Dew point calculator: will that surface sweat?

The percentage on its own is close to meaningless for predicting condensation, which is unfortunate, because the percentage is what every cheap hygrometer shows and what every dehumidifier control is set in. The useful move is to convert once — read the temperature and the humidity, get the dew point, and then compare that single number against every cold thing in the space.

The rule, and it is the whole rule

Water condenses wherever a surface is colder than the dew point of the air touching it. Above the dew point, nothing happens. Below it, water leaves the air and collects, and it will keep collecting as long as both conditions hold. There is no third case and no threshold of humidity at which it starts — a surface one degree below the dew point is wet, and a surface one degree above it is dry.

The margin is what matters in practice. A surface sitting 15°F above the dew point is genuinely safe; you would need a large change in conditions to threaten it. A surface 3°F above is not safe, it is merely dry at this instant, and a cool night or an open door will take it under. The tool prints the margin for exactly this reason, and treats anything under 5°F as unstable rather than fine.

It also prints the humidity at which the surface would start to sweat, which is the most directly actionable of the three numbers. If it says the surface sweats above 58%, then 58% is your real humidity target for that space — not a general number copied off an article, and not whatever the machine came set to.

The classic failure: summer air in a cool crawl space

The single most common moisture problem this arithmetic explains is a vented crawl space in summer, and it is worth walking through with real numbers because the conclusion runs against ordinary intuition.

A crawl space is in contact with the ground. Soil a few feet down changes temperature slowly and lags the season badly, so through most of the summer the floor, the piers, the ducts and the bottom of the joists in a crawl space are substantially colder than the outdoor air. Meanwhile the vents in the foundation wall — put there deliberately, by a building code that was trying to help — are letting hot, wet outdoor air in.

Why venting a crawl space in summer makes it wetter

Foundation vents exist because of an entirely reasonable idea: moisture accumulates under a house, and air movement dries things out. That is true when the incoming air is drier than the air already inside, which in much of the United States it is for a good part of the year. In July it is not. In July you are pumping air with a 76.2°F dew point into a space whose surfaces are in the mid-sixties, and every cubic foot of it deposits water on the way through.

This is why a homeowner who notices a damp crawl space, opens all the vents and puts a fan down there in August often finds it visibly worse a fortnight later. The intervention is not failing to work; it is working in reverse. More airflow through a cold space in humid weather means more water delivered, not less.

It is also why practice has moved the other way. Sealed, insulated crawl spaces with a sealed ground vapour barrier and a dehumidifier — rather than vents — have become the standard remedy where summers are humid, and the model codes have followed with provisions for unvented conditioned crawl spaces. We are not in a position to tell you what your local jurisdiction has adopted or what your inspector will accept, and you should check that before closing anything permanently. The physics, though, is not jurisdictional. Warm wet air on a cold surface makes water everywhere.

The same mechanism, in miniature, is behind most of the moisture complaints in an ordinary house: the cold water pipe that drips in July and is blamed on a leak, the toilet cistern that seems to sweat, the window that streams on a winter morning, the duct that soaks the insulation around it, the corner of a closet on an outside wall that grows mould while the rest of the room is fine. Each one is a surface below the dew point. Not one of them is a plumbing failure.

Using the number to decide whether you need a dehumidifier at all

The reason to compute this before shopping is that it splits into three outcomes, and only one of them ends with buying a machine.

The margin is comfortable and the humidity is in band. If the surface you were worried about is 10°F or more above the dew point and the room is inside the 30–50% range, there is nothing here to fix. Damp smells with these readings usually point at a source — a slab wicking, a failed gutter, a bathroom fan vented into the attic — and a dehumidifier will run continuously against that source, cost money, and never win. Find the water.

The margin is thin but the humidity is reasonable. Here the cheapest fix is often to change the surface rather than the air. Insulating a cold water pipe costs a few dollars and moves that pipe above the dew point permanently. Sealing and insulating a duct run does the same. Adding a vapour barrier over bare crawl space soil cuts the moisture arriving in the first place, and is the step practitioners take before sizing any equipment. A machine is not the only way to raise a margin, and it is rarely the cheapest.

