Humidifier vs dehumidifier

A humidifier adds water to the air. A dehumidifier takes it out. Run both at once and they fight; run the wrong one and you drive the problem further in the direction it was already going.

We should say plainly what we sell and what we do not. This site stocks and reviews dehumidifiers, and does not stock humidifiers. So if the answer below is a humidifier, the useful thing we can do is tell you that clearly and get out of the way.

The EPA publishes the same figures in two separate documents: “Indoor relative humidity (RH) should be kept below 60 percent — ideally between 30 percent and 50 percent, if possible.”

That band is the whole decision procedure.

Everything else — how the air feels, whether your throat is dry, whether the window is wet — is a proxy for that reading, and the proxies disagree with each other in winter. Which is the next section.

This is the single most common confusion in the category, and it is genuinely confusing rather than a failure of attention. In winter the same house can be:

• Too dry by the reading. Cold outdoor air holds very little water. Heat it to 68 F and the relative humidity falls, because warm air can hold far more than it is carrying. Hence static, cracked furniture and January humidifier sales.
• Too wet for its own glass. The pane sits close to outdoor temperature. The EPA: “When warm, humid air contacts a cold surface, condensation may form.” A surface at 40 F needs far drier air to stay clear than a wall at 68 F does.

Both statements can be true in one room on one night. The resolution is that relative humidity is relative to a temperature, so a single house has as many effective humidities as it has surface temperatures.

The practical consequence: do not buy a humidifier because the air feels dry while a window is streaming. Adding water will make the glass worse and the mold risk in the cold corners worse, for a comfort gain you could get more cheaply by stopping the draft that is drying the room. What to do about the glass.

That asymmetry is worth dwelling on, because it changes how you have to shop. A dehumidifier can be compared against every other dehumidifier on a common federal test: 9.1¢ per liter removed at IEF 2.01, 11¢ at the bottom of the field, 8.2¢ at the top. For a humidifier there is no equivalent figure at all, so every comparison is between manufacturers’ own claims.

Seasonally, plenty of houses do: a dehumidifier in a damp basement through the summer, a humidifier upstairs through the heating season. That is two machines solving two problems at two times of year, which is ordinary.

Simultaneously, in one space, is always a mistake. They will cycle against each other indefinitely, each one giving the other something to do, and the electricity bill is the sum of both. If you find yourself wanting both in one room, the reading is telling you something about air movement or a cold surface rather than about humidity.

There is one genuine exception worth naming, and it is not a humidifier: in a cold space a desiccant dehumidifier returns its heat to the room while drying it, which is why it is the machine that works where a compressor unit frosts. The winter case, in full.

• Reading above 60 percent: dehumidifier, and look for a source as well — the EPA says to “act quickly” on visible condensation.
• Reading 50 to 60: dehumidifier on a humidistat, set inside the band. Lower settings cost more and achieve less.
• Reading 30 to 50 and a wet window: neither. Warm the glass, move air across it, and stop insulating it with tight curtains.
• Reading below 30 and static everywhere: humidifier — and we will not pretend to have picks for you.
• Reading below 30 and a wet window: check the hygrometer against a second one. These two findings in one room are usually a bad sensor or a reading taken in the wrong place.