Is Crawl Space Encapsulation Worth It in the Pacific Northwest?

Sometimes, and less often than the sales pitch suggests. The research that best matches this region, a Washington State University study of crawl spaces in Washington, found that with proper site water management, both sealed and vented crawl spaces can be built successfully in the Northwest. So the honest order is: fix where the water comes from, get a proper ground cover down, and seal the crawl space when there is a specific reason to.

We do encapsulation, so we have every reason to sell it. Here is when we would tell you not to, and when we would.

Why the national advice does not fit Whatcom County

The federal warnings about vented crawl spaces are framed for humid climates. The Department of Energy explains that vents were meant to let outside air dry the space, but in humid climates the outside air is often wetter than the crawl space air, and that has led to structural, mold and pest problems.

Whatcom County is not that climate. Washington’s energy code places it in climate zone 4C, where the C means marine. Under NOAA’s 1991 to 2020 climate normals, Bellingham International Airport averages about 35 inches of precipitation a year, and about half of it falls from October through January. Summers there are dry: July averages under an inch. That pattern matters. The WSU researchers credited relatively dry summers with low dew points for letting vented crawl spaces perform adequately.

Federal guidance agrees. EPA’s Indoor airPLUS program says crawl spaces should be insulated, sealed and conditioned, but lists homes in a marine climate among the exceptions. That program does not call for a sealed crawl space in Bellingham.

One caution: the WSU study monitored four new, code-built homes per climate for about 18 months. An older Bellingham house with a torn ground cover, a plumbing leak or a hillside sending runoff toward it may behave differently.

What the case for sealing gets right

Sealing a crawl space is not a gimmick. It changes how the space behaves.

Steadier conditions

In the WSU study, sealed crawl spaces ran warmer, with steadier temperatures and slightly lower humidity, and never reached dew point. The vented ones reached it only rarely and briefly. DOE says that once a crawl space is insulated and air-sealed at the perimeter with a continuous sealed ground cover, its temperature and humidity stay stable and close to indoor conditions.

Less crawl space air in the house

In a heated house, warm air escapes near the top and pulls outdoor air in through lower parts of the building. One Washington study measured about 40 percent of winter house air coming up from a vented crawl space. In the sealed, power-vented crawl spaces in that test, the share fell to about 5 or 6 percent, but that depended on an exhaust fan running all the time, and the authors note the benefit is lost if the fan fails.

An expert view

Building science researcher Joseph Lstiburek argues that a crawl space works best either fully outside the house or fully inside it, as a conditioned space insulated at the walls rather than the floor. It is a respected view, though his example is Washington, DC, not a marine climate.

The downsides nobody puts in the brochure

It changes radon, and Washington code knows it

In the WSU study, radon in sealed marine-climate crawl spaces averaged almost nine times the level in vented ones, which the authors linked to less dilution and to running the sealed crawl spaces under negative pressure. Those homes were in a higher-radon area than ours, and living areas tested below EPA’s action level. EPA rates Whatcom County as low potential for radon, but says every home should be tested regardless. Washington’s code requires a radon system in an unvented crawl space. A sealed crawl space is not a radon mitigation system. We recommend a radon test, and if levels are high, a certified radon mitigation company.

More moving parts

In Washington, a crawl space can be built without foundation vents only when the ground is covered with a continuous, sealed Class I vapor barrier that runs up and is sealed to the foundation wall, a radon system is installed, and an exhaust fan runs continuously to the outdoors at 1 cubic foot per minute for every 50 square feet of crawl space floor. That is a fan that has to keep running, and the WSU study noted that a continuously running crawl exhaust fan could have a significant energy cost.

Energy savings are not a given

The Washington study found the modeled energy impact of sealing a crawl space was minimal, and only significant when supply ducts in the crawl space were quite leaky. Savings depend on the house, especially where the ducts run and how leaky they are.

It does not fix water

A liner over a wet crawl space hides the problem. DOE’s guidance is that moisture problems must be taken care of before insulating, with a drainage system where there is a flooding or leak history and no working perimeter drain.

Should you encapsulate a crawlspace?

It is often worth it when

• Supply ducts run through the crawl space and leak, which is the case where the WSU modeling found sealing made a real energy difference
• You want the crawl space brought inside the house, insulated at the walls, with steadier conditions year round
• The drainage is fixed and you want to cut how much crawl space air reaches the house in winter, accepting that in the Washington test this depended on an exhaust fan running all the time

A vapor barrier is often enough when

• The crawl space is vented, the framing is dry, and there is no standing water
• The ground cover is missing, torn or too short at the walls
• Gutters, downspouts and grading are doing their job

It is someone else’s job when

• The foundation is cracking or settling: call a structural engineer
• There is extensive mold: call a mold remediation company
• There are live rodents: call a pest control company first
• A radon test comes back high: call a radon mitigation contractor

Fix these first, whatever you decide

EPA’s advice for any home applies under the house too: fix plumbing leaks as soon as possible, keep gutters clean and repaired, and make sure the ground slopes away from the foundation. DOE suggests piping downspout water to sloping grade more than 5 feet from the foundation. Floor insulation that has sagged off the subfloor needs putting back or replacing, because the energy code requires it to stay in contact with the subfloor.