Chapter 5: Crawlspace Mold

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Why Do Crawlspaces Get So Moldy?

If your home has a crawlspace, you absolutely must pay attention! Did you know that water enters your crawlspace in several ways, causing mold, only to compromise the air quality of your home?

In this chapter, I discuss why mold grows in crawlspaces and the best ways to prevent it. Crawlspaces are designed to hold mechanical equipment, pipes, wires, structural members and that’s it.

Most people don’t like going into their crawlspace. It is dark, musty, damp, dangerous, difficult to navigate, and inhabited by creatures like spiders and rodents. There are several possible causes of mold in the crawl space.

Crawlspace mold is typically caused from outside moisture migrating in from below, beside or above. One of the main clues of a mold problem in the crawlspace is a musty smell inside your home. 

Reasons For Crawlspace Mold

Poor soil conditions/high water table

Not all land is appropriate, or solid enough to support a home.
For example, a home built on or beside clay will prevent drainage. This will result in water pooling against the foundation wall.

This image shows excessive pooling against the foundation wall, likely due to poor soil conditions or drainage.

As a result, water will slowly migrate into the crawlspace, which will eventually lead to mold.

Broken, disconnected or damaged drain pipes

Expired drainage materials and vegetation can cause the drain pipes below the ground to become damaged. The damaged pipes allow water to seep into the crawlspace through the foundation wall.

Leaking, disconnected downspouts and gutters

This issue causes pooling around the foundation wall. If the home is not sloped to divert water away, the problem becomes worse.

Installed sprinkler systems

Many people like to have sprinklers set up around their home to feed their gardens. Sprinkler installation companies may not want to hear this, but installing sprinklers against the side of the home is never a good idea.

The pipe connections for sprinklers systems are buried under the ground. If and when they break or become damaged, the issue may go undetected for quite some time, causing water to slowly seep into the home.

In other cases, the sprinklers spray water against, rather than away from the home, especially when they haven’t been installed correctly. Further, soil beds and gardens along the side of the home draw moisture like a sponge, causing the water to sit or slowly seep through the foundation wall and into the crawlspace.

Cracks in the foundation wall and floor

Unsealed foundation walls will encourage moisture to enter the crawlspace, often resulting in mold.

Modern standards call for foundation floors to have a vapor seal below a concrete slab to prevent moisture seepage from below.

Older houses, by contrast, typically have unprotected, porous foundation walls, encouraging water to seep into the crawlspace through hydrostatic (water) pressure. The problem is magnified when there is a crack in the foundation wall.

Dirt floors

Unsealed, earth exposed floors will support mold growth. There are millions of mold spores in the soil.

Condensation along the ceiling joists

It is common for mold to grow along the ceiling joists where there is enough moisture present.

When condensation develops, any organic debris that settles along the joists becomes mold food. The photo clearly illustrates this.

If your home sits above a crawlspace and it smells musty, there is a strong possibility that mold is either growing on the ceiling joists, sheathing or somewhere else in the crawlspace.

Crawlspace wall vents

Conventional crawlspaces called for exterior vents to be placed along the perimeter foundation wall. Overtime, we have learned that this set up only contributes to humidity and condensation in the crawlspace, resulting again in mold growth.

Bathroom leaks

Leakage from bathrooms above into the crawlspace is common. However, they are often concealed below toilets, tubs and showers and not always detected quickly enough to prevent a mold problem. 

Pipe condensation and leaks

Older copper pipes eventually develop pin-hole leaks resulting in slow dripping.

Condensation on copper pipes in the crawlspace is another common issue and source of moisture.

Outdated piping materials such as Polybutylene installations have been responsible for thousands of leaks, and in some cases significant water damage.

Incorrect venting

When laundry dryers and kitchen exhaust fans discharge humidity into the crawlspace, rather than outside, mold growth is likely the outcome.

Ceiling insulation in the crawlspace

This is not a wise idea. People typically do this because they want to keep the floors warm in the home above.

Here is the problem… When the insulation gets wet from a leak or condensation, mold will grow undetected on the wood sheathing and joists. Furthermore, gas lines, pipes, junction boxes or wires will be hidden or inaccessible for inspection. 

Cold crawlspaces and storage

Cool crawlspaces take longer to dry, which can further accelerate mold growth.

A cold, damp, crawlspace is not the best place to store your items, especially items like documents, photographs, cardboard or any porous fabrics. 

Cross contamination from the crawlspace

When people bring their stored items (with mold on them) into the home from the crawlspace, they will accidentally cross contaminate their homes with hundreds of thousands of mold spores.

Efflorescence (a white powdery substance left behind on concrete, stone, brick, etc) is a telltale sign of moisture in the crawlspace. This is evidence of seepage, but not necessarily mold. At the same time, it is helpful to know when moisture ingress is happening.

Is Mold In The Crawlspace Dangerous?

Many people complain about health issues from their moldy crawlspaces. People may suffer adverse health effects, similar to cold or flu symptoms.

Mold spores are so tiny that they can easily move through small gaps between the crawlspace and the home above. Typically these voids are located in the mechanical room, laundry room, kitchen and bathrooms.

When enough of these spores enter the living space, the residents of the home will often complain that they smell a strong moldy odour. Often they do not know where it is coming from. They may not realize that it is coming from the crawlspace, until it has been inspected for water damage, moisture issue or mold.

