Why Do I Have Mold in my Crawl Space?

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Why Do I Have Mold in My Crawl Space?

Barry C. Westbrook, CIH, PE

Home inspectors have become very picky about mold in vented crawl spaces. If mold is discovered on floor joists or sub floors in the crawl space, you will definitely see it in the final report. The reason is simple – LIABILITY.

Considering the abundance of publicity surrounding the perils of “toxic mold”, the last thing any home inspector wants is a law suit. Fair enough. However, once mold is identified, what is the appropriate response?

Here is an actual excerpt from a home inspector’s report:

Mold growth is visible on many of the floor joists and sub floor in the crawl space. The level of mold growth is low to moderate. The presence of certain mold and mold spores can result in mild to severe health effects in humans and pets and can deteriorate the structure of the dwelling. Health effects include, but are not limited to: sinus congestion, blurry vision, sore throat, dry cough, aches and pains, skin irritation, bleeding of the lungs, headaches, memory loss and fever.

This type of wording can create friction between the buyer and the seller of the property. Considering all the nasty health effects described above, is a moderate exposure acceptable? Even if I were willing to expose myself to this risk, is it appropriate to expose my children even to moderate levels of mold?

The home inspector is doing the prudent thing by stating the known health effects of mold exposure and trying to establish a relative description of the magnitude. In effect, the inspector is passing the decisions about risk on to someone else: either the buyer of the property or another professional with more experience in assessing health exposure. The report goes on as follows:

As a practical matter, there are several things that can be done to minimize mold growth: (1) Keep the crawl space well ventilated. (2) Divert downspouts away from the foundation wall and keep gutters clean.

Is this an effective plan for avoiding mold in the crawl space? While getting storm water away from the foundation and keeping the gutters clean are certainly good things to do, they are not likely to prevent mold from growing in the typical crawl space. The reason this is true relates to two of the biggest myths in building construction:


Myth # 1. Standing water is the main cause of mold in the crawl space.

Myth # 2. Mold can be prevented by keeping a crawl space well ventilated.


Let’s see why these statements are not true.

While different mold species need varying amounts of moisture, let’s stick to the basics: mold cannot grow without moisture. For mold to grow on floor joists and sub floors, water vapor must condense out of the air and onto the surface of the wood.

Where does this water come from? The prevailing wisdom is that this moisture comes from the ground, but reflecting on some basic science principles will show why this is not usually the case.

Take a look at the chart below prepared by the non-profit Advanced Energy Corporation. You will see that the relative humidity in a vented crawl space is almost totally a function of the relative humidity of the outside air.


When the outside humidity is high, the humidity in the crawl space will also be high. In absolute terms, the amount of moisture entering the crawl space from the outside is many, many times the amount of moisture evaporating from the ground surface. (Note: The blue line in the graph represents the relative humidity in a closed crawl space assembly which is what we promote at DocAir.)

To understand the origins of mold growth in a crawl space you must become familiar with the term dew point. For water vapor to condense onto a surface, the temperature of that surface must be below the dew point of the surrounding air. At the dew point, the air is 100% saturated and can no longer hold any more moisture. When the temperature of the air reaches its dew point, water vapor changes from a gas to a liquid: the process of condensation. The colder the surface, the more condensation will form. That is why there is usually more mold.

Why Do I Have Mold in My Crawl Space?

What about underneath bathrooms and kitchens? These floors are not insulated as well as carpeted floors, and can become very cool in the summertime. Moisture forms on the cool surface in the crawl space, and mold proliferates. We may also see damage to hardwood floors when the wood absorbs condensation and expands outward. This swelling creates tremendous compressive forces, and when the wood reaches a limit, it will buckle or cup.

What Can Be Done?

So is mold in the crawl space a problem? The answer depends on your expectations.

In any discussion of mold exposure, it is important to note that there is a wide spectrum between unsafe and pristine. There is also large variability in human sensitivities to mold spores and fungal particulates depending on sex, age, genetic makeup, and overall health. With these considerations in mind, the environmental condition of most homes with vented crawl spaces should not be considered unsafe. However, air quality can definitely be improved by keeping crawl spaces dry throughout the year.

Although standards are beginning to change, building codes have traditionally required crawlspaces to be ventilated. The intent is not to prevent moisture from entering the crawlspace (which is really what happens during the summer months) but to allow excess moisture under the building to evaporate. It is assumed that this moisture is coming up from the ground. If the outside air is sufficiently dry and the amount of moisture is not too great, this process is adequate. Unfortunately, during the summer months, the outside air is usually too humid, and vents in the foundation transport moisture from the outside to the inside rather than the other way around.

Condensation in the crawlspace can best be prevented by sealing it off both from the ground and from the outside using a continuous liner such as the one shown below.


Why Do I Have Mold in My Crawl Space?

As a final measure to ensure a dry, energy efficient crawl space, we recommend the installation of an electronically controlled dehumidifier in the crawl space that will maintain the relative humidity below 60% throughout the year.


It is predictable that vented crawl space will have some mold growth on wood floor joists and sub floors exposed to the ambient air. DocAir installs closed crawl space assemblies for both new and existing homes. A closed crawl space delivers major benefits in terms of air quality, energy efficiency, and structural sustainability. However, we always emphasize that there are millions of vented crawl spaces in existence throughout the southeastern United States. The decision to install a closed crawl space assembly should be governed by the sensitivities, preferences, and budgets of the home owner.


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