Ideal crawl space humidity level

The short answer is 30–50% relative humidity, with 45% as the number to aim at and 60% as the line you do not want to sit above. The longer answer is that relative humidity is the wrong measurement for the thing most people are actually worried about, and swapping to the right one changes what you do about it.

We have never crawled under your house, and we have never run any of these units in a crawl space. What we do instead is publish the arithmetic, so you can check it against your own readings. Everything computed on this page comes out of one shared file.

Why each of those four bounds exists

A band with no reasoning behind it is just a number someone copied. Each of these four is doing a different job.

60% is the growth line. Mold on wood and paper-faced materials needs liquid water at the surface, and surfaces sitting in air above roughly this figure hold enough adsorbed moisture for long enough to supply it. This is the number that appears in building guidance and on manufacturers’ own humidistats, and it is the only one of the four with a mechanism you can point at. It is a ceiling, not a target.

50% is the working ceiling. The gap between 50% and 60% is margin. Crawl space humidity is not steady — it moves with the weather, with the ground, and with whether anyone opened a vent. If you set your target at the growth line, then every humid week spends time on the wrong side of it. Setting the ceiling ten points lower means a bad week costs you nothing.

45% is the aim point. It sits in the middle of the band, which is the only reason to prefer it. There is no benefit to 45% over 50% that anyone can demonstrate; there is a benefit to having ten points of headroom in both directions.

30% is the floor, and it is nearly theoretical down there. Below it, wood gives up moisture, joints and flooring above can move, and you are spending electricity to remove water that was doing no harm. In practice a crawl space with soil under it does not reach 30% on its own. If yours reads that low, read the section on wrong-in-the-other-direction below before you change anything.

Relative humidity cannot tell you whether a surface will sweat

Here is the problem with the percentage. Relative humidity is relative to temperature — it says how close the air is to saturation at the temperature it is currently at. A crawl space is not at one temperature. The air near the joists is warmer than the air near the ground, an uninsulated duct running through it is colder than both, and a metal pipe is colder still. The single figure on your hygrometer is a statement about the air it is sitting in and nothing else.

Dew point is the measurement that answers the actual question. It is the temperature at which that air becomes saturated. Anything in the crawl space colder than the dew point will collect liquid water on it, whatever the humidity reading says. That is why ducts drip in a space whose hygrometer reads a perfectly ordinary number, and it is why two crawl spaces at the same RH can behave completely differently.

The table below is the conversion, computed across the temperatures and humidities a crawl space actually runs at. Shaded cells are the combinations whose dew point is at or above 62°F — a plausible temperature for a shaded, ground-cooled surface or an air-conditioning duct in summer. In those cells, that surface is wet.

Read the 80°F row. At 80°F and 70% RH the dew point is 69.3°F — everything in the crawl space below that is wet, and 70% is not an alarming-looking number to most people. Now read the 60°F row: at the same 70% RH the dew point is only 50.2°F, which is below almost anything down there. Identical percentage, completely different situation. This is the single strongest argument for buying a hygrometer that reports dew point, or for doing the conversion yourself.

Summer is the problem. Winter usually is not.

Most people expect a damp crawl space to be worst in winter, because winter is when houses feel damp. In a crawl space it is the other way round, and the dew point table explains why in one step.

A crawl space is coupled to the ground, so it lags the outside air. In summer that means it is colder than the air being pulled into it through the vents. Warm outdoor air carrying a high dew point meets a cold floor and cold joists, and it does what the table says it does. In winter the same lag makes the crawl space warmer than outside, so the air coming in warms up, and warming air raises its capacity — the same moisture becomes a much lower relative humidity.

This is why “open the vents to dry it out” is such durable and such bad advice for a humid summer climate. For half the year the vents are the source. It is also why a crawl space that has been fine for years can go wrong after a single exceptionally humid stretch: nothing about the house changed, the outdoor dew point did.

Where the hygrometer goes, and why one reading is misleading

If the crawl space is not at one temperature, it is not at one humidity either. A single sensor tells you about one cubic foot of air. That is worth knowing and it is not worth treating as the state of the space.

Put it in the middle of the space, at joist height, away from the walls and away from the dehumidifier. A sensor near a vent reads the outdoor air. A sensor on the ground sheet reads the wettest air in the room and will make you chase a problem that exists in one corner. A sensor sitting on top of the dehumidifier reads the driest air in the room, which is the failure mode that lets a space stay damp while the display insists it is fine — the machine satisfies its own humidistat.

Two sensors are worth far more than one. One in the middle, one in the worst corner you can find — the one furthest from the access, or the one over the wettest ground. The difference between them is the number that tells you whether you have a humidity problem or a localised water problem, and no single reading can tell you that.

Read it over a week, not once. A crawl space at 55% at nine in the morning can be at 70% by late afternoon in July. A cheap logging hygrometer that keeps a minimum and maximum is enough; what you want is the shape of the daily swing and the worst hour, because the worst hour is the one that decides whether anything grows.

If the number is too high

In order, and the order matters — every step below is cheaper than the one after it, and each one reduces the load on the next.

