What a Hygrometer Tells You — and the Number the Sources Disagree On

A hygrometer is a small screen that tells you the relative humidity in the room. They cost about a tenner, they need no installation, and they turn an argument about whether the flat is damp into a number you can both look at.

Which makes it slightly awkward that the guidance does not agree on what the number should be.

The two figures

EPA's short page, Ten Things You Should Know about Mold, says to reduce indoor humidity to 30–60% to decrease mold growth.

EPA's longer document, A Brief Guide to Mold, Moisture and Your Home, says to keep indoor humidity below 60 percent, and ideally between 30 and 50 percent.

Same agency, same subject, two different answers — and the gap between them, 50% to 60%, is exactly the band a lot of homes actually sit in. We are not going to average them into a single tidier figure, because that would be inventing a number neither page prints.

What to do with that

Read it as a ceiling and a preference. Both pages agree that 60% is the line you do not want to be above. The longer document additionally prefers you under 50%. If you are at 55%, one page is content and the other would rather you were drier; nobody thinks you have an emergency.

The reading matters less than the pattern

A single number tells you very little. The same room can be 45% at eight in the morning and 70% twenty minutes after a shower, and both are normal.

• Leave the meter in one place for a week before concluding anything.
• Check it after the things that add water — showering, cooking, drying laundry — and see how long it takes to come back down. The recovery time is the real finding.
• Compare rooms. A bedroom sitting consistently higher than the rest of the flat is telling you something about that room.
• Cold surfaces matter more than room averages. Condensation forms where the surface is coldest, which is why a wall behind a wardrobe can be wet in a room reading a comfortable 50%.

And the number that is not on the hygrometer is still the important one: whatever gets wet needs to be dry within 24 to 48 hours. That window appears on both EPA pages, and unlike the humidity target, they agree on it exactly.

Read the sources

• EPA — Ten Things You Should Know about Mold — the 30–60% figure; original at https://www.epa.gov/mold/ten-things-you-should-know-about-mold
• EPA — A Brief Guide to Mold, Moisture and Your Home — the below-60%/ideally-30–50% figure; original at https://www.epa.gov/mold/brief-guide-mold-moisture-and-your-home