Relative Humidity (RH)

The amount of moisture in the air as a percentage of the maximum it could hold at that temperature.

Relative humidity (RH) is expressed as a percentage — 0% is bone dry, 100% is air fully saturated with moisture (the point at which condensation starts forming). It's "relