Our indoor environments play an enormous role in our ongoing health, even if we don’t always factor them into the wellness equation. If they’re filled with contaminants (like mold!) and poor indoor air quality, that can cause a long list of chronic symptoms and illnesses. Knowing key information, like what causes mold in homes, can help you protect your body from unwanted exposures.

Indoor mold growth can also wreak havoc on the structural integrity of your home by damaging walls and ceilings. Not to mention, it can ruin personal belongings as well. The money spent on remediation and repairs can be hefty, not only hurting your wallet but also causing a serious amount of stress.

That’s why knowledge is key! If you know what causes mold in homes, you can take proactive steps to prevent it. Preventing indoor mold growth not only helps avoid health issues caused by exposure but also saves you money. It’s the ultimate win!

Here are the top 12 culprits for what causes mold in homes to help you identify potential problems and get started on solutions to resolve them.

What Causes Mold in Homes?

1. What Causes Mold in Homes?: High humidity

The ideal humidity level in a home should be between 35 and 50 percent. Too high or too low humidity indoors can cause not only wellness issues but also problems with the health of the home as well. 

When the humidity is too high, it can cause microbial growth, poor indoor air quality, and structural issues in the building. However, you also don’t want it to be too dry indoors! That can lead to adverse health reactions and structural issues like wood separation.

Ideal humidity should be maintained not only for comfort but also for a safe indoor environment that promotes ongoing wellness. Check out this post for tips on safely lowering or raising your indoor humidity levels.

Pro tip: Use a hygrometer in these rooms to monitor humidity levels in real time and address issues as they pop up.

2.  What Causes Mold in Homes?: Leaks

Leaks are a common issue that causes mold in homes. Not all leaks are massive events, though. Oftentimes, ones that remain undetected for some time are the biggest culprits for microbial growth.

That’s why it’s important to regularly check for current leaks or problems that could lead to issues in the future. Issues such as malfunctioning appliances or holes in the attic can allow for unwanted water intrusion.

Places to take a look at include:

  • Underneath sinks
  • Inside cabinets
  • Appliances
  • Attic
  • Basement
  • Around the toilet and shower
  • Hot water tanks

Pro Tip: Invest in waterproof under-sink mats to help prevent water damage from making its way deep into your cabinets and flooring and give you more time to fix the problem.

structural issues)

3.  What Causes Mold in Homes?: Structural issues

Problems such as cracks in the foundation can allow moisture to intrude during weather events. Not only can this be dangerous for the integrity of the building, but it also creates the perfect opportunity for microbial growth.

Ensuring the home's exterior is prepped to act as an appropriate barrier is a huge aspect of avoiding water intrusion and one of the problems that causes mold in homes. With that in mind, regularly take a close look around your home to determine if there are any cracks, gaps, damage, or other issues that could allow moisture inside. 

Places to check include:

  • Foundation 
  • Roof
  • Windowsills 
  • Door frames 
  • Siding 

If you find any problems, take care of them ASAP. The sooner you can catch and resolve the problem, the better. 

4.  What Causes Mold in Homes?: Lack of Cleaning

Not keeping up with cleaning is what causes mold in homes for a couple of reasons.

Thanks to modern building practices pushing for net-zero energy efficiency, there is not a lot of airflow between our indoor and outdoor environments. Since we can’t put a bubble around our homes, a majority of the particles that make their way inside will stay there until we remove them. The more spores that are in an environment, the higher the chance that one will stumble onto a habitable environment and begin to grow. Not to mention, the more microscopic particles there are in your home, the lower the indoor air quality will be.

On the other hand, a lack of cleaning also allows for a buildup of material that can be used as food for microbial growth. Mineral buildup in the showerhead or organic material in the coffee maker are a few examples. This buildup can then trap moisture as well, creating ideal conditions for a contamination situation to develop.

That’s why it’s important to set up a routine cleaning schedule to ensure your home remains in tip-top shape. Deep cleaning regularly helps remove particles like spores, bacteria, and toxins, as well as organic matter that can be used as food sources. This improves air quality and decreases the likelihood of microbial growth.

Building the right cleaning product arsenal is a key piece of the puzzle. You want to make sure that the tools in your toolkit are up to the task of actually removing microscopic particles from surfaces.

A few great options to get you started include:

Check out our blogs for cleaning tips depending on specific surfaces and appliances.

HomeCleanse also offers packages packed full of cleaning products we use to decontaminate our clients’ homes. These can ensure you’re getting the most out of your cleaning extravaganza.

Prioritize dusting

Microscopic particles settle where dust settles. Dusting can help reduce the number of particles that get kicked up into the air that proper air filtration must deal with. Make sure to use slightly damp microfiber cloths during this cleaning process, as they are 100 times better at wiping away small particles than regular rags, as well as a HEPA vacuum cleaner.

