This is the first article in a two-part series on crawl spaces in residential construction.
Read part two at Crawl-Space-Mold.

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 Certified Industrial Hygienist Barry Westbrook, president of DocAir (www.docair.net), 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