Should Crawl Space Vents Be Open or Closed?

Closed. All year. Vented crawl spaces are a disproven building practice, and modern science is clear on it. The only way to improve the air quality in your home is to isolate that environment from the rest of the world so you can control it, and you cannot control an environment that is open to the outdoors.

I know that contradicts what a lot of people were taught. Here is what changed.

Where the old advice came from

The historic explanation was to open your crawl space vents in the summer to allow for airflow, then close them in the winter to keep your plumbing from freezing.

That made a certain amount of sense in an older house. It does not make sense in most houses built in the last several decades, and the reason is sitting right under your floor.

What changed: the HVAC unit under your house

Modern homes mostly have their HVAC system in the crawl space. That changes two things at once.

First, it eliminates the airflow the old advice depended on. The ductwork and the unit fill the space and interrupt the movement of air that opening a vent was supposed to create.

Second, and this is the real problem, it creates temperature swings. Warm humid summer air comes in through an open vent and hits cool HVAC ducts. That produces condensation, the same way a cold glass of tea sweats on a porch in July.

That condensation raises the relative humidity in your crawl space. Once relative humidity gets high enough, you have an environment that is conducive for mold and for pests to thrive in. And because of the stack effect, that air does not stay under the house. It gets pulled up into your living area.

So the vent you opened to help the crawl space breathe is the thing feeding the humidity problem.

What to do instead

Keep them closed, and then actually control the environment rather than just sealing it and hoping.

Closing a vent by itself is not a solution. It is one piece of a system. A real encapsulation isolates the crawl space with a quality woven vapor barrier on the walls, the piers and the floor, and then conditions it with a properly sized dehumidifier. We also like to install two dehumidistat fans, which push stale or smelly air out and help redistribute the dry air from the dehumidifier out to the far corners of the crawl space.

That is the difference between closing a door and controlling a room.

One thing to be careful about

If you close your vents and do nothing else, you can trap moisture that was already down there. Sealing a wet crawl space without conditioning it is not an improvement.

That is why I would rather come look before you change anything. Ten minutes under your house tells me more than an hour of guessing from up here.

Not sure what is going on under your house? Call me at 910 580 1836 and I will come find out.