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Why Do I Have Mold In My Crawl Space? Part Three

Date Added: July 23, 2009 06:06:05 PM
Author: American Home Inspector Directory
Category: Real Estate & Home Inspection Articles

Why Do I Have Mold In My Crawl Space?

Part Three

This is the third article in a series on crawl space mold.

Mold myths

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.


According to Certified Industrial Hygienist Barry Westbrook, founder and President of DocAir (www.docair.com), a building systems evaluation and indoor air quality consulting firm, “While different types of mold 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 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 the case.


Westbrook adds, “It takes a lot of energy in the form of heat to change water from a liquid to a gas. If you were a colony of mold looking down from the sub floor of your home, you would see an occasional molecule of water evaporating from the ground and becoming mixed with the air in the crawl space. Meanwhile, you would also see millions and millions of water molecules coming into the crawl space through the foundation vents already mixed in the outside air. The actual amount of moisture in the outside air varies directly with the temperature, but in the heat of summer, air can hold a tremendous amount of moisture.


Understanding some basic science will totally undo these myths. Consider the following.


Relative humidity

Research prepared by the non-profit Advanced Energy Corporation indicates that the relative humidity in a vented crawl space is almost totally a function of the outside relative humidity. 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 within the crawl space.


Dew point

Dew point is the other scientific principle necessary to understand the mold phenomenon. For water vapor in the air to condense onto a surface, the temperature of that surface must be below the dew point. At that point, the air is 100% saturated and can no longer hold all the moisture. The air begins to drop some of the water onto the surface as condensation. The colder the surface, the more condensation will form. That is why there is usually more mold underneath bathrooms and kitchens. These floors are not insulated as well as carpeted floors, and become very cool in the summertime. Moisture forms on the cool surfaces in the crawl space and mold proliferates. We also see cupping and buckling of wood floors for the same reason.

The air that you breathe

So is mold in the crawl space a problem? The answer depends on your expectations. The Advanced Energy Corporation findings conclude that much of the air we breathe in our “naturally ventilated” homes comes from our crawl spaces through penetrations in the floor. The flow of air from the crawl space is driven by the difference in building pressure between the first floor and the outside. Research shows that depending on the structure, well over 60% of the air we breathe inside the living space of our homes comes from the crawl space (based on Radon infiltration data).

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