Chapter 7: Basement Mold

Do You Have A Moldy Basement?

This chapter describes the many sources of basement mold, why it can be such a problem, how to deal with it, and effectively how to prevent basement mold from coming back in the future.

There is some overlap in this chapter with the chapter on crawlspace mold, as the sources for moisture ingress into the home can be similar.

Sometimes the cause of mold in your basement is straightforward and easy to identify.

In other cases, there are a number of factors and conditions to consider before you can get to the root of the cause.
It is important to understand that basements, by design, are conducive to mold growth.

Many basements are affected by mold because this part of the home is partially or completely underground, which means limited natural light, poor ventilation, high humidity levels, excess dust, dirt, and debris, along with cooler temperatures. 

These conditions all contribute to mold growth in the basement. People don't always spend a lot of time in their basements. This allows for dust build-up and all types of mold spores to grow, including black mold.

Mold growth in the basement is most common in dark, cool areas, such as behind furniture, in closets, or behind items that have been placed up against exterior (outside facing) walls.

Further, concrete foundation walls are porous, especially in older homes. This gives rise to moisture slowly seeping into the basement. When this happens, often the floor and walls become saturated. 

mold on cracks on basement wall

Wet building materials will allow mold to begin to consume organic debris. At this point, it is common to smell the musty odour associated with mold growth, which is often a clue that mold spores have begun to contaminate the air.

It only takes 24-48 hours for mold to start growing in your basement. After mold colonies have established a presence, they will continue to grow and spread more quickly.

Moisture ingress can be an expensive undertaking, especially when it goes undetected for an extended period of time. 

If you didn’t read the chapter on crawlspace mold, I have listed some of the most common ways moisture can get into the home:

  • Pooling water against an unprotected foundation wall (A foundation wall without a waterproof membrane - very common in older homes)
  • Inverted slope into the home, which allows water to pool against the side of the home
  • Older, broken drain tile materials like clay, concrete, and corrugated plastic pipe
  • Damage caused by tree roots growing too close to the home
  • Cracks and voids in the foundation wall

This picture is an example of an outdated, plastic corrugated drainpipe, typically referred to as "Big O", by those in the know.

Clues to look for when diagnosing moisture ingress into the basement of an older home include bubbling or chipped paint along the walls, texture changes along the baseboards, discoloration (usually black, brown, or yellow) wet carpets, wet floors, wet building materials, musty odours and mold of course.

Newer homes with basements experience issues associated with condensation-related mold because they are tightly sealed with vapour barriers, which trap moisture inside the walls, rather than allow the home to breathe.

outdated plastic drain pipe

Mold grows on stored items such as cardboard boxes, old photos, books, documents, or furniture with particle board backings. These items serve as the perfect mold food due to their organic makeup.

Sports equipment and camping gear typically become infected by mold. When these items are stacked up against a wall in the basement, the issue may go undetected for quite some time.

When mold grows on stored items made from paper or cellulose-based materials you will have to discard them.

When dealing with mold-infected possessions, you should hire a mold remediation contractor to assist you, or you will risk cross-contamination of mold throughout the home. 

Excess humidity in the basement is usually the main cause of mold growth on furniture. One cause of high humidity is air drying laundry in a confined space.

In this case, you may not notice any clues on the walls or floors.

The sofa in the picture (mold patches) is an example of what can happen to furniture when mold spores land on the surface, while it is exposed to high levels of humidity and dust particles. This indicates a larger overall mold problem that needs to be further investigated by a mold inspector or remediation contractor. 

mold on sofa in basement

Carpeting on basement floors can be contaminated by mold from moisture below or undetected spillage from planters, pets, etc.

Floods from clogged up drains and sumps commonly contribute to basement mold.

If the basement has been saturated for over a week, then the scary Stachybotrys (aka: black mold, or what people like to call toxic black mold) can wreak havoc in your basement.

stachybotrys mold on basement wall

How To Prevent Basement Mold

Every attempt should be made to keep the basement warm and dry. One of the best ways to monitor the humidity in your basement is to use a hygrometer, as noted in the chapter on bathroom mold.

Ideally, indoor humidity levels should not exceed 55%. If the humidity levels in the basement continue to be a challenge, I recommend buying a dehumidifier.

You’ll need to dump out all accumulated water and clean the dehumidifier daily as the components can also become dirty and moldy inside the machine.

To prevent a potential flood, or backup, have a drainage company scope the perimeter (outside) below ground drain pipes to determine the condition of the system.

