Humidifier or dehumidifier: read a hygrometer first, because the 30 to 50 percent band means you need neither

If your only evidence is the dial on the last machine you owned, you do not have a reading yet, and the reading is the answer.

What does the hygrometer reading tell you?

A hygrometer is a gauge that reads relative humidity, the share of water vapour the air holds against the most it could hold at that temperature. EPA’s Mold Course says humidity can be measured with a humidity gauge or meter. The hygrometer comparison lists six on maker-printed accuracy; the three shown above are its top picks. Put one in the room with the complaint, leave it a week, and read the table.

What do EPA’s numbers mean?

EPA prints the same guidance twice. Its Mold Course Chapter 2 says indoor relative humidity should be kept below 60 percent, ideally between 30 and 50 percent, if possible. Its Brief Guide to Mold, Moisture and Your Home repeats the figures and lists air conditioners and dehumidifiers for when they are needed.

The 50 to 60 percent band is where readers overspend. EPA prints no machine for it, so this page reads it as a watch zone: a dehumidifier earns its place once the Brief Guide’s signs appear, condensation or moisture collecting on windows, walls or pipes.

NDSU Extension prints two summer ceilings, below 65 percent for mold and below 50 percent for dust mites; EPA’s 60 sits between them and is the number this page uses.

When is a humidifier the right machine?

Only in winter, and only below 30 percent. CEE says a humidifier should be set between 30 and 40 percent in winter, and NDSU’s recommended winter range is the same 30 to 40 percent. EPA’s Mold Course lists humidifiers among the sources that raise humidity in a building: set too high, one is a moisture source like any other.

The ceiling then drops with the weather, because warm humid air that touches a cold surface condenses, as EPA’s Mold Course puts it. CEE’s winter table caps indoor RH at under 40 percent when it is 20 to 40F outside, under 35 percent at 10 to 20F, under 30 percent at 0 to 10F, under 25 percent at -10 to 0F, under 20 percent at -20 to -10F and under 15 percent at -20F or below. The indoor humidity by outdoor temperature calculator interpolates that table for tonight’s forecast.

So a humidifier that is right in November fogs the glass in January. NDSU says a relative humidity above about 40 percent increases the potential for condensation on windows and cool surfaces. The fix is a lower setpoint, not a second machine; the window condensation guide reads the glass.

Which humidifier depends on how many rooms read low. One bedroom is the bedroom humidifier comparison; a whole floor is the large-room humidifier comparison; every room on a forced-air furnace is the whole-house humidifier comparison, where units with an outdoor-temperature control track the CEE table for you. The humidifier cleaning guide covers the maker cleaning steps.

When is a dehumidifier the right machine?

Above 60 percent, which in most houses means summer upstairs and the basement in any season. EPA’s pages give no basement-specific figure, so the same below-60-percent guidance applies there.

ENERGY STAR sizes by floor area and dampness. Under 2,000 square feet it lists 20 to 30 pints per day for a slightly to moderately damp room at 50 to 75 percent RH, 25 to 40 pints for very damp at 75 to 90 percent and 30 to 50 pints for wet at 90 to 100 percent; over 2,000 square feet the figures are 30, 40 and 50 pints and up. ENERGY STAR says it is better to oversize than undersize. The dehumidifier size calculator runs that chart, and the capacity ratings guide explains why an older label reads higher than a current one.

Two ENERGY STAR points save money. Every ENERGY STAR dehumidifier has a built-in humidistat that cycles the unit at the set RH. And extending downspouts and correcting the soil slope at the foundation can reduce basement humidity and the dehumidification needed: a weekend job before it is a purchase.

What should you do first?

The first four steps cost a gauge and nothing else.

1. Put a hygrometer in the room with the complaint and read it morning and evening for a week; a logging model does this for you.
2. Compare the reading with EPA’s bands: under 30 percent, 30 to 50 percent, 50 to 60 percent, above 60 percent.
3. In winter, check the reading against CEE’s outdoor-temperature ceiling with the indoor humidity by outdoor temperature calculator before you raise a humidifier.
4. In summer or a basement, fix the downspouts and grading first, as ENERGY STAR describes.
5. Buy the machine the table names, sized by the linked page, and keep the hygrometer in the room to confirm.

If the reading never leaves 30 to 50 percent, you have saved the cost of an appliance you did not need.

Frequently asked questions

Sources

1. EPA, Mold Course Chapter 2 (government or regulatory source)
2. EPA, A Brief Guide to Mold, Moisture and Your Home (government or regulatory source)
3. Center for Energy and Environment (CEE), Managing indoor humidity levels all year round (manufacturer or vendor document)
4. NDSU Extension, Measure humidity level (university or extension service)
5. ENERGY STAR, Dehumidifiers (government or regulatory source)