What Is Ideal Indoor Humidity

The usable band

For living spaces, comfort and building health broadly sit between about 40% and 60% relative humidity. That is not a regulation, it is a compromise: low enough to keep mould and dust mites down, high enough to avoid dry air symptoms and static.

What goes wrong below it

Dry air pulls moisture out of skin, airways and timber. Occupants report sore throats, dry eyes and static shocks; instruments and furniture move. Heating in a cold climate is the usual cause, because heating cold outside air without adding moisture drives RH down sharply.

What goes wrong above it

Above roughly 60% the problem changes from comfort to damage. Condensation on cold surfaces, mould in corners and behind furniture, dust mite populations rising, and a general feeling of stuffiness at temperatures that would otherwise be fine. Bathrooms, kitchens and drying rooms push the rest of the dwelling up if they are not ventilated.

Why one reading is not enough

A house is not one humidity. The nursery, the bathroom, the cellar and the north-facing bedroom can be 20 points apart. That is the argument for a multi-channel thermo-hygrometer: put a sensor in each room that has ever been complained about, read them on one console, and the problem room identifies itself in a day.

Acting on it

Below range, add moisture or reduce over-heating. Above range, ventilate at the source and warm cold surfaces. In both cases set the console’s high and low alarms once you know the room, so the next excursion announces itself instead of being discovered as a stain.