The humidity itself is the problem. If the air is at or above 60% relative humidity, you have a mould-supporting environment whether or not anything is visibly wet — mould does not need liquid water, only sustained humidity at the surface. That is the case where removing water from the air is the actual answer, and it is worth doing properly rather than with whatever unit is on sale.

What a dehumidifier changes, and what it cannot

A dehumidifier removes water from air. It lowers the dew point, which raises the margin on every surface in the space at once. What it cannot do is warm a surface up. If a duct is running at 55°F because there is chilled air inside it, no amount of dehumidification makes it warmer — it simply lowers the dew point of the air around it until 55°F is no longer cold enough to matter. Both routes end in a dry duct. Only one of them costs electricity forever.

Set the target on evidence rather than habit. If the tool says your coldest surface sweats above 55% humidity, then a machine set to 50% has a five point margin, and one set to 60% is not solving your problem no matter how much water ends up in the bucket. Where a space contains something you actively care about — stored paper, instruments, tools, ammunition, anything steel — the target usually needs to be lower than comfort alone would suggest.

One arithmetic trap sits between this page and the buying decision. In 2019 the US Department of Energy moved the dehumidifier capacity test from 80°F to 65°F. A dehumidifier removes less water from colder air, so the same machine now earns a smaller number on its label: a modern 50 pint unit removes roughly what a pre-2019 70 pint unit did. Every sizing chart written before that change — and most of the ones circulating now, which were copied from them — is on the old scale, so following one with a current machine undersizes by about a third. Check which scale any capacity figure is on before you trust it.

Cold spaces change the machine, not the physics

The same temperature sensitivity that forced the rating change decides which kind of machine belongs in a space. A refrigerant dehumidifier works by chilling a coil below the dew point and collecting what runs off it, which is the process on this page performed deliberately. In a cold room that coil approaches freezing, frost forms on it, and the machine spends an increasing share of its time defrosting instead of drying. Below roughly 65°F a refrigerant unit is already losing output, which is exactly why the test was moved there.

That is why an unheated garage, an unconditioned workshop or a cold basement is a different buying problem from a warm one, and we have kept it on its own page: choosing a dehumidifier for a garage or workshop goes through the temperature limit, the humidity band that stops rust on tools, and the cases where insulating is the honest first move.

What this calculation cannot tell you

The dew point itself is a settled piece of physics and the arithmetic here is standard — this page computes it with the Magnus approximation, from one shared function used by every number on the site, so the tool and the tables cannot disagree. What it cannot do is know your building.

It assumes the air you measured is the air touching the surface, which is often not true: still air in a closed closet or behind stored boxes stratifies, and a corner can sit several degrees colder and considerably more humid than the middle of the same room. It assumes your instruments are honest, and inexpensive hygrometers are not precision instruments — treat a reading as approximate and prefer the trend over the digit. And it assumes a steady state, when a real space swings across the day as the sun moves and the ground lags.

We should also be plain about what this site is. Damp Down has never run any of the dehumidifiers it writes about in a crawl space, a garage or anywhere else, and does not claim otherwise. What we do instead is publish the arithmetic, on both rating scales, with the derivations in the source, so you can check every figure rather than trust it. Product data here is Amazon listing data as recorded on 21 August 2026, and capacity figures are the manufacturers’ own tested numbers. Coverage claims in square feet are not comparable between brands and we do not rank anything on them.

When the answer is no

If you came here because a room feels muggy and you were about to buy a machine, run the numbers first. A comfortable margin and a humidity inside the band means the problem is not in the air, and the most useful thing this page can do is send you away without spending anything. Look for liquid water, check where your bathroom and dryer vents actually terminate, and put a hygrometer in the space for a week before deciding.

If the numbers say otherwise — a surface below the dew point, or a room sitting above 60% for weeks — then the case is real, and the next question is capacity in pints per day on the modern scale, which is the one number that is actually tested and the only one worth comparing across brands — the dehumidifier sizing calculator works it out on both scales.