The advice reversed for a reason, and the reason is physics about humid air meeting cool surfaces. What actually decides it in your climate.
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The crawl space: moisture, vents and encapsulation
Most of a house's moisture problems start in the space nobody goes into. And the standard advice about ventilating it has been quietly reversed in much of the country.
The ground is the water source, not the air
A crawl space is a room. It is a low, dark, usually unfinished room, and the air in it is connected to the air you breathe upstairs far more than people expect. What happens down there does not stay down there.
In a dirt crawl space, bare soil is a continuous source of moisture: water vapor moves up out of the ground day and night, with no meaningful upper limit. That vapor condenses on the coolest surfaces available (the underside of the floor, ducts, pipes, the framing) and that is the mechanism behind most of what people find down there: damp insulation falling out of the joists, a musty smell that migrates upstairs, and surface growth on wood.
Which brings us to the argument about vents. For decades the standard was to ventilate a crawl space year-round, on the theory that airflow dries it out. In hot, humid weather that logic can invert: outdoor air holds a great deal of moisture. And when it enters a cool crawl space it cools down and drops that moisture onto the coldest surfaces. Ventilating with humid air can make a crawl space wetter, not drier. This is why encapsulation (sealing the ground with a heavy vapor barrier, closing or sealing the vents, insulating the walls rather than the floor, and controlling humidity mechanically) became the mainstream approach in much of the US. It is also why the right answer genuinely depends on climate, and why a blanket rule from a forum is unreliable.
Encapsulation is not automatically the answer, either. If there is standing water down there, sealing plastic over it solves nothing: the water has to be dealt with first, usually with drainage and a pump, exactly as in a basement. And a crawl space with a combustion appliance or ductwork in it has requirements of its own that a plastic liner doesn't address. This section covers what the liner does, what it doesn't, and the order the work has to happen in.
Crawl space guides
Liner, sealed vents, wall insulation, dehumidification. And the one condition that has to be solved before any of it goes in.
Both sit under the house and both get damp, but they fail differently and the fixes are not interchangeable.
Soil vapor, humid air or liquid water. Find the source before you pay for a fix.
Rust, leaks and air supply for equipment that lives in the dampest room of the house.
The low gap under the floor: how it is built, the main types and why moisture collects there.
Crawl space, piers or slab: what is down there, why it gets damp and what to look at first.
What concrete walls and a poured floor do for moisture, and what they leave untouched.
Water on the ground under the house is a symptom. How to tell the source and what to fix first.
Why bare soil gives off moisture, and what a ground cover does and does not do.
A musty smell points to moisture. How to trace it before you cover it up.
What sits under the floor of your house, what goes wrong and what to check from the hatch.
A fan moves air but does not remove moisture. When it helps and when it makes things worse.
What it means to close a crawl space off from outdoor air, and why the water comes first.
Drying only lasts if the source is fixed. The order of work that holds up.
Ground cover, sealed walls and closed vents: the parts, and when it is the wrong first move.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.