Do you need a dehumidifier in winter?

The intuition is that winter is damp, so winter is dehumidifier season. For the heated part of a house that is backwards, and understanding why saves most people the purchase.

Relative humidity is a ratio: how much water the air holds against the most it could hold at that temperature. The second half of that fraction grows quickly with temperature, so the same quantity of water in warmer air reads as a lower percentage.

Cold outdoor air in winter contains very little water in absolute terms. Leak it into a house, warm it to 68 F, and its relative humidity falls — which is why heated houses in winter are the ones with cracked furniture, static and dry skin, and why humidifiers sell in January.

So the honest general answer for a heated room is no: if anything you may have the opposite problem. Which of the two machines you actually need is worth settling before buying either.

Everything changes where the air is not heated. The EPA puts the mechanism plainly: “When warm, humid air contacts a cold surface, condensation may form”, and “Mold is frequently found on walls in cold corners behind furniture where condensation forms.”

In winter the cold surfaces are the ones that matter, and they cluster in four places:

1. A crawl space or under-floor void. Cold, in contact with wet ground, and ventilated with outdoor air. The clearest case for a dehumidifier running all winter. The crawl-space case.
2. An unheated or part-heated basement. Masonry below grade stays cold and damp while the floor above is heated, which puts the dew point line inside the wall. Basements.
3. A garage or workshop. Usually in the forties in winter, often storing metal that rusts and material that warps. Garages.
4. A closed-off spare room. Heating turned off, door shut, so the surfaces track outdoor temperature while moisture from the rest of the house migrates in. It shows up as wet glass and a smell when you open it in December. What a shut-up spare room needs.

Here is the trap this page exists for. The places that need drying in winter are cold, and a standard portable dehumidifier — the kind almost everyone already owns — stops working in exactly those conditions.

A compressor machine chills a coil below the dew point of the air so water condenses on it and runs off. In a cold room the coil has to go below freezing to get there, and the condensate becomes frost instead of drainage. The unit then alternates between drying and defrosting, and the water in the tank after a week tells you how little it achieved.

The specification sheets are no help, because the certified figures are measured warm:

Read the gaps in that table rather than the numbers. The one class with a published low-temperature limit is the desiccant, and it is the one class with no certified efficiency. You can know that it works in the cold or you can know what it costs to run. Not both.

1. Measure, in the room that has the problem. The EPA target is below 60 percent relative humidity, ideally 30 to 50. A reading from the heated living room tells you nothing about the garage.
2. Take the room’s temperature at the same time. Below about 60 F, a compressor unit is off the table regardless of what the humidity says.
3. Look for the source before the symptom. Winter damp in a cold room is usually the ground, a failed gutter or a leak rather than room air. A dehumidifier run against active water ingress is a permanent electricity bill with no end state.
4. Try heat and air movement first in a small space. Cheaper, quieter, and it addresses the cold surface rather than the air above it.
5. Then match the machine to the temperature, not to the square footage. The capacity question is secondary to whether the unit functions at all. Sizing, once you know the type.