Closed vs Vented Crawl Spaces

The short answer

A vented crawl space is open to outdoor air through foundation vents. The theory is that airflow carries moisture out. In cold, dry weather it does.

A closed (sealed, unvented, encapsulated) crawl space has the vents shut, the ground and often the walls covered with a sealed liner, and the space brought inside the building's moisture-controlled envelope. It has to get its humidity managed some other way, because you have removed the mechanism that was doing it.

Neither is universally correct. But in a climate with mild, humid summers and a long wet winter, ventilation alone frequently fails to keep a crawl space dry, and can add moisture during warm weather. That is why modern codes — Oregon's included — permit an unvented crawl space, provided you replace ventilation with something that actually controls the air.

Why vents were required in the first place

Foundation vents solve a real problem. A crawl space over bare soil continuously receives moisture evaporating out of the ground. Without a way for that moisture to leave, it accumulates, condenses on framing and insulation, and creates conditions for mold and eventually decay.

Air exchange is one way to move it out, and for a house in a cold, dry winter climate it works well. Outdoor air in January holds very little moisture in absolute terms, so bringing it into the crawl space and letting it leave carries water vapor away with it. Nearly every Portland home built before the 1990s uses this approach, usually paired with no ground cover at all.

Why vents can work against you in this climate

The mechanism that dries a crawl space in winter can wet it in summer, and this is the part that surprises people.

Warm air holds substantially more water vapor than cold air. When warm, humid outdoor air enters a crawl space that has stayed cool — as crawl spaces do, being shaded, below grade, and in contact with earth that warms slowly — that air is cooled. As it cools toward its dew point, the moisture it carries can condense on the coolest surfaces available: ducts, pipes, the underside of the subfloor, the joists.

So the vent that dried the space in January can be delivering moisture into it in July. This is the core building-science argument for sealed crawl spaces, and it is why "just add more vents" is frequently the wrong response to a damp crawl space.

Two other realities in older Portland housing stock compound it. Vents get blocked by landscaping, stored material, or added insulation, so the ventilation that was designed often is not the ventilation that exists. And a vented crawl space in a heated house loses heat through the floor all winter, which is why cold floors and a vented crawl space so often appear together.

What Oregon code actually requires

The Oregon Residential Specialty Code addresses this in Section R408. It is worth reading the actual requirements rather than a contractor's summary of them, because the specifics decide what a legitimate bid contains.

If the crawl space stays vented (R408.1)

• Ventilation openings are required through foundation or exterior walls.
• The ground surface must be covered by a Class I vapor retarder or other approved material, lapped not less than 12 inches at the joints and extended not less than 12 inches up perimeter foundation walls.
• Minimum net ventilation area is not less than 1 square foot for each 150 square feet of under-floor area.
• That may be reduced to 1 square foot for each 1,500 square feet where the ground is covered by the required Class I vapor retarder.

Note what that last point means: covering the soil lets you cut required ventilation by a factor of ten. The code already treats ground cover as doing most of the moisture-control work.

If the crawl space is closed (R408.3)

Ventilation openings are not required where both of the following are provided:

1. Exposed earth is covered with a continuous Class I vapor retarder. Joints overlap by 6 inches and are sealed or taped, or overlap a minimum of 12 inches where joints are not sealed or taped. Edges extend not less than 12 inches up the stem wall and are attached and sealed to the stem wall or insulation.
2. One of these three for the under-floor space:
   • Continuously operated mechanical exhaust ventilation at 1 cfm per 50 square feet of crawl space floor area, including an air pathway to the common area, and perimeter walls insulated per the code's insulation section; or
   • A conditioned air supply sized at 1 cfm per 50 square feet, including a return air pathway, and perimeter walls insulated; or
   • Dehumidification sized to provide 70 pints of moisture removal per day for every 1,000 square feet of crawl space floor area.

This is the single most useful thing on this page for evaluating a quote. "Encapsulation" that seals the vents and provides none of those three is not a complete system under any of the code's paths. If a bid closes the vents and does not name an exhaust rate, a supply air rate, or a dehumidifier capacity, that is the question to ask.

The Portland metro radon exception

Oregon adds provisions that apply with unusual directness to this region, and they turn on one list of counties: Baker, Clackamas, Hood River, Multnomah, Polk, Washington and Yamhill, where radon-mitigating construction is required.

In those counties, for new construction, the 2023 ORSC does not permit the reduced ventilation area for a naturally ventilated crawl space — the 1-in-1,500 allowance described above is off the table, and the crawl space is held to the full 1-in-150. Oregon has also restricted unvented crawl spaces in these counties absent an approved mechanical ventilation system meeting the radon appendix alongside the R408.3 requirements.

Confirm this one against the current code text. The radon provisions have been revised across recent editions and are the part of R408 most likely to have moved. The county list and the restriction are stated in the Building Codes Division material cited below; the exact operative wording in the edition applying to your project is worth reading directly, or asking your building department for.

Clackamas, Multnomah and Washington counties are the core of the Portland metro. The reasoning is straightforward: sealing a crawl space changes how soil gas moves into a house, and radon is a soil gas. Ventilation that was incidentally diluting it is no longer there.

Two things this does and does not mean. It governs new construction, so it is not a prohibition on retrofitting an existing Portland home — the code section that applies to your retrofit is R408.3 above. But it is a clear signal from the state that closing a crawl space in these counties is a decision with a radon dimension, and the sensible response for any homeowner here is to test for radon before sealing and again afterward. Test kits are inexpensive, and the Oregon Health Authority publishes guidance on testing and on what results warrant action.

Which is right for your house?

If you close it, what has to be true

A closed crawl space done properly is a durable improvement. Done partially, it is worse than the vented space it replaced, because you have removed the drying mechanism without installing its replacement. The non-negotiables:

• No active water intrusion. Drainage before liner, always.
• Continuous Class I vapor retarder over all exposed earth, sealed at seams and run up the stem walls per R408.3.
• One of the three humidity-control methods, correctly sized — not "the vents are shut and it will be fine."
• Perimeter wall insulation where the exhaust or supply-air paths are used.
• Combustion safety. If a gas appliance draws combustion air from the crawl space, sealing it changes that. This is a safety item, not a detail.
• Radon tested before and after, particularly in Clackamas, Multnomah and Washington counties.
• An access hatch you can still get through, and a plan to check the space periodically. Sealed does not mean forgotten.

In the Portland metro, a full closed-crawl-space system runs $10,100 to $12,325, reaching $14,875 to $20,400 with a dehumidifier and premium scope — see encapsulation cost for the breakdown, and how long encapsulation lasts if you are weighing that against its service life.

Portland Crawlspace Pros is a lead-matching service connecting homeowners with independent licensed contractors; we do not perform construction and we do not issue code interpretations. Confirm requirements for your specific project with your local building department, and verify any Oregon contractor's license with the Construction Contractors Board (how to do that, and what it does and does not prove). A free in-home inspection is how to find out which configuration your crawl space is actually a candidate for.