The Dust Test

Speaking of dust, a good idea is to test your dust annually. Testing this dust with a product like The Dust Test will help to determine if there are hidden issues causing unwanted problems in your home. Highly contaminated dust is not only a health hazard, as all of those particles can enter the body when they’re kicked up into the air when the dust is disturbed. It also indicates an underlying contamination problem somewhere in the home.

5.  What Causes Mold in Homes?: Attic Issues

As an area of the home that is unfrequented, this is a top location that causes mold in homes.

Some of the top factors that lead to attic mold include: 

  • Roof Leaks: Holes from a storm blowing debris around, broken shingles, improperly installed gutters, and chimney damage can all lead to moisture making its way into the attic.
  • Improper Exhaust: These systems are designed to take moisture-filled air out of areas like kitchens and bathrooms. If they lead to the inside of the attic instead of directly outside, that wet, hot air will increase the humidity inside of the attic and can form condensation wherever the exhaust vent ends.
  • Ventilation Issues: Attics typically have passive ventilation systems, where the outside air comes in through soffit vents and then releases this air through vents at the top of the space. If the vents are blocked or if there aren’t enough for the size of the space, this airflow will not occur, and the hot, humid air will remain inside, allowing moisture to build up and mold to grow.

Any of these scenarios can allow moisture in, and with food sources in abundance, mold spores that make it into the attic space will think they won the lottery.

Take a look at this post for a list of things you can do to prevent mold in your attic.

What Causes Mold in Homes?

6.  What Causes Mold in Homes?: Faulty Gutters

What causes mold in homes? Clogged and faulty gutters are one of the top routes. Their job is to take all of that precipitation and divert it away from the building. However, when they’re blocked, broken, or improperly installed, it allows water to pool in crawlspaces, windows, the roof, and along the foundation of the building. As basements are below ground level, these spaces will be the first to experience water damage from moisture intrusion from the water-logged surrounding ground.

That can cause some serious structural damage and allow all of that water to make its way inside, opening the door for microbial growth.

Cleaning them regularly throughout the year, ensuring that they’re installed properly, and directing them away from the foundation of the home helps prevent water from intruding inside and causing mayhem. Consider a downspout extension if there’s often heavy rainfall in your local area.

7.  What Causes Mold in Homes?: Improperly Installed Windows

Another common issue that causes mold in homes and water damage is windows that are not installed correctly. You can find this in:

  • New construction
  • Older buildings
  • Recently renovated homes

That’s why it’s important to understand how windows should be installed. Whether you’re about to start on a new construction project or just want to make sure your current windows are up to the job, having an idea of what to look for can help ensure your home is protected from indoor mold growth, is safe for you and your family, and avoid costly remediation projects down the road.

Check out this blog post for steps on proper window installation to prevent indoor mold growth and how to assess if your windows may be a problem. 

Prevention doesn’t just stop with installation, though! There are other steps you can take to ensure that mold growth doesn’t pop up along the windowsill or deep inside the foundation of the home. 

A few additional actions include: 

  • Keeping windows closed on rainy or humid days
  • Making sure the interior materials stay dry
  • Creating airflow in kitchens and bathrooms
  • Maintaining indoor humidity levels between 35-50%
What Causes Mold in Homes?

8.  What Causes Mold in Homes?: Bad Insulation

Insulation is one of the less-thought-of factors when it comes to an issue that causes mold in homes.

When there’s too little or no insulation, it can create condensation problems in the walls since the material helps separate the wide ranges of hot and cold temperatures. Take the summer as an example. It’s hot outside, but with the A/C running, the interior of the home is cold. Condensation can begin to develop when the two differences in temperature meet. Insulation helps prevent this situation from occurring so that this moisture does not get the opportunity to develop.

Spray foam is another aspect to consider since it’s the most popular form of insulation in modern times. Why? It has the best R-value per square inch of any other form of insulation. There’s one caveat with it, though. Spray foam creates such a great seal that if water or moisture gets into the wall, it’ll trap that water or moisture between the two layers of material. This means that if your walls are made of cement or plywood, the material will remain wet for a long time before the leak is discovered. This can lead to mold, wood rot, bacteria, and all sorts of other things that can impact indoor air quality over time. If this form of insulation is present in the home, routine inspections for moisture intrusion will be key because there won’t be any obvious interior signs that would normally be present when leaks occur.

Always carefully consider the type of insulation you’re using when either building a new home or remodeling. Today, they have antimicrobial alternatives such as mineral wool, wool, or denim insulation. Staying away from cellulose products that can easily grow mold is a good idea. From there, always look up the insulation codes in your region. Certain regions require higher R-values if the region is known for large temperature swings. This R-value is meant to protect you from those temperature swings to ensure you don’t have issues like condensation developing. Not to mention, it helps you save on your heating and cooling bills. 

9.  What Causes Mold in Homes?: Flooding

Unfortunately, no home is immune to flooding. Natural disasters, excessive rain, damaged pipes, and structural issues are a few of the common issues that can lead to a home experiencing severe water damage. And with this watery event comes a list of problems, one of which is increased indoor contamination.

