Closed (Sealed) Crawl Spaces

This BLOG presented with permission from  Metrospec Services LLC

Open (wall-vented) crawl space foundations are widely used in building construction throughout the United States, with approximately 250,000 new homes built on crawl spaces every year and an estimated 26 million homes already in existence.

Crawl spaces are inexpensive to build, functional in terms of providing a level foundation for on sloping sites, and convenient spaces in which to locate mechanical systems. Unfortunately, traditional wall-vented crawl spaces can also host serious moisture issues, as well as a variety of other problems. It is an all too common problem for the building inspection professional to enter a wall-vented crawl space and find beads of moisture on the floor insulation above, high wood moisture content in the substructure and visible mold growing on surfaces. Homeowners can complain of high humidity, musty odors, buckled hardwood floors, and mold damage in the home.

Since the 1940s, residential building codes and conventional wisdom have prescribed passive ventilation from the outside as the means for providing crawl space moisture control. However, in our humid climate, this typically contributes to moisture problems rather than preventing them. The expense of moisture repairs and the increase in complaints, let alone legal action related to mold growth in homes has made homeowners more aware of the need to control moisture. This awareness is helping to drive demand from a growing number of consumers to invest additional efforts to incorporate closed crawl (sealed) space systems in both new and existing homes. The moisture control provided by a properly closed crawl space can dramatically reduce the risk and associated liability of mold and moisture damage.

As a performance goal, crawl space foundations should certainly be free of all rot and ideally should also minimize the conditions that lead to moisture and mold growth. While well built wall-vented crawl spaces may be able to control rot, they cannot maintain relative humidity below the 70% threshold that supports mold growth. They also routinely experience condensation on a variety of surfaces in the crawl space. Closed crawl spaces can maintain relative humidity below the 70% target and dramatically reduce the potential for high moisture on surfaces in the crawl space.

Key Components of a closed crawl space

• Exterior water management systems to prevent intrusion of liquid water (bulk water)

• Air sealed walls to minimize the entry of humid outside air (water vapor)

• Vapor retarders to minimize the evaporation of water from the ground or perimeter wall (capillary)

• Mechanical drying systems to provide ongoing, active removal of humidity

  The term "closed" is used to describe the alternative to a wall-vented crawl space design, but several other terms are commonly used.

"Unvented" In fact, several closed crawl space designs are vented, just not with outside air. "Sealed" The N C residential code does not define a closed crawl space as requiring a sealed vapor barrier but to accomplish the goals of moisture control it is certainly needed.

"Conditioned" Not all closed crawl spaces are conditioned. A closed crawl space that is truly conditioned must have insulation located at the perimeter wall and a thermal barrier covering any foam plastic insulation as well as a sealed vapor barrier.

The words "closed," "sealed," "unvented" or "conditioned" will likely continue to be used interchangeably to refer to a variety of crawl space designs that do not have ventilation with outside air.