What Causes Mold in a House?

Find out what causes mold in to grow in the home and how to handle the issue.

Mold on a wall in a house

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Most people have seen some type of mold throughout their lives because mold can appear in a variety of places, like food that has gone bad, growing across wet stones, or showing up in the corners of a damp cellar. While you can toss rotten food and walk around moldy growths outdoors, when you find mold growing inside your home, the problem isn't as easy to deal with. You will need to be able to identify what is causing the mold to grow and spread in order to take steps to resolve the issue.

Mold is a form of fungi. It thrives in damp, dark environments where there is organic matter, like wood, to feast on. Common areas around the home to find mold include bathrooms, kitchens, basements, attics, doors, and windows. Use this guide to find out what causes mold to grow in a house and how to eliminate it.

Insufficient Ventilation

One of the main reasons mold has an opportunity to grow is because areas of the home that are prone to high levels of moisture, like the bathroom or kitchen, are not properly ventilated. Poor ventilation keeps the excess moisture trapped in one room where it settles onto the ceiling, walls, floors, appliances, counters, sinks, and any other surfaces. Airborne moisture can even seep through narrow cracks and gaps, where it accumulates in dark, dank spaces, creating the ideal environment for mold to grow and spread.

Turning on a fan, opening a door, or opening a window are all simple ways to increase the ventilation in the home. If you have just finished taking a shower, turn on the bathroom fan and open the door to allow the moisture to clear. Similarly, if you are washing dishes or boiling water, turn on the fan above the stove or open a window to increase the airflow throughout the space. For areas that may be difficult to ventilate, like a basement, consider investing in a dehumidifier to help pull the moisture out of the air.

Persistent Humidity

Mold and mildew need a damp or moist area to grow and thrive. When the home experiences a high level of ongoing humidity, the moisture in the air can seep into the walls, floors, ceilings, and furniture, creating a prime area for mold to develop. Persistent humidity can be a result of poor ventilation in areas of the home where moisture is common, like the kitchen or bathroom. However, it can also be caused by humid weather conditions outdoors, continuous rainy weather, or even as a result of living near a large body of water.

Due to the high potential for humidity problems coming from the surrounding environment, you will need to take steps to reduce the humidity level in your home, keeping the relative humidity below 55 percent. Proper ventilation with a fan or by opening the windows can help reduce humidity levels throughout the home, but if the main source of humidity is coming from outdoors, then the best option you have is to invest in a dehumidifier to capture the moisture in the air.

Leaking Pipes

Water damage is often caused by leaks behind the drywall or under the sink because the leak can go largely unnoticed for a long period of time. Once you notice the leak, you will still need to repair it, unless plumbing falls outside of your skill set. At which point, you will have to contact a plumber and wait for your scheduled appointment. This damp, dark area is perfect for mold to grow and spread, especially if the water damage is extensive.

Once the mold begins to spread through the walls and into the insulation, the cleanup and elimination of the mold significantly increases in time, difficulty, and cost. This is because you need to get rid of all of the mold and any affected material that cannot be adequately sanitized, such as insulation, drywall, particleboard, and even floor boards. In most cases, widespread water damage and mold growth will require gutting the affected area, cleaning and sanitizing any remaining materials, then renovating the space to replace the damaged or destroyed walls, flooring, ceiling, and any furniture or appliances.


Another factor than can lead to mold growth in your home is heavy condensation. Condensation occurs when warm air comes into contact with a cold surface, like an exposed cold water pipe. The water vapor in the air rapidly cools upon contact with the cold surface, condensing into water droplets. This process can be seen by observing a glass of ice water on a hot summer day. The extreme difference between the temperature of the air and the temperature of the glass quickly leads to water forming on the exterior of the glass.

Low levels of condensation are easy enough to wipe away, but heavy condensation can result in enough water that many people actually believe the condensation is an active leak from a water line. This level of moisture can quickly cause water damage to drywall, flooring, and appliances, leading to the growth and propagation of mold colonies. Ventilation and a dehumidifier can help to reduce condensation by removing excess water from the air, but it's also recommended to insulate any objects or surfaces that tend to be significantly colder than the ambient air temperature, such as cold water pipes.

Home Flooding

When a flood occurs in the home, there are many different concerns you will need to address, including the elimination of widespread mold growth. Flooding can affect the floors, walls, furniture, appliances, and even the home infrastructure, resulting in significant and widespread water damage. While you attempt to remove the excess water, pull out any affected items, and start the arduous process of ripping up your damaged flooring or taking out water damaged drywall, mold has the opportunity to spread into deep, dank pockets of the home.

From this point, the mold can move through the walls and up into new areas of the home if it is left untreated. For this reason, it's important to remove all damaged items and infrastructure from the flooded space. Use a shop vac and towels to clean up excess water, then set up fans and a dehumidifier to help dry the area. Once the area is dry, use soap and hot water to scrub all surfaces to remove any mold growth. Treat the area with a mold spray or a disinfecting solution to ensure that any remaining mold spores are killed before they can spread.

Roof Leaks

While floods mostly affect the basement or the cellar, leaks in the roof seep into the attic. The moisture soaks into the insulation, walls, ceiling boards, and may even cause water damage in the main area of the home if the issue is not resolved. This excess moisture provides an ideal place for mold spores to grow and spread into vast colonies that are difficult to get rid of unless you dispose of any affected insulation and items.

Once mold has taken hold in the attic, you need to take similar steps as you would after a flood to clean and dry the space. Scrub the area with soap and hot water, then spray any affected surfaces with a disinfecting solution to kill any remaining mold spores. It isn't enough to simply dry out the attic insulation. You will need to remove and replace any insulation that shows signs of mold growth to ensure that this doesn't become a recurring problem. It's also necessary to make repairs to the roof to prevent future leaks.

Damp Basements

Basements and cellars often have high levels of moisture and humidity due to their location underground. Any appliances that use water, like a washing machine, also increase the moisture in the air, but because these spaces are difficult to ventilate, the airflow through a basement is limited. The damp, dark space quickly becomes a great location for mold to form, grow, and spread.

To get rid of mold in a damp basement, start by cleaning any visible mold with hot water and soap. Follow up this initial cleaning by spraying the affected areas with disinfectant to kill any lingering mold spores. After treating the immediate problem, you will need to take preventative measures to stop the mold from returning. Invest in fans to ventilate the space and set up a dehumidifier to help draw excess moisture out of the air. If there are any specific areas that are prone to mold growth, ensure that they remain uncovered for the maximum benefit.

Mold Remediation

Finding the source of the mold, like a leaky pipe or condensation buildup, is the first step to resolving the problem. After determining the cause of the issue, you need to assess the extent of the mold growth. In extreme cases, the mold infestation may be too much for the average DIYer to handle, so you may need to call in a professional contractor to deal with the situation.

If you have inspected the affected area and determined that you can tackle the problem on your own, the next step in the process is to remove and discard any contaminated materials, then dry the area by setting up fans and dehumidifiers. Removing the moisture is key to preventing mold growth and propagation.

Finally, you need to clean and disinfect the mold areas. Wash affected surfaces with hot, soapy water, then use a mold treatment spray or a disinfecting spray to kill any remaining mold spores. Make sure that when you are cleaning mold and mildew, you wear protective clothing that can be discarded, a mask, gloves, and goggles that fit tight around the eyes.