All outdated drain pipes will eventually need to be replaced. This includes clay, concrete, or the plastic corrugated (sometimes referred to as “big O”) materials - with the modern PVC drainpipe.

Unfortunately, this can be an expensive project when the pipes are damaged or outdated. If you live in a wet climate (like I do in the Pacific NorthWest) every effort should be made to seal all cracks in the foundation wall.

Install a proper moisture barrier as well. Place drainage stone around the perimeter to allow the rainwater to seep into the ground below the home, rather than allow water to pool against the foundation wall.

It is best to speak with a drainage contractor for the ideal solution based on the type of soil, age of your home, current drainpipe, and elevation of the land surrounding your home.

If your basement is susceptible to floods, you will need to call in a drainage contractor who specializes in flood prevention. Typically a sump and pump will be recommended if you do not already have one.

The sump is designed to collect excess water. A pump is used to push the water up and out of the home through pipes. This is especially important when you have a basement well below ground.

If you already have a sump on your property, it should be inspected and serviced annually by a drainage contractor. Interior sumps should never be covered up with flooring or left alone.

Also, if you have a sump in the basement, rather than outside, make sure that the sump cover is secure and sealed to prevent mildew odours from seeping into the home. The cover will need to be removable for the annual inspections.

Have a mold or experienced home inspector come in to take moisture readings along the inside exterior facing walls, to help identify any concerns.

If your inspector has a thermographic camera, even better! Leaks and hidden moisture issues can easily be found with the use of Infrared technology.

However, before you hire an inspector to do a thermal investigation, make sure the inspector has been adequately trained. The best time to use the camera to identify moisture ingress is during the winter.

Buying a camera doesn’t make one a skilled thermographer. A level 1 certification from a reputable institution is the way to go.

Be sure to ask for proof. Thermography is complicated and it is easy to misinterpret what the camera reveals unless you have adequate training.

Investigate all corners for evidence of moisture. Bugs in corners leave clues, especially spiders. Spiders like to set up shop in cool moist areas, as they are perfect for attracting other insects looking to hydrate.

Take a look at the ceilings below bathrooms and kitchens. Leaks will usually look like dark round circles or ovals. Active leaks will eventually turn dark brown or black.

Suspended cellulose tiles will also show leaks. Here, you can lift them and check where the leak is coming from. Check the pipes and connections when searching for the leak.

Any active leaks should be addressed as soon as possible with the help of a plumber. The photo shows evidence of a past leak along a suspended ceiling tile. 

leak on ceiling tile

Mold-damaged walls, ceilings, and floor surfaces should be removed in short order with the help of a remediation contractor. I cover the topic of sourcing out and hiring a professional mold removal company in this article.

Rather than storing your paper items like books, documents, photos, and or any other organic materials in cardboard boxes, put them in sealed plastic bins, along with moisture-absorbing products from DampRid. This will protect them from moisture damage and mold.

It is best to put them on shelves, rather than the ground and away from the wall to allow for airflow and ventilation.

When making renovations to the basement, use mold-resistant building materials and paint. I don't recommend carpets as basement flooring. They will absorb water and become moldy if there is a flood or leak.

Of course, if you are concerned with mold in your basement, it is best to hire an experienced mold inspector. Often they will be able to find mold in areas that may not be obvious to you.

They should be using a flashlight and inspect the furniture, as well as the baseboards, wood trim, and door frames. They will be looking for any blotchy mold colonies where dust settles and accumulates.

Dark colonies will be fairly obvious, but lighter colour molds will be more difficult to find. Remember, mold comes in several different colours other than black mold.

There are white, green, brown, blue & pink molds. The exposure may be dangerous to those with mold allergies, or respiratory issues. 

Hard surfaces can be cleaned with a commercial mold cleaner. However, when furniture with fabric or other porous materials have been mold affected, they will likely need to be discarded.

A remediation contractor can give you the best advice on which items you may keep or need to purge.

All good mold inspectors should be using a flashlight, moisture meter, thermographic camera (as discussed above), and appropriate air testing and or mold sampling equipment to find sources of mold and moisture.

mold infected bookcase

When vacuuming, use a HEPA (high-efficiency particulate air) vacuum cleaner. For best practices, do so weekly.

These tips should give you a solid understanding of what you should look for and how to deal with mold in your basement.

In the next chapter, we move to the garage to learn about what a mold problem looks like there and how to deal with it.