Walk around almost any older house in the Puget Sound and you'll see small screened openings along the foundation. Those are crawl space vents, and homeowners ask us all the time whether to keep them open or close them up.
What vents are supposed to do
The idea is simple: let outside air move through the crawl space to carry away moisture coming up from the ground. For decades, building codes required them, and most homes here were built with them.
Where it gets complicated
Vents work best when the outside air is drier than the crawl space air. In our winters, outdoor air is often cold and damp. In summer, warm, humid air can come in and condense on cooler surfaces under the house. Vents help some of the time, and not much the rest of the time.
They're also entry points. A vent with a torn or rusted screen is one of the most common ways rats get under a house.
What matters more than the vents
The biggest source of moisture in a crawl space is the ground itself. A dirt floor gives off water vapor all year. A good ground cover, at least 6-mil polyethylene, overlapped and sealed, stops most of it. In a vented crawl space, that's the single most important thing.
So, open or closed?
- If you have a standard vented crawl space: keep the vents open and screened, and make sure the ground cover is in good shape.
- Don't just block the vents with foam or plywood. Without a plan for humidity, that can make things worse.
- If you want the vents closed, that's encapsulation: a sealed liner, sealed walls, and humidity control.
Not sure what's going on under your house? We'll inspect it and tell you. See vapor barriers and encapsulation.