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Commentary by Don McGonagil
May 07, 2004

This is the final article in a two-part series on crawl spaces in residential construction. For more information about DocAir’s Proof Positive™ technology designed to solve the crawl space moisture problem, please contact us.

Toxic Mold

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.

DocAir (www.docair.com), 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

For more information about DocAir’s Proof Positive™ technology designed to solve the crawl space moisture problem, please contact us.

Commentary by Don McGonagil
May 07, 2004

This is the first article in a two-part series on crawl spaces in residential construction. For more information about DocAir’s Proof Positive™ technology designed to solve the crawl space moisture problem, please contact us.

The Myth

Crawlspaces in building construction have been widely accepted for years. They have enabled homebuilders to provide a level building surface for sloped lots cheaply, and they are a convenient place for plumbing, electrical conduits, and heating, ventilation, and air conditioning (HVAC) systems. However, they are a frequent source of moisture, mold, and other indoor air quality problems.

Homebuilders have long known that moisture from outside sources can accumulate in crawlspaces, so nearly all homes built with crawlspaces in the last 75 years have vents installed in the foundation walls. The idea is that the passive ventilation will allow humid, musty air to escape to the outside. It sounds like a good idea on paper, but in reality, vented crawlspaces create many more moisture problems than they solve. Understanding how vented crawlspaces work requires little more than grade-school science, but homebuilders and homeowners alike have failed to grasp the ramifications of the subtle mechanisms at work underneath their feet.

According to DocAir (www.docair.com), a building systems and indoor air quality consulting firm, “The concept of relative humidity is the single most important issue that should be considered by homebuilders when building a home with a crawlspace.” Relative humidity is defined as the amount of water contained in the air at a specific temperature. As air cools, it loses its ability to hold moisture. When air contains all the water it can hold, it is said to be at 100 percent relative humidity, or saturated. The temperature of air at the saturation point is called the dew point. At the saturation point, moisture will begin to condense onto any surface whose temperature is at or below the dew point.

Vented crawlspaces tend to be several degrees cooler than the outside air due to shading and contact with surfaces cooled by HVAC systems in the living space above. During humid Southern summers, the relative humidity of outside air is often 70 percent or more. As this outside air moves into the crawlspace through the foundation vents, it quickly cools. If the temperature of the crawlspace or surfaces within the crawlspace is at or below the dew point of the outside air, condensation will occur. Slowly but surely, this moisture will accumulate inside the crawlspace, fostering mold growth, wood rot and a host of other problems.

Many crawlspaces have conditions that actually accelerate this process. Un-insulated floors, torn or damaged ductwork insulation, and leaky supply ducts lead to colder surfaces, meaning increased condensation. In effect, the crawlspaces of many homes actually serve as dehumidifiers for the outside air. The cold sub-floor and/or exposed metal ductwork act like the coils in your household dehumidifier, and the crawlspace itself becomes the drip pan. By encouraging the movement of humid air into the crawlspace, foundation vents actually increase the chances of having unwanted moisture build up underneath the home.

The Solution

First, all exposed dirt inside the crawlspace must be completely covered with a vapor barrier made of at least 6-mil polyethylene. Many homes have vapor barriers, but few are installed properly. The joints must all be taped and sealed. The barrier should be extended up the foundation walls, covering the vents. Ideally, the vapor barrier would extend across the floor joists, resulting in a completely encapsulated crawlspace. Any moisture that percolates up through the soil moves along the underside of the vapor barrier and can exit through the foundation vents. Humid outdoor air can no longer reach wooden surfaces underneath the home. In areas where radon gas is a problem, the encapsulated crawlspace is also useful in that any soil gases originating underneath the home are routed harmlessly to the foundation vents without ever having the chance to enter the living space of the home.

Don McGonagil
The Home Inspection Company
(615) 582.2296
www.thehomeinspectioncompany.com
donmcgonagil@comcast.net

For more information about DocAir’s Proof Positive™ technology designed to solve the crawl space moisture problem, please contact us.

Doc Air provides a fresh and innovative approach to solving indoor air quality problems. One size-fits-all solutions are a headache you don’t need. We design and install custom solutions for residential and commercial clients that are practical, cost-effective, and guaranteed to work. 

Doc Air applies our knowledge of building physics and the best-in-breed construction methods to create the optimum in energy effiency, air quality, and sustainability. 

Your family need not settle for less. Breathe the fresh DocAir!