Indoor humidity: what the number tells you, and what it doesn't
The usual guidance is to keep indoor relative humidity below 60%, and ideally between 30 and 50% (US EPA). It's a useful target for a whole room. It isn't a test of whether a home is damp.
Why a normal reading can still mean damp
Relative humidity depends on temperature. The air in the middle of a room can read 50% while the air against a cold outside wall, behind a wardrobe, or in a closed cupboard is much damper, because that surface is colder. That is one reason mould can appear in corners and behind furniture in rooms that read fine. A musty smell, water marks or mould that comes back are evidence too, whatever the hygrometer says.
How to measure it
- A cheap digital hygrometer is enough. Put it at head height, away from windows, heaters and the kitchen.
- Note the reading in the morning and evening for a week. One reading tells you little.
- If a cupboard or wardrobe smells musty, leave the hygrometer inside it overnight with the door closed and compare it with the room.
- Readings that swing a lot can follow showers, cooking or drying clothes, but also the weather, heating or the air conditioner. Note what was happening around each change before deciding what to fix.
Why it matters for allergies too
Dust mites thrive in warm, humid homes, and reducing indoor humidity is one of the National Asthma Council's recommendations for limiting them, along with washing sheets and pillowcases weekly in water hotter than 60°C (National Asthma Council Australia). The same guidance suggests avoiding evaporative coolers, which add moisture to indoor air, and unflued gas heaters.
What lowers it
Start with the sources: drying clothes indoors, a dryer that vents into the room (Sustainability Victoria advises venting dryers outside so the moisture doesn't make the house damp; source), showers without extraction, and cooking without a rangehood that vents outside. When the outdoor air is clean and not muggy, ventilation helps carry indoor moisture out; in cold weather a short burst of airing often does this well, because cold outdoor air holds little water. Skip it when there's smoke or poor air outside. A dehumidifier helps once those are dealt with and readings still sit above 60%; the litres-per-day guide explains how to read one.
Sources
Each source below was opened and checked against this page on 30 October 2026.