Fixing the Most Common Source of Mold: Your Crawlspace

Jeff Bookout
March 11, 2021

Reading Time: 6 minutes

We asked BioBalance expert Jeff Bookout about the most common mold problems he finds in homes. 


The Crawlspace Problem

In my experience of helping people with mold issues, crawlspaces are the number one reason why people get sick from mold exposure. If you have a dirt floor crawlspace, it is apt to have many problems. When many people visualize a mold problem they think of a patch of black mold on a surface. The larger the patch, the bigger the problem. So, imagine a mold patch not the size of a wall, but the size of a house footprint, because that is how big your crawlspace is and that is how big a mold problem can be.

Now your family doesn’t live in a crawlspace, but contaminants in your crawl space have a great chance of joining with room air. Imagine you cleaned your crawlspace or basement with a cleaner with a strong smell like Pine-Sol. When you go back into your home, do you smell the Pine-Sol? Yes, you do. If the smell can get into your home from gaps, vents, or just seep through the floor, mold and the mycotoxins and other volatile organic compounds (VOCs) they produce can get into your home too. It’s like when I walked into a house yesterday for a real estate transaction, as soon as I walked in, it had that “musty old basement smell” and they don’t have a basement, they have a crawlspace. 

So, what do we do about the crawlspace problem? Here is a list of the top issues with crawlspaces and ways to fix them. 

1. Insulation

Problem: Any time you can see the pink or yellow parts there is an opportunity for mold and mycotoxins to get trapped in the insulation. Once in, there is no way for them to get out. While they are trapped in the insulation, they can release VOCs. Foam sealant below the soil like on an outer wall can sometimes trap moisture, smelling like an old dirty mop, and it’s likely full of contaminants. 

Treatment: For pink and yellow paper insulation, remove it and replace it with either plastic, foam board insulation, plastic back insulation, or if it is above the soil level, spray foam insulation. That is as long as you don’t react to the chemical smell of the spray foam. For the foam sealant on sidewalls below the soil line, if it smells like a dirty mop, rip it out. 

2. Contaminated Wood

Problem: There is a good amount of wood in a typical crawlspace, floor joists, and the subfloor. If you have any moisture in the crawlspace due to a leak, you’re likely to have visible mold growth on the joists and subfloor.  

Treatment: If the wood is wet or has dry rot, remove and replace it. If for some reason you can’t remove it, treat it with a 10-12% hydrogen peroxide solution, then apply a sealant once dry. I recommend AFAM, Safeco, Zinser, or any product that doesn’t produce VOCs because they don’t have a strong paint or chemical smell. 

3. Pipes & Tubs

Problem: Many pipes and water lines coming up from the crawlspace are not sealed. Condensation on these pipes can be a breeding ground for mold. Bathtubs are also problematic, there is typically a cavity under the tub that can harbor contaminants and come up to the room area several ways through a wall penetration (e.g. – light switch, outlet, or plumbing access area). This is called the “stack effect” meaning any time you have warm air below, it will always get driven upwards. 

Treatment: Use Foam Sealant for going around all pipes, holes, cavities, electric lines, and floor vents. If you see a hole that contaminants can come up through, seal it up. 

4. Water

Problem: If you have a water leak in your crawlspace, or even just high humidity, you’ve likely got mold growing there, and it’s getting into your soil as well. We rarely go into the crawlspace, so checking for water leaks is important. Check for water standing next to your foundation, or other sources like under your kitchen or bath. 

Treatment: Pure and simple, find the source and stop the leak. If you find moisture standing up against your foundation, you might need to trench a French drain system or get better guttering sloping away from the foundation, or whatever it takes to keep water out. If water always comes in your crawlspace and you can’t fix it, install a sump pump just like someone would have in their basement. If you have high humidity without a water source you can find, consider installing a dehumidifier made for a crawlspace. 

5. Ducts and Vents

Problem: One of the first places that contaminates can pass from your crawlspace to your home air is your heating and cooling vents. When your system kicks on and those floor vents aren’t completely sealed, it creates so much negative air pressure that it sucks the air from in your crawlspace and brings it up into your breathable air. There is also a phenomenon called the Venturi Effect. It happens when a larger duct is connected to a smaller duct. At the connection point there is often a small gap. When your system comes on it creates a vacuum and sucks the crawl space air into the ductwork, then out your registers into your breathable air. 

Treatment: Use Foam Sealant to going around floor vents and duct sealant to seal gaps in the ducts.

6. Poor Air Circulation

Problem & Solution: I want you to envision that crawlspace to be its own separate home and prevent whatever is down there from getting up in your home. You’ll never have it perfectly seperated, but it is a good goal. One way to do that is to make sure that the crawlspace itself is properly ventilated. I recommend a fan that goes over the existing foundation vent and has two small fans that are temperature controlled. If it gets below 40 degrees, they are going to shut off so it doesn’t freeze your pipes. Otherwise, it forces air out of the crawl space and prevents it from going up into your home.

7. Mold in the Soil

Problem: Quite simply, if there is any moisture content in your soil at all, it is going to grow mold. If you have soil that doesn’t dry after 48 hours, it is a problem.

Solution: Install a plastic vapor barrier of at least 20 mils. Code is often going to be thinner, but I recommend 20 mils or higher. The plastic will encapsulate the soil and there’s no way for that soil to ever communicate with the air above it. It goes over the soil, goes up the sides of the foundation, over posts, etc. And make sure each plastic sheet overlaps the other by at least 4 inches.


BioBalance Products for Crawlspaces

I get asked often if there are any BioBalance products to help with crawlspace issues. Treating the air in a crawlspace with our fogging solution, HavenFog, or if you have a small crawlspace, will indeed improve the air, but it is not a long term mold fix. The source of mold growth in your crawlspace needs to be stopped. I hope the problems and solutions listed above will help you with a long term solution. 


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