Any moisture in a home offers a perfect opportunity for indoor contaminants like mold to move in and begin causing mayhem. Flooding, in particular, creates a massive chance for this fungus to move in thanks to the level of water in question. Tack on the overall low awareness and consideration for this event, and it’s the perfect storm for a toxic environment to develop.

Avoiding this dangerous situation should be at the forefront of everyone’s home maintenance plans. Regardless of whether you live in a home or an apartment, near a body of water or right smack in the middle of a dry zone, and have a basement or no basement, knowing how to keep your home safe after any water-damaging event can ensure you and your family will continue to breathe easy. 

Check out this ultimate guide to preventing mold before flooding and after an event occurs.


10. What Causes Mold in Homes?: Failure to Maintain the HVAC

The HVAC is essentially the lungs of a home. As such, it’s important to ensure that it’s operating properly and not bogged down with issues like microbial growth. When the system flips from cool air to warm air, condensation can build up within the ducts or the unit itself, which offers the perfect home for things like mold. 

If a lucky spore lands in this moisture, it will begin to colonize the space, leaving you with hidden mold growth within the home. This growth will just continue to thrive undisturbed until those warmer or cooler months occur. When you flip the thermostat to the alternate setting (heat or cold), all of those moldy particles will be blown throughout your home. These particles will decrease your indoor air quality and contaminate the surfaces inside. 

The change in temperature can also lead to mold growth on other parts of the unit, such as the condensation coil and drip pan.

Routinely scheduling a professional to come in and service the HVAC in the spring and fall, before the warmer or cooler weather settles in, can help avoid this contamination catastrophe. This individual should test for microbial contamination while assessing the unit so that if there’s a problem, it can be resolved before turning on the unit and blowing it throughout the home. They should also clean the coil, make sure the blower, furnace, and cabinets are clean, and ensure that everything is operating correctly. Think of it as a biannual tune-up and cleanup.

A clean and healthy HVAC system helps maintain a clean and healthy home.

11.  What Causes Mold in Homes?: Improper Installation of HVAC 

Another common contamination problem is due to poor insulation techniques around HVAC components, particularly around the register boxes or boot connections, which are boxes that our HVAC grilles sit inside. When these boxes, which are typically metal, get cold or hot while the unit is in use (this will vary depending on the season and if the AC or heat is on) and the air around the box is the opposite temperature, condensation can form. It’s important to make sure all of these boxes are well insulated to prevent microbial growth from developing in the condensation present. The usual culprits are the ones located in the attic. These can begin to develop condensation, allowing mold to grow in the drywall around them, on the vent, etc.

Not having ductwork and using wall cavities as ductwork is another issue that causes mold in homes. During those warm, humid summer months, moisture-rich air will be brought into the walls. This will more than likely cause condensation as that air hits the cooler indoor surfaces.

Improperly ducted returns are another problem. Not only is improper ductwork bad for the unit itself thanks to restricted airflow, which can lead to money spent fixing or replacing the unit, but it also allows microbial growth. The lack of airflow can lead to a buildup of humidity in rooms, and as we’ve mentioned, more moisture means more opportunities for mold. It can also lead to growth in the ducts themselves.

squeegeeing shower

12.  What Causes Mold in Homes?: Not Limiting Moisture

Indoor contaminants like mold can grow in as little as 24 to 48 hours if they’re provided with a wet surface and food source. Once established, it will release microscopic particles into the surrounding air for as long as the colony exists. This lowers indoor air quality and creates a toxic environment. Aka, it’s something we want to avoid!

That’s why one of the best ways to improve indoor air quality is to reduce any moisture-rich opportunity that causes mold in homes.

A few of the top steps to reduce moisture includes:

  • Wiping up spills
  • Squeegeeing the shower after use
  • Allowing appliances to dry thoroughly after use
  • Making sure bathroom items dry out completely (towels, loofahs, bath mats, toothbrushes, etc.)
  • Separating the shower curtain from the liner
  • Not putting away wet dishes in cabinets
  • Creating airflow in moisture-rich rooms like bathrooms and kitchens

You’ve Got This!

All of this information is not meant to make you stressed out or fearful. Knowledge is power! Indoor mold growth and air quality are little-discussed topics. The more aware you are of these issues, the more you can do to prevent any problems from popping up in your home. 

Also, don’t feel like you have to jump in and knock out this entire list in a weekend! Any step taken to improve your indoor air quality and environment is a huge leap in the right direction. Remember to breathe and allow yourself grace as you navigate your journey to a healthier home.

If you have any questions, schedule a consultation call with us, and we will be happy to work with you to ensure you're on the path to success. 

Still Have Questions?

A member of our team is here to help!  Click on “Get Started ➤” below to book a consultation with a member of the HOMECLEANSE team. We have a few quick questions that will help us put together a roadmap to solve or prevent all of your mold problems.

Two minutes of your time could lead to better health for you and your family.