Feature: Closed crawlspaces

Feature: Closed crawlspaces

By Maria Mauceri on 03/22/2010

Each year, tens of thousands of homes are built on crawlspace foundations in the Southeast. In most of these homes, the spaces are built with wall vents, which are supposed to reduce moisture problems — but recent research has shown this approach actually brings in more moisture than it lets out. Building scientists have consistently found that when warm, moist air (typical in humid climates) enters a crawlspace, it cools and significantly increases the relative humidity (RH).

High-moisture levels can cause many problems in homes, including:

  • Mold or moisture damage in the crawlspace or living area 
  • Musty odors in the living area 
  • Condensation on air-conditioning ductwork or equipment 
  • Condensation on crawlspace insulation, water pipes or truss plates 
  • Buckled hardwood floors
  • High humidity in the living area
  • Insect infestations 
  • Rot in wooden framing members

Research by Advanced Energy indicates that a properly closed crawlspace system (also known as a sealed or unvented crawl system) — with no vents to the outside — can provide greatly improved moisture control when properly installed. Depending on climate, duct location and insulation placement, closed crawlspaces can also provide significant energy savings.

From 2001 to 2005, Advanced Energy conducted a study in North Carolina to test the theory that the closed crawlspace design really can solve moisture problems and save energy. The study compared three groups of homes with different crawlspace designs: one group of homes with wall-vented crawlspaces and two groups of homes with two different closed crawlspace designs. The study showed that RH in the wall-vented crawlspaces often exceeded 80 percent in the summer months. On the other hand, the RH in closed crawlspaces barely exceeded 60 percent and reduced energy consumption by 15 to 18 percent.

The results of the research supported a complete revision of the North Carolina Residential Code to allow closed crawlspaces and improve minimum requirements for vented crawlspaces.

In a more recent study, Advanced Energy showed that closed crawlspaces are a great moisture-control strategy and can reduce energy consumption in varying climates nationwide.

An increase in complaints and legal action related to mold growth in homes has made homeowners and builders more aware of the need to control moisture levels. More and more often, homeowners and builders are investing the additional time and money to install closed crawlspaces in both new and existing homes.

It is critical, however, that professionals interested in installing closed crawlspaces choose the right materials and tools and get the training necessary to ensure proper installation.

Recommended design components include:

  • Moisture management
  • Pest control 
  • Combustion safety 
  • Thermal insulation 
  • Fire safety 
  • Radon safety

An Advanced Energy publication — Closed Crawl Spaces: An Introduction to the Southeast —features sample closed crawlspace designs and descriptions of each design component. Altogether, the design components presented are not definitive specifications, but they have proven their performance in real-world field tests. To ensure success, installers will likely adjust designs to accommodate local site conditions, code requirements, home design, construction processes and occupant needs.

The biggest challenge to getting a properly closed crawlspace is finding a qualified installer. Advanced Energy is developing a training curriculum that it plans to offer through electric utilities and other building groups.

The cost for a closed crawlspace in new North Carolina homes is typically less than $2 per square foot. While the air quality and durability benefits alone can make this a worthwhile investment, an added benefit is that the energy savings may pay for the system well before the owners pay off the mortgage.

For more information about closed crawlspaces and Advanced Energy’s study results, visithttps://www.crawlspaces.org.

Advanced Energy is a Raleigh, N.C.-based nonprofit committed to a future in which energy needs are met at reasonable costs and with the least negative consequences. The organization continues to work collaboratively to demonstrate that industry, government and nonprofits can successfully work together to improve the environment and encourage the economy. For almost 30 years, Advanced Energy has created economic, environmental and societal benefits through innovative and market-based approaches to energy issues. For more information, visit https://www.advancedenergy.org.

[Maria Mauceri has been a building science associate with Advanced Energy’s Applied Building Science team since 2006. She provides critical analysis of residential structures, conducts field research, data collection and diagnostic testing. Contact Mauceri or any of Advanced Energy’s on-staff building science experts at (919) 857-9000 or moreinfo@advancedenergy.org.]