Short answer: the US EPA recommends keeping indoor humidity below 60% relative humidity, ideally between 30 and 50%[1]. That range applies to living rooms, bedrooms and offices alike.
Relative humidity is only half the story: what matters for condensation is whether any surface falls below the air's dew point. In winter, windows, outside-wall corners and the backs of wardrobes against outside walls get cold. The same 55% RH that is harmless in summer can condense on a cold pane in January. Use the condensation risk calculator with a surface temperature reading to check a specific spot.
| Room | Typical issue | What helps (source) |
|---|---|---|
| Bathroom | Short peaks near 100% after showers | Run the fan or open the window when showering[1]; extract ventilation rates in this guide[2] |
| Kitchen | Cooking and dishwashing moisture | Exhaust fans or open windows when cooking or running the dishwasher[1] |
| Bedroom | Overnight moisture from occupants, cold outer walls | Keep RH in the 30–50% band[1]; don't push furniture tight against cold outside walls (EPA lists walls behind furniture as a place hidden mould grows)[1] |
| Basement | Persistent damp | Dehumidifier sized per ENERGY STAR[3]; see basement humidity |