In homes that have crawl spaces, a very high percentage of mold issues that we see in the living spaces have a connection to the conditions in the crawl spaces. This is due to two inescapable realities:

1. Crawl spaces in humid climates routinely have mold growing in them.

2. Crawl spaces are not completely isolated from the living space of the home.

Let’s consider both of these realities in more detail before looking at possible solutions.


Whenever relative humidity (RH) is 70% (or higher) for extended periods of time, mold is almost certain to grow. That being said, the RH in crawl spaces in humid climates is routinely well above 80% during the summer months. This is due to the fact that warm, moist air migrates through the foundation vents into the crawl space where it cools off (thereby raising the RH of the air). In other words, if the temperature of the outside air is 90o F with a RH of 60%, and the temperature inside the crawl space is only 75o F, the RH of that air may easily rise above 75% when it migrates under the house and cools off. This is due to the fact that when you cool a given volume of air you also raise its RH. Therefore, during the summer months, crawl spaces in humid climates provide mold with ideal growing conditions due to the high RH and the presence of wooden building materials on which the mold can “feed”. Moreover, the presence of HVAC ductwork in the crawl space exacerbates the situation by providing relatively cool surfaces on which condensation can form.

Please note: This is true even for crawl spaces that appear to be “dry”. In other words, a crawl space does not have to be “wet” in order to constitute a mold- growing environment. It simply needs to have a high RH (i.e. 70%+ for long periods of time).


A crawl space is not completely isolated from the living space above it. This is due to at least three realities:

1. The presence of plumbing, wiring, and other penetrations that form passage ways for the movement of air between the crawl space and the living space.

2. The dynamics of “stack effect” whereby warm air exiting the attic through the structure’s roof vents results in a slight drop in air pressure in the living space below that, in turn, draws air into the living space from the crawl space below.

3. The presence of HVAC air handlers and ductwork in crawl spaces allow for air in the crawl space (along with any mold toxins in that air) to be dispersed throughout the living space simply because in “real life” there is no such thing as a perfectly sealed HAVC system.

These inescapable realities mean that what “goes on” in the crawl space has a significant impact on the environment of the living space above it. Yet, for the most part, home owners (and, unfortunately, many home builders) do not give crawl spaces a second thought. However, anyone who is serious about their health—and the health of their loved ones—needs to make sure that their crawl space environment is not one that is effectively poisoning their home with mold toxins.


In order to insure that you crawl space is not a “mold factory” that is pumping toxins into your home, it is imperative that you control the conditions in the crawl space. As illustrated above, passive foundation vents alone are not sufficient. In fact, they are usually part of the problem (i.e. because they facilitate the movement of warm humid air into the cool crawl space). The best solution that we have found to the problem of moldy crawl spaces is full encapsulation coupled with dehumidification.

There are many “fly by night” outfits and amateurs that offer crawl space encapsulation at “bargain” prices. However, we have seen first hand the costs associated with such “bargains” (i.e. costs in terms of shortcuts and poor quality work). The proper approach to crawl space encapsulation includes the following:

1. Making sure that outside water is being directed away from the foundation of the house (this requires considering the grade of the land and proper gutter function).

2. Installing French drains along the inside perimeter of the crawl space where needed (i.e. the need for such being determined by a competent crawl space specialist).

3. Installing a sump pump to deal with any water captured by the French drains.

4. Sealing up all of the passive foundation vents.

5. Installing a plastic liner on the floor of the crawl space (i.e. that covers the French drains and the sump pump) as well as running that plastic all the way up the crawl space walls (i.e. effectively making the crawl space a sealed compartment).

6. Installing a dedicated crawl space dehumidifier in the crawl space.

When these steps are taken, your crawl space will now be a controlled environment in which the RH can be kept at 50% (thereby insuring that mold in your crawl space will never be a problem).

Please note: Many contractors try to take short cuts in this process by attempting to condition the air inside the encapsulated crawl space with the HVAC system (and, thereby, save money by not installing a dedicated crawl space dehumidifier). Based upon our experience of inspecting homes for mold, we can say with confidence that this approach simply does not work. Not only does the HVAC system fail to adequately control the RH in the crawl space, it effectively results in the crawl space and living space being combined into a single air “envelope”. This constitutes an unmitigated disaster if mold starts growing in the crawl space (i.e. because the HVAC system then effectively pumps the mold toxins throughout the entire home). Therefore, never settle for anything less than an encapsulated crawl space with a dedicated dehumidifier. If your home has an encapsulated crawl space in which the HVAC system is used to condition the crawl space air, we urge you to have that situation corrected (it is a very easy “fix”).

Finally, while crawl space encapsulation with dehumidification is the best and most effective way to insure that your crawl space is not a “mold factory”, it can be rather expensive. If, therefore, encapsulation is not financially feasible, it may well be that crawl space exhaust fans and other corrective measures may suffice in particular circumstances (i.e. depending on the specifics of the crawl space scenario).

For more information on crawl spaces and how to best address them, we strongly recommend the Crawl Space Ninja website:

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