Side of the house, stop 4 of 4

Foundation vents: open in summer, closed in a freeze?

For crawl space vents in winter, Clemson’s advice is to close foundation vents and reopen them in spring.¹ Outside of that, EPA wants a crawlspace kept well ventilated.² Closing helps limit cold air on the pipes, but it doesn’t replace insulation.

Stand outside and walk the foundation wall with a flashlight, counting the vents as you go.

Updated November 4, 2026, Midlands House Walk

Work these from outside. Don’t crawl under to reach one. If your flashlight shows standing water, sewage or live wiring, stay out and get help from someone trained.

Rusty rectangular metal vent in a brick foundation with its louvers shut and pine straw at its foot — A rusty vent with its louvers shut, pine straw below. Rust like this can leave a louver stuck where it is. AI-generated picture.

Metal vent in a brick wall with louvers open, dark space visible behind, grass below — Louvers open and a dark gap behind. In most of the year, this is the setting you want. AI-generated picture.

How are yours set right now?

Look for: Go round and note each one: open, shut, stuck, or buried in mulch or soil. Clear anything piled against them.
What it tells you: Open with a freeze coming? Clemson says to close them.¹ Shut in June? Reopen them, since EPA wants air moving under there.² A sealed crawlspace is different, so ask whoever sealed it. A blocked vent does neither job.

Gray foam pipe insulation with taped joints along the floor joists of a crawlspace — Pipes in gray foam with taped joints. This, more than any vent, is what stands between the water and the cold. AI-generated picture.

What are the vents protecting?

Look for: From the hatch, put your flashlight on the pipes. See if they’re wrapped, and if the joints are taped or gaping.
What it tells you: Insulated exposed pipes generally handle mild freezes down to 20°F. Cold spells of two days or more, or single digits, may still freeze pipes under homes.³ Bare lines need wrapping.

Flashlight beam from an open hatch across plastic sheeting, brick piers and wrapped ducts and pipes — A flashlight from the hatch, on plastic sheeting and brick piers. Stay at the opening and let the beam do the walking. AI-generated picture.

Is it dry under there when the vents are open?

Look for: Look for plastic over the dirt. EPA suggests a plastic cover,² and Clemson says 4 mil polyethylene covering 70 percent keeps lumber from getting too moist.⁴
What it tells you: EPA wants indoor humidity below 60 percent.⁵ A musty, wet crawlspace is a different problem from cold, and it starts at why a crawlspace stays damp.

Concrete slab edge at ground level below vinyl siding, a pipe rising beside the wall to a tap, no vents in sight — A slab house: concrete edge at the ground, siding above, no vents. If your wall looks like this, the vents question doesn’t apply. AI-generated picture.

No vents anywhere?

Look for: Walk all four sides. No vents and a concrete edge at the ground suggest a slab, not a crawlspace.
What it tells you: Nothing to open or close, then. The slab house stop lists what you can see from outside.

Setting the vents through the year
When	Vents	Why
Winter	Closed	Clemson’s advice for freeze season
Spring	Reopened	Clemson says to reopen them then
Spring through fall	Open and clear	EPA wants the crawlspace well ventilated

Yours to do

Close and reopen the vents you can reach from outside.
Clear mulch, straw and soil away from them.
Add a reminder for fall and spring, and look at a freeze is coming when the forecast turns.

For a licensed pro

Vents that are rusted stuck, broken or missing.
A wet crawlspace, or anything that needs someone under the floor.

Where this comes from

Keep walking to under the house (AI picture)What to look for from the crawlspace hatch