Foundation Vents:
Open, Closed, Blocked, Buried or Missing Their Screens

What foundation vents are for

A crawlspace sits on bare ground (ideally covered with a plastic vapor barrier). That ground gives off moisture all year. Foundation vents let outside air move through the crawlspace and carry some of that moisture out, so it doesn’t end up in the joists, the subfloor and the insulation.

The residential code used in Oregon and Washington sets a minimum: 1 square foot of net vent area for every 150 square feet of crawlspace floor, reduced with a vapor barrier. It also calls for a vent within 3 feet of each corner, so air crosses the whole space instead of pooling in dead corners.

Russ wrote up crawlspaces with too few vents in Portland, vents along only one wall in another Portland house, and a crawlspace area under a Portland bathroom with no vents at all.

Closed and plugged vents

Closed vents were the most common finding. Some vents have built-in doors; others get stuffed with foam blocks, insulation or plywood. Owners close them in winter to keep the floor and pipes warmer, and they often never get opened again.

Russ’s report wording leaves room for that: while vent closures may be intentional during colder months, closed vents reduce airflow and can trap moisture. In Sandy, the vents were plugged outright. In Vancouver, fallen floor insulation was blocking the vents from inside. The post on crawlspace moisture covers what too little airflow leads to.

A sealed or conditioned crawlspace is a different thing. It’s built with no vents, a sealed liner, insulated walls and conditioned air or a dehumidifier. Closing the vents on a vented crawlspace doesn’t make it one.

Screens, rodents and debris

Every vent needs a screen. The residential code calls for corrosion-resistant mesh with openings no larger than 1/4 inch. The CDC notes rats can get through holes larger than about half an inch.

This is where most vent findings land. Damaged or loose screens in Troutdale, Vancouver, Ridgefield, Fairview and many more. Missing screens in Portland. Oversized mesh in Beaverton. A poor screen repair around a gas line in Troutdale. A vent installed backwards in La Center, with the exterior face pointing into the crawlspace. Russ recommends repair because damaged vent mesh can let rodents into the crawlspace. The post on rodent activity in crawlspaces covers what they leave behind.

Debris was the other one: leaves, soil and bark in the vent openings in Troutdale, Vancouver and Wilsonville. A vent full of bark mulch isn’t venting.

Vents at or below grade

When the soil, a flower bed or a patio is higher than the bottom of a vent, rain and soil go straight into the crawlspace. Russ found vents below grade in Lake Oswego and Beaverton, and moisture at the vents inside crawlspaces in Portland and Gresham. In Portland and Lake Oswego, the stem wall under the vents was stained where water had been running in.

A vent below grade needs a vent well, a curved liner that holds the soil back. Russ found vent wells that weren’t secured in Vancouver and wells built from untreated wood in Fairview, which will rot in the ground. In Vancouver, a patio slab poured above the vent line blocked most of the openings along that wall. The post on crawlspace water intrusion covers the grading side.

What Russ checks

He walks the outside of the foundation and looks at each visible vent for a screen, blockage, closure and its height above the soil. Inside, he notes vent locations, staining under vents, insulation or debris blocking them, and signs of animals. He doesn’t calculate net vent area. Most vent repairs are simple; persistent moisture and rodent problems go to a qualified contractor or pest control.

For the rest of the space under the house, the guide to what to look at in the crawlspace before you make an offer covers vapor barriers, insulation and framing. To set up an inspection, you can book online.