1. Find liquid water first. A dehumidifier cannot out-run a leaking supply line, a downspout discharging next to the foundation, or graded soil that sends surface water under the house. If there is standing water or a wet patch that has an upstream direction to it, that is a plumbing or drainage job and nothing on this page substitutes for it.
2. Cover the ground. Bare soil evaporates continuously, and it is the largest single moisture source in most crawl spaces by a wide margin. A sealed vapor barrier over the ground, lapped and taped, removes that source rather than treating it. Doing this before sizing any equipment usually moves the space down a whole dampness class.
3. Then deal with the air. Only once the ground is covered and the outside air is controlled does a dehumidifier get to do the job it is good at, which is holding a space that is already close, rather than fighting an open supply of water.

For sizing, do not use the coverage claim on the box. Square-foot coverage figures are not measured to any common standard and are not comparable between brands — in our own product data one crawl space unit publishes 1,300 sq ft while a portable publishes 5,000 sq ft, and the crawl space unit is the larger machine. Both figures are the manufacturer’s own. Size on pints per day, which is tested.

Our arithmetic for a 1,500 sq ft crawl space with damp spots on the ground sheet gives 20 pints/day on the modern scale — which is 28 pints/day if you are reading a chart written before 2019, and most charts online were. That figure already includes the uplift practitioners apply to crawl spaces for running cold and taking moisture up through the soil continuously. The sizing guide walks through both scales.

If step three is where you are

ALORAIR ALORAIR 120 Pints Energy Star Crawlspace Dehumidifier, Sentinel HD55S

120 pints/day tested, which is 169 on the old scale. Far more than the 20 pints/day our arithmetic asks for — and capacity is not why you would buy it. It is built to sit in a sealed crawl space, run at crawl space temperatures and drain continuously, which a room portable is not.

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$486.49 · 1,346 reviews

That honesty cuts both ways: if capacity were the whole story, a cheap portable would do the job, because the tested numbers say so. It is not the whole story. A room portable is rated at 65°F, loses capacity as the space gets colder, fills a bucket that nobody is under the house to empty, and has no sensible way to send water anywhere. Check the operating temperature range on any listing before you buy — we do not publish one, because our product data does not carry it and we are not going to invent it.

Drainage decides whether it keeps working. A crawl space rarely has a floor drain below the machine, and gravity only helps if there is somewhere lower to go. Where there is not, a condensate pump lifts the water out to a drain or to daylight — a Little Giant condensate pump is the usual answer, and at that price it is the cheapest part of the job. Whatever you use, run the discharge somewhere it drains away from the foundation, or you have built a loop.

If the number is too low

This is a much more common reading than it is a real condition, and the usual answer is that the number is wrong rather than the crawl space.

Suspect the sensor first. Cheap hygrometers drift, and they drift dry more often than wet. A reading below 30% in a space with soil under it is close to implausible; check it against a second sensor before acting on it.

Then suspect the location. A sensor sitting in the discharge of a dehumidifier reads the driest air in the space, and a sensor in a fully conditioned, encapsulated crawl space that is being supplied with house air will read whatever the house reads, which in a cold, dry winter can be genuinely low.

If it is real, it is a winter phenomenon and it is mostly benign. Very cold outside air, warmed as it enters, arrives dry — the January rows in the table above land at around 40% and 32% RH. The thing to avoid is a dehumidifier grinding away all winter in a space that was already below the band, which costs money and dries structural wood for no benefit. A humidistat that switches off below the ceiling handles that by itself.

What this page cannot tell you

It cannot tell you whether what you found under your house is condensation or a leak. Every number here describes air, and air problems and water problems look identical from the access hatch. If the wet is in one place, has a direction, or does not track the weather, it is not a humidity problem and a dehumidifier will not fix it.

It also cannot tell you whether existing damage matters structurally. Sustained high humidity puts moisture into joists and subfloor, and whether that has gone far enough to matter is a question for someone standing in the space with a moisture meter — a building inspector or a structural engineer, neither of whom sells you a machine. If you have soft wood, that call is worth more than anything you can buy.

Where we can help is with the arithmetic, and the arithmetic on this page is all published: the band, the dew point conversion, and the load in pints per day on both rating scales. Last reviewed 21 August 2026. Damp Down earns a commission if you buy through a link here, which does not change what the numbers say.

Where this leaves you

Cover the ground first. Then hold the band.

• Target 30–50% RH, aim at 45%, never sit above 60%.
• Our arithmetic for a 1,500 sq ft damp crawl space: 20 pints/day modern scale, 28 on the pre-2019 scale.
• 120 pints/day tested (169 old scale), with continuous drainage rather than a bucket.
• Watch dew point, not just the percentage — a duct below the dew point drips whatever the hygrometer says.

We have never run this unit in a crawl space and make no first-hand testing claim. Price and rating are an Amazon snapshot taken on the review date above.

ALORAIR

ALORAIR 120 Pints Energy Star Crawlspace Dehumidifier, Sentinel HD55S

$486.49

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