This is the final article in a two-part series on crawl spaces in residential construction.

Toxic mold is a monumental problem for homeowners, commercial property managers, schools and insurance companies. Moisture trapped in wall cavities promotes the growth of fungi and other microbial contaminants, which can result in significant health problems and major property damage. Billions of dollars are spent each year to treat the symptoms. However, unless negative building pressure is corrected, the mold will return.

Indoor air quality professionals must be able to identify moisture pathways into buildings; however, they often overlook the crucial role that negative air pressure plays in causing mold problems by forcing moisture into a building.

Barry Westbrook, founder and President of DocAir (www.docair.net), an indoor air quality consulting firm, said, “The volume of air entering a building must equal the volume of air leaving. This is basic physics. Because homes, offices and schools are constructed more tightly than ever to save energy, air lost to the outside through various routes (often through ventilation fans) results in negative air pressure. Negative air pressure sets the stage for a number of IAQ problems, such as excess moisture, mold and even radon gas.

The problem

There are three things that destroy materials in general, and wood in particular — water, heat and ultraviolet radiation. Water is by far the most important. A dirt crawl space under a home is a very bad idea. The earth has very high humidity in the soil. This water vapor moves easily into the crawl space and upwards into the house. Mold, insects and structural damage are the consequences of moisture intrusion.

Mold thrives and reproduces by producing airborne spores by the millions, and some molds are toxic. Mold spores are in the air everywhere. All they need to grow is organic material such as wood, the right temperature range and moisture. Dirt crawl spaces have the humidity necessary for mold growth. Humidity levels from 50 percent to 90 percent are easily found in dirt crawl spaces that have never flooded.

Mold can grow on dirt, insulation, framing and even under carpet. Mold destroys organic materials as it feeds on them. Mold reproduces by producing millions of floating airborne spores, which some people are very sensitive to. Spores can be dormant for many years, waiting for the right conditions to grow. Some molds produce mycotoxins, such as the black mold Stachybotrys. There is a solution. Mold will not grow when the humidity levels are reduced to below 40 percent.

The natural airflow in a house is from bottom to top. This sucks the moist air and everything in it up into the living areas of the home. Mold spores, odors, humidity and critters create a very unhealthy environment for people. Many people are allergic to these things, and don’t realize that their dirt crawl space is affecting their health.

Dirt crawl spaces are great places for bugs to live and reproduce. Insects and critters of all kinds love damp environments and wet materials. Rot and decay occur in damp environments causing structural damage.

Moisture is the prime factor that causes structural damage. Over time, rot and decay from a moist environment damage framing and steel structural members. Dirt crawl spaces do not have to be wet or flooded to be extremely unhealthy. Heating and cooling costs are higher in a home with a vented crawl space.

The solution

The encapsulated crawlspace can be combined with other devices and techniques to prevent indoor air quality problems. A dehumidifier equipped with a humidistat can be placed in the crawl space to further protect the homeowner against moisture intrusion. With a few minor adjustments to the HVAC system, the home can be placed under positive air pressure, meaning conditioned air is forced out. In many homes, negative building pressure actually sucks air from the crawl space into the home, providing an avenue for mold spores and radon gas to enter. Under positive pressure, these potentially harmful agents are forced to the outside and exit through the foundation vents or the building skin.

Until the construction industry fully grasps the concept of relative humidity and the damage that vented crawl spaces can cause, homes will continue to be built with foundation vents. The best way to prevent moisture problems in your crawl space is to encapsulate it. Crawl space condensation is both costly and preventable. Ask yourself, what reason is there to allow your home to rot away beneath your feet?

Don McGonagil, The Home Inspection Company 615.582.2296 www.thehomeinspectioncompany.com

donmcgonagil@comcast.net