Indoor Air Quality and The Stack Effect

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. 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 65% of the air on the first floor in a home originates from the crawl space or basement.


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. 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 eventually structural damage. 

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.

Common Moisture Related Air Quality Problems: 


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 also 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.


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.


The level of indoor air humidity affects the indoor levels of house dust mites and house dust mite allergens. These mites are microscopic arthropods (a family of related animals) that live indoors and feed on skin flakes and other organic materials in dust.

Bedding, upholstered furniture, and carpets are among the sites where dust mites live. The allergens, present primarily within the mite's fecal pellets or their fragments, may be inhaled when airborne. Dust mites do not drink water; they absorb water from the surrounding air. If the relative humidity of the air is maintained below approximately 50% for an extended period, mites will not survive. As humidity increases mite levels tend to increase.  However, it is the relative humidity in the microenvironment around the mites, not the average humidity of indoor air, that affects dust mite levels and the relative humidity where mites live can be higher than the average indoor humidity. For example, the relative humidity near a carpeted floor located over a cold, damp crawl space will be higher than the relative humidity in the air at the center of a room.  Thus, high relative humidity in the crawl space promotes the growth of dust mites and associated allergens through higher indoor air humidity levels.