In other cases, mold spores enter the home from compromised or rust damaged ductwork, connected to the forced air heating system. 

Preventing Crawlspace Mold

Essentially, you want to keep the crawlspace warm and dry, which is much easier said than done. There are a number of maintenance and proactive repairs that you can do.

From the outside, the home should have upgraded drain tiles installed, with a waterproof membrane to protect the foundation wall.

For foundation walls without a waterproof membrane, any cracks on walls and floors should be inspected, properly repaired (sealed) and monitored annually.

Sprinkler systems and soil beds (beside the foundation wall) should be removed. Perimeter drainage stones (sometimes known as aggregate) rather than soil will encourage drainage.

All soil beds and flower gardens should be placed at least 1-foot away from the foundation wall.

If the home does not have an underground drainage system, then divert all downspouts away from the foundation wall (ideally six feet to prevent pooling) by using downspout extensions or splash blocks. If the land slopes towards the home, call a landscaper to improve the situation.

If the home has older outdated piping materials ~ clay, concrete or plastic corrugated drain pipes, call a drainage specialist to scope it for adequacy, service life, damage, clogs etc.

In most cases, older drainage systems will need to be replaced with a modern PVC pipe installation.

This photo shows a broken outdated clay drainpipe that was eventually replaced with a current PVC pipe and waterproof membrane. 

Although potentially expensive, It is best to deal with these types of issues proactively when possible. Contrary to conventional wisdom, crawlspaces should actually be conditioned, or heated like a basement, not vented.

The reason for this is two-fold.

First, vents allow moist cold air into the crawlspace during the winter. Second, in the summer the vents allow warm air in.

When the newly introduced air comes in contact with the cooler materials inside, condensation forms, resulting in moisture. So, regardless of the weather, foundation vents contribute to crawlspace mold, so remove or cover the vents. Remember, we don’t want moisture in the crawlspace.

When inside the crawlspace, take a good look at all pipes, drains and floorboards especially under bathrooms, kitchens, washing machines or water heaters. If you notice any leaks or moisture, call a plumber for immediate repairs.

I don’t recommend storing items in the crawlspace. However, if you absolutely need to do so, use sealed plastic bins combined with a moisture-absorbing product to prevent humidity. Never use cardboard boxes, as they will be consumed by mold when exposed to moisture.

If you see mold infected items, I always recommend calling in a trained, licensed & certified mold Remediation Company to remove them. They have the equipment and resources to do so safely.

Unfortunately, people often move moldy items from the crawlspace into the home, which will cross contaminate the house with thousands of unnecessary mold spores. It is not worth compromising the health of you and your family.

If you notice flooding (usually in the winter) then you will need to call a company specialized in dealing with basement or crawlspace flooding.

A sump pump may be required to remove excess ground water. Or the exterior drainage might be the problem. Regardless, call in an exterior drainage company so it can be dealt with quickly. If your dryer exhaust duct vents into the crawlspace, have it removed and properly vented outside.

If the laundry vents through the crawlspace and to the outside, ensure that it is properly insulated and sealed at the penetration point through the wall. If possible, crawlspace ceilings should not be insulated with fiberglass batts, as this too can contribute to condensation, and mold.

The only time a crawlspace ceiling should be insulated is when the floor is dirt, does not have a vapor barrier, and the ceiling is completely sealed with an air barrier such as spray foam insulation. (Assuming you do not want to upgrade the crawlspace to modern building standards)

The goal is to create a complete environmental separation between the crawlspace and the rest of the home.

For the best advice, speak to an insulation contractor experienced with crawlspace upgrades and repairs.

Ideally, the entire floor space should be covered with vapor barrier under a concrete slab and walls properly insulated.

If the home has a dirt floor, it should be covered with a heavy-duty vapor barrier and completely sealed up the walls. A basement waterproofing or crawlspace repair contractor can help you with this.

If you have mechanical equipment in the crawlspace, have it maintained annually. Check for leaks. All ductwork should be insulated to prevent rusting and condensation.

The best way to deal with high humidity is to install a dehumidifier and possibly an exhaust fan. This photo is an example of what can happen to the heat ducts from excess humidity.

Remember: Reducing the ongoing humidity inside the crawlspace is extremely important.


Check the pipes in the crawlspace for leaks. In colder climates, your pipes should be covered with a foam insulation to prevent condensation.

Copper pipes will typically last from 25-50 years, based on the installation, water pressure and chemicals in the water. When copper pipes age, they will develop pinhole leaks.

You will see a dark green section with a slow drip. If this is the case, there is a strong possibility that other areas inside the home will also have pinhole leaks. (heads up!)

At this point, you will need to call a plumber in to replace all leaky pipes. You should ask the plumber to take a look at all other visible piping in the home to determine where any other pipes will need to be replaced.

Each visit from a plumber will cost you money, so it is much better to ask the plumber to check as much as possible rather than calling him back time and time again.


If you have any reason at all to suspect mold or find it in your crawlspace, call a professional mold inspector or mold remediation contractor to help you search for the moisture source as soon as possible.

Taking these measures will help you reduce crawlspace mold and the associated issues caused by mold exposure.

In Chapter 6, we are going to look at how to prevent mold in the bathroom.