Aim to keep indoor relative humidity below 60%, and ideally in the 40 to 55% range. Between 50 and 60% is a watch zone — fine in open living space but worth monitoring in wardrobes, bathrooms and against cool external walls. Persistent readings above 60% mean moisture is coming in faster than it is being removed, and that is when condensation and mold risk climb.
Key takeaways
- Target: below 60% RH, ideally 40 to 55%.
- 50 to 60% is acceptable in open space, watch it in enclosed spots.
- Above 60% for long periods is the mold risk zone.
- Measure where the problems are, not just in the living room.
Why 60% is the line
Mold needs liquid water on a surface, not just humid air. But the higher the relative humidity, the warmer a surface can be and still reach its dew point — so at 65 to 70% indoor humidity, ordinary cool spots like the back of a wardrobe on an external wall, a cold water pipe, or the ceiling near an AC diffuser start to condense moisture. Keeping the air below 60% pushes the dew point below the temperature of those everyday cool surfaces.
The ranges explained
- 40 to 50% RH — preferred. Comfortable and low risk.
- 50 to 60% RH — acceptable in open, well-aired rooms; monitor enclosed spaces, wardrobes, bathrooms and cool perimeter walls.
- Above 60% RH — investigate. Something is adding moisture faster than the AC removes it: infiltration of humid air, a leak, poor AC performance, or too little run time.
How to measure
- Use an inexpensive digital hygrometer — more than one if you have several problem areas.
- Place them where growth appears or where you smell dampness, not only in the main living room.
- Read over several days and note the pattern — overnight, when the AC cycles, after showers or cooking.
- Inside a closed wardrobe on an external wall is a revealing spot to check.
If readings stay high
Clean or replace AC filters and book a coil and drain service if overdue; run the AC at a moderate setpoint for longer periods rather than very cold in bursts; add extraction discipline in the kitchen and bathrooms; keep balcony doors closed; and consider a dehumidifier for a specific stubborn room. If it still will not come down, have the system checked — it may be oversized or faulty — and rule out a leak. See humid even with the AC on.
If your living room reads 52% but the inside of a wardrobe on the external wall reads 68%, the wardrobe is where mold will appear. Measure the problem spots.