CRAWL SPACES
Crawl Space Foundations
A crawl space foundation is a shallow unfinished space beneath the first floor of a building that allows access to the wiring and plumbing. Often, the heating and cooling ductwork are also located here. The access to the crawl space is either from the interior of the home or from the outside, possibly from the garage area. Crawl spaces are seldom entered except by the occasional serviceman or home inspector.
Impact of the Crawl Space on Air Quality in a Home
A crawl space is one of the more important areas of a building, yet one of the most neglected and misunderstood areas. Although not part of the living or occupied area, it is part of the building envelope, and thus has a profound impact on the air quality in the occupied area of the building. As air warms within the building envelope, it moves upward (Figure 1). This is known as the stack effect. The air within a crawl space travels upward, infiltrating the living area through the ductwork, gaps in the flooring, through and around holes from wiring, conduit, and plumbing. If there are indoor air quality issues in the crawl space, they will impact the living areas in the floor above. Up to 40% of the air on the first floor in a home originates from the crawl space or basement.
The Problem: Moisture
Crawl spaces are susceptible to moisture intrusion and excessive relative humidity by virtue of their design. The real culprit is water vapor, or moisture in the air, often measured and reported as the relative humidity (RH). A RH <60% will keep mold from growing. There are numerous entry points for moisture intrusion into a crawl space (Figure 2). The crawl space design is fundamentally flawed and prone to failure and to facilitate mold growth unless steps are taken to keep moisture out..
Occasional or mild to moderate moisture problems will result primarily in air quality problems due to mold growth. Chronically damp crawl spaces will result in air quality problems due to mold growth, but also will allow the wood decay fungi to thrive, resulting in wood rotting, and eventual structural related issues.
Moisture intrusion into a crawl space can occur from flooding, but a tremendous amount of moisture enters the crawl space as water vapor, or moisture in the air. This often is a major source of moisture, driving mold growth, air quality problems, and structural issues. Water vapor enters the crawl space through the ground, the concrete block walls, and through the crawl space vents.
So, moisture intrusion (liquid water or water vapor) occurs at several entry points in a crawl space:
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While excessive moisture is the root cause of most crawl space problems, homeowners often just deal with the symptoms of excessive moisture, rather than with the problem itself.
Common Moisture Related Air Quality Problems
Mold
Mold in a crawl space is probably the number one symptom we see. Where there is mold, there is, or has been, excessive moisture. Moisture intrusion into a crawl space can be evaluated by a visual inspection and by measuring the RH of the air in the crawl space. A visual inspection will determine whether there is standing water or whether flooding has occurred in the past. The RH can be measured, providing a snapshot of the actual conditions existing at the time of the inspection. The RH should be maintained at <60% to inhibit the growth of mold. When it ranges from 60% to 65%, mold growth is possible, and when it exceeds 65% mold growth is probable. The extent of the mold growth is influenced by several factors, but two important ones are how far the RH exceed 65% and for how long a period of time.
In the presence of moisture, mold grows in the ground and/or on the wood components in the crawl space. Mold reproduces by forming spores (like tiny seeds), and disseminating these spores into the air – a common means of exposure for people.
Odors
Moisture allows mold to grow. When mold grows, a by-product of their metabolism is mold volatile organic compounds (MVOCs). These compounds are gaseous and enter the air, and are responsible for the mildewy or moldy odors associated with damp environments. Sometimes homes with a damp crawl space will smell musty or have an “earthy” odor. This moldy odor can infiltrate into the living or occupied areas of a home.
| Common Moisture Related Structural Problems |
Crawl spaces that are chronically wet or damp will develop a problem in addition to mold growth. Excessive moisture and a chronically moist environment will cause the moisture content of wood to increase. Wood with a moisture content of 19% or greater provides the perfect conditions for wood decay fungi to grow. The activity of wood decay fungi results in wood rotting, destroying the structural integrity of the wood components in the crawl space. Sagging and rotting sub-floors, weakened and rotting floor joists, rotting band joists, and rotted and crushed sill plates are all symptoms of extremely moist crawl spaces.
Sealing or Encapsulating the Crawl Space
Several steps may be necessary to control excessive moisture in a crawl space. First, crawl space flooding must be dealt with. Depending on the extent and source of the flooding, several options are available to the homeowner for dealing with the flooding, including: installing a gutter system on the home’s roof, improving drainage around the home , installing a sump system, and installing drainage tile in the crawl space that collects ground water and direct it to the sump system.
Figure 3. and Figure 4. Crawl space with dirt floor
After addressing crawl space flooding, the crawl spaces should be sealed or encapsulated with a vapor barrier to minimize moisture intrusion to manageable levels. Surprisingly, some homes, even new homes, have only a dirt crawl space floor (Figure 3 and Figure 4). This is a serious construction deficiency that will have long term negative consequences for the homeowner.
Simply lining the crawl space floor with 6 mil plastic sheeting, or visquene, is not sufficient protection against moisture intrusion (Figure 5). In terms of a vapor barrier, unsealed plastic sheeting is only marginally effective at keeping moisture out of a crawl space. Moisture, as water vapor, enters around the perimeter or edges of the plastic sheeting when it is not sealed, through the crawl space block walls, and through the crawl space vents. If the crawl space floods at all, the water will often get on top of the plastic sheeting and become trapped, prolonging the time that the crawl space is exposed to an excessive relative humidity. Often, large areas of a crawl space floor are not covered with plastic sheeting. The exposed ground allows more moisture to enter the crawl space as well.
Figure 5. Crawl space floor lined with black
plastic sheeting
Properly sealing or encapsulating the crawl space completely seals the crawl space floor and crawl space walls, stopping moisture intrusion into the crawl space through these areas (Figures 6 & 7). The edges or perimeter of the vapor barrier is sealed to the block wall and all seams are taped with an industrial sealing tape, forming a good barrier that stops moisture intrusion into the crawl space. Vents are sealed, stopping hot humid air from entering the crawl space during the summer. During the winter, the sealed vents keep cold winter air out, keeping your home warmer
Figure 6 and Figure 7. Crawl space sealed on the perimeter with white vapor barrier.
Additional Reading/Resources
Sealing or encapsulating a crawl space is an important step homeowners can take to prevent mold growth in a crawl space. Unless these steps are taken, after a period of time, mold will grow. How extensive the mold growth is and whether any wood rotting has occurred is dependent upon numerous other factors that were discussed to some extent above.
While we are huge proponents of encapsulated or sealed crawl spaces, this is a relatively new way of thinking. It is probably fair to say that the majority of people involved in the residential or small commercial construction industry are unaware of these changes in the thinking about keeping a crawl space dry. In fact, some involved in the industry may realize that crawl spaces are damp and that they should be drier, but their advice is to install additional crawl space vents or add power ventilators in a attempt to exhaust the moisture from the crawl space. However, these efforts only serve to worsen the moisture problem and the symptoms associated with excess moisture.
To learn more about IAQ and the crawl space, visit our Resources and Links page.