Ideal indoor humidity

The range

The EPA gives one range for homes, in its guide to mold and moisture:

ENERGY STAR says the same in its dehumidifier guidance: the optimum for a building is generally 30 to 50%. The upper end matters most. Mold needs moisture, and the EPA’s advice is to dry wet areas within 24 to 48 hours.

The same top applies to humidifiers. The EPA warns against running one above 50%, since higher levels may encourage biological growth in the home.

In winter

Cold weather lowers the target. ENERGY STAR puts it this way:

Cold glass is the first surface where water shows. When drops form on windows, walls or pipes, the EPA says to act quickly and dry the wet surface.

Basements and crawl spaces

Below ground, readings run higher. The ENERGY STAR sizing table sorts spaces into three states by their humidity before a dehumidifier runs. Here they are on the same scale as the EPA’s target:

The goal stays the same as upstairs. A basement at 70% still counts as damp in that table, far above the 50% top of the EPA range. For crawl spaces, the Department of Energy’s Building America guide calls relative humidity above 70% bad.

How to measure it

With a hygrometer. The EPA notes that they are sold at hardware stores. Read it in the room you care about, at more than one time of day.

A dehumidifier measures too. Every certified model has a built-in humidistat, ENERGY STAR says. Once the room reaches your setting, the unit cycles on and off to hold it.

How to bring it down

Start with the water, then size the machine. The EPA’s list, in order:

1. Outside firstClean the gutters, and make the ground slope away from the foundation.
2. Then insideVent clothes dryers to the outside, and run the bathroom fan when showering.
3. Then a machineUse air conditioners or dehumidifiers when needed.
4. Size itOur size calculator applies the ENERGY STAR table to your floor area and reading, then lists the certified models at that size. A setting of 50% sits at the top of the EPA’s range.

Sources

Every figure and quote on this page comes from these, each with the date we read it.