometimes this is not the case, and water can collect around the home’s foundation. When this happens, mold growth can occur in the moist environment.

Leaking plumbing/pipes

Pipes exist inside the walls of our homes, in spaces are dark and hidden from our view. Water leaks inside walls create optimal conditions for mold to grow. If you live in an area that experiences temperatures below the freezing point, you need to be careful about your pipes freezing.

If the pipes are not properly insulated, or if they are near outside walls or areas that are not heated, this could happen. Water expands when it freezes, which can cause pipes to burst. That leads to water leaks, which can lead to mold growth.

Roof leaks

A roof keeps a home protected from the elements. When there is a leak in the roof, water can seep in. If the water is not removed quickly, the resulting dampness can lead to a mold problem. If you see evidence of a leaky roof, act quickly to prevent mold damage.

dampness and water from leaky plumbing
Dampness and water seeping through from a leaky roof or pipe.

Water seepage through basement walls

Basement walls should be clean and dry. If you find they are damp or wet, it’s important to find the cause and stop water from seeping through. Basement walls are made of concrete, which is a strong building material. It can, however, crack for a number of reasons.

After your foundation was created, your home was built on top of it. That weight put pressure on the foundation and the soil beneath it. Seasonal changes bring freezing and thawing, which can lead to shifts in the soil beneath and around a foundation, especially if that soil is damp.

Tip: Once a mold has been found, mold containment (sealing off the affected area) can prevent airborne mold spores and mold on clothing or equipment from making its way into other areas of a home.

Condensation from an air conditioner

Air conditioning systems can provide the essential ingredients for mold growth. In the course of normal use, condensation occurs, creating the moisture mold needs. Optimum temperatures (between 68 and 86 degrees Fahrenheit) are common in and around an AC system. Finally, continuously deposited atmospheric dust deposited in the system provides a food source for mold.

Sump pump issues

Many homeowners have a sump pump in the basement. If groundwater starts to accumulate, a sensor on the device will trigger, and the pump will quickly drain the water, keeping it away from the foundation and any items stored in the area. It’s a great way to protect your home, but it’s important to check often to make sure it’s working properly.

If your home is in a rainy area, the pump has to work hard and may malfunction at some point. If you don’t realize this has happened, water could collect, creating a potential habitat for mold.

Failed water heater

Water heaters, by definition, heat water. A leak in a water heater will provide mold the moist environment mold seeks. An old water heater is likely to leak near the end of its life. An indoor water heater being used outdoors where it is exposed to harsh conditions could leak.

A water heater in a system with high water pressure can develop leaks. A clogged drain on your water heater could cause mold issues. Finally, a water heater in a closet with a lack of air circulation can lead to a rise in humidity, leading to mold growth.

Malfunctioning refrigerator or ice maker

Refrigerators and ice makers can develop water leaks, leading to a mold problem. Frost-free refrigerators with a freezer on top sometimes develop blocked drains due to food debris or old ice buildup.

Refrigerators that dispense water or ice have a plastic water tank that can crack or leak with age. These appliances also have a water supply line and a water inlet valve that can leak. All of these issues can cause water to pool underneath, behind, beside, or in the bottom of your refrigerator, providing the moisture mold spores need to grow.

Conditions required for mold to grow indoors

  1. Darkness – Mold, unlike plants, does not need sunlight to grow. Sunlight can actually harm some types of mold, so many types prefer darkness.
  2. Food Source – Mold can grow on almost any organic substance present in buildings, including: cardboard, carpet, ceiling tiles, cloth, drywall, insulation, paper, tile, wallpaper, and wood.
  3. Moisture Source – Molds thrive in moist conditions. It’s important to keep indoor areas dry for this reason. If your home has water damage, it’s imperative to dry the areas within two days to prevent mold growth.
  4. Mold Spores – Molds reproduce by releasing spores into the air. These are similar to plant seeds, but these tiny spores can be transported for large distances in the air.
  5. Oxygen – Like us, molds need oxygen to survive. It needs very little oxygen, though, so you’d need to eliminate all oxygen in a space to keep mold from growing there.
  6. Time – When ideal conditions are present, mold can begin growing in just a day after coming in contact with a source of moisture. Within two weeks, mold spores will be created and start to spread. Within three weeks, you would be able to spot mold visually.
  7. Warmth – Mold can grow in both warm and cool conditions, but there are limits. Temperatures below freezing and above one hundred degrees Fahrenheit will cause mold spores to die.