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Crawl Space

Closing Homes Crawlspace Helps Air Quality

The remedy for indoor air quality concerns may be below your house.

Most houses in this region of the country are built on a crawlspace – “an open area under the home that allows for grade changes and room for plumbing and mechanical systems.”

To allow for ventilation of these crawlspaces, vents are installed through the walls to the outside. This allows for air circulation and to replace stale air with fresh air from the outside. This system is commonly referred to as a “vented crawlspace.”

Recent studies have found that foundation vents bring in more moisture than they let out. A type of crawlspace system known as a “closed crawlspace” has no vents to the outside, and can provide greatly improved moisture control and significant energy savings.

Excessive moisture in a vented crawlspace is the primary cause associated with a host of serious crawlspace concerns. It is estimated that 60 percent of the air you breathe on the ground floor of a home with a vented crawlspace home actually comes from the crawlspace. Common symptoms of excessive crawlspace moisture include:

  • Mold or moisture damage in the crawlspace or living area.
  • Musty odors in living areas.
  • Family members suffering from allergies, asthma or headaches.
  • Condensation on air-conditioning ductwork or equipment.
  • High humidity in the crawlspace and living area.
  • Condensation on insulation, water pipes or metal (rust) in the crawlspace.
  • Buckling hardwood floors.
  • Doors and windows that stick because of swelling.
  • Insect infestations.
  • Mold leading to rot in wooden framing members.

When warm, moist outside air enters a crawlspace, it instantly cools and drastically increases the relative humidity of the crawlspace. When the relative humidity goes over 100 percent, the moisture is released into the crawlspace atmosphere, with condensation accumulating on the walls, floors and building components.

Moisture can pass from a crawlspace into a building through cracks in walls, floors and ceilings. This movement of air is known as “stack effect,” a natural phenomenon of constant movement of air through the house. Here’s how it works in crawl spaces: When hot air rises, most of it finds ways to escape into the upper areas of a house and then outside. As the hot air leaves the building, cool air rushes in through leaky windows, doors and crawlspace vents to replace it and repeat the cycle.

The air in the crawlspace that gets sucked up into the living area brings with it moisture, dust, allergens, mold spores and radon. If the stack effect is causing air to be drawn into the house through the lower levels, then any possibility of meaningful cross-ventilation is negated.

According to Billy Tesh, owner of Pest Management Systems Inc., a Greensboro company that provides services including closed crawlspace installation, two factors have led to moisture problems in crawlspaces:

Air conditioning: Central air conditioning creates a larger difference in the temperature between the inside of the house and the crawlspace. Without air conditioning, the ambient temperature inside a home was about the same as in the crawlspace. This did not lead to condensation in the crawlspace. Since early air conditioning systems were not very efficient, most people did not experience major problems. Newer high-efficiency models, coupled with tighter building construction, have magnified the temperature difference that has led to many of the moisture problems today.

Waterproofing: In our area, waterproofing of foundations was not made a standard practice until 1985. As a result, many homes have moisture present in the crawlspaces because of poor grading, malfunctioning gutter systems (or no gutter system), and leaks from plumbing and waste systems. Most of these problems can be repaired with interior drains, downspout piping, waterproofing systems, and proper grading around the house.

Two primary methods used to close a home’s crawlspace are the wall insulated system and floor insulated system. Find out more about how to “close” your existing home’s crawlspace in next week’s New Triad Homes.

Meanwhile, you can review additional information about crawlspaces online. To learn more about research and building techniques, visit the Web site of Raleigh-based nonprofit Advanced Energy. Go to, click on “Buildings” in the top menu, then on “Knowledge Library” in the menu on the left side of the page. Indoor air quality and crawlspaces top the list of topics.

Scott Allred is chairman of the Triad Green Building Council and owner of Precept Construction.

He can be reached at or 286-6811.

For additional resources on green building, visit or

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