Should crawl spaces be vented? It's up for debate

April 08, 2007

I was speaking with a builder friend of mine recently, and he mentioned he was working on a new house in Cowlitz County with a sealed crawl space that did not have vents in the foundation wall. I thought crawl-space venting was a code requirement, but he said it wasn't a problem. Doesn't venting the crawl space prevent moisture accumulation and help avoid mold and mildew problems in houses? Have the codes changed to allow unvented crawl spaces?

You're right - for years, building codes have required a minimum level of ventilation in a home's crawl space for moisture control. The International Residential Code, the current building code for Washington state, continues to require crawl-space ventilation, although it allows several options for providing it. In some instances, local code officials may interpret the code differently.

Several years ago, a study conducted by Advanced Energy, a Raleigh, N.C.-based nonprofit firm, created a stir in the home building industry by providing scientific evidence of the benefits of sealing crawl spaces for the control of energy and moisture in humid climates.

Research on 12 identical homes in North Carolina between 2001 and 2004 showed significantly reduced crawl-space moisture condensation and energy savings of 15 percent to 18 percent. While the study is not complete, it did hold some promise for new techniques for building less-moisture-prone, more energy-efficient homes.

While this study did not affect the building code, it was enough to convince code officials in some jurisdictions to allow builders to build homes with unvented crawl spaces.

One builder, New Tradition Homes of Vancouver, wondered if sealing up crawl-space vents in homes built in the Pacific Northwest would produce similar results.

"We had heard the buzz at home builder conventions and read stories in trade publications about sealed crawl spaces and wondered if it would work in our homes as well," says Steve Tapio, quality control and building science team lead for New Tradition.

"We were put in touch with the Washington State University Energy Program. They were looking for a builder-partner for a conditioned crawl-spaces study they were doing. One thing led to another, and we are now evaluating energy use and moisture in the crawl spaces of four of our houses here in Clark County," Tapio says.

The research study, "Indoor Air Quality and Energy Effects of Conditioned Crawl Spaces in a Maritime Climate," being conducted by WSU with funding from the Department of Energy's Building America project, examines the moisture impacts of crawl spaces in vented-unconditioned versus unvented-conditioned configurations.

Another aspect of the study is to determine the energy impacts of placing ductwork for forced-air heating systems inside a conditioned space versus leaving it in a vented, unconditioned crawl space.

"This is a two-year study, and it's too early to determine anything categorically concerning moisture, but early results suggest strongly that there are pretty significant energy impacts to getting the ducts out of the vented crawl space," claims Michael Lubliner, a building science specialist with WSU.

"We have known for a while that poorly insulated and leaky ducts in the crawl space and attic can impact heating and cooling costs by up to 40 percent. The research seems to bear this out, and it appears that just moving the ducts into conditioned space, whether it's in a conditioned crawl space or into the house, can have a big impact on heating and cooling costs," Lubliner adds.

David Hales, WSU project manager for the study, confirms that it's too early to know whether sealing the crawl space will have an impact on moisture.

"These are new houses, which tend to have a lot of moisture in them anyway. We are not seeing problem moisture levels in the crawl spaces at this point, but it's just too early to tell. We'll have a better idea after the houses dry out and we get second-year data," Hales says.

Lubliner surmises that if sealing crawl-space vents proves not to increase moisture levels in the crawl space or house, homeowners with leaky ductwork would be better off sealing their foundation vents.

"We should know in a couple of years whether the North Carolina experience holds true for houses in the Northwest as well," Lubliner says.