Consider closing the crawl space

By Hannah McKenzie
Consider closing the crawl space

A fully closed crawl space with sealed walls and a polyethylene vapor retarder on the ground

Last month we talked about how a wet crawl space can lead to a musty-smelling home. By eliminating liquid water entry and controlling moisture inside the crawl space, the musty smells should subside. Your next thought should be considering closing the crawl space.

A closed crawl space is somewhat like a basement because it is an enclosed space beneath the home but not conditioned. Often they look like an empty swimming pool – with a drain as well as sheets of thick white plastic covering the floor and extending up the foundation walls and piers. You'll also hear the occasional hum of a dehumidifier or HVAC system that keeps the crawl space air sufficiently dry.

Closed crawl spaces offer numerous benefits:

  • Additional moisture control, which means reduced opportunity for mold growth
  • Lower relative humidity within the home during summer months
  • Less dry-feeling air during winter months
  • Fewer opportunities for pests to enter the home
  • Potential improvement of the indoor air quality
  • And if your HVAC system is in the crawlspace, possible savings of up to 15 percent on your heating and cooling bills

It is an all-too-common experience in the building profession to enter a wall-vented crawl space during a Carolina spring or summer and find beads of moisture in the floor insulation, high wood moisture content, visible mold growing on surfaces, condensation on metal truss plates, plumbing pipes, air conditioning equipment or ducts, and in some cases, rot in the wood framing. Homeowners often complain of high humidity, musty odors, buckled hardwood floors, and mold damage in the home above.

Like replacement windows, the expense of closing a crawl space on an existing home can rarely be justified by energy savings. However your comfort and the long-term durability of your home are strong arguments for the effort.

In a five-year research study that began in 2001, Advanced Energy documented the benefits of closed crawl spaces in 12 central North Carolina homes. A large quantity of data was gathered, and homeowners were interviewed. As we suspected, moisture levels and mold growth were drastically reduced in homes with closed crawl spaces. The study also revealed energy savings if the HVAC system was located in the crawl space. One homeowner was delighted to report that the day after her crawl space was closed she had to adjust the thermostat from its usual position because she was cold in her home on a hot day! This was something that had never happened to her before. Closing her home's crawl space had lowered the relative humidity inside her home, thus making her more comfortable with the air conditioner set to a higher temperature.

Because of this study and a lot of community team work, the North Carolina Building Code Council adopted new crawl space code language in December 2004 and approved the installation of closed crawl spaces. The popularity of closed crawl spaces has been steadily increasing in existing and new homes since that time.

To be successful, a closed crawl space must consist of parts that work together as a system to control the variety of water sources.

  • Exterior water management prevents intrusion of liquid water
  • Air-sealed foundation walls minimize the entry of humid outside air
  • Vapor retarders such as polyethylene sheeting minimize the evaporation of water from the ground or perimeter walls
  • Mechanical drying systems provide ongoing, active removal of water vapor
  • Drains or pumps remove water coming from plumbing leaks or floods

These components are not like a cafeteria meal, where you pick and choose what suits your mood or pocketbook. They all must be included for the closed crawl space to offer full benefit and not cause harm. Pest control, combustion equipment, insulation, fire safety, and radon must also be addressed to ensure success.

All of the parts of a closed crawl space are discussed in detail at www.crawlspaces.org. The website also includes research findings, construction tips and closed crawl space contractors.

As a native North Carolinian, I love humidity but I don't want to experience it inside my home. Now you, too, know how to make a change.

About the Author

Hannah McKenzie is a residential building science consultant at Advanced Energy in Raleigh who specializes in working with nonprofit developers like Habitat for Humanity to make new affordable housing energy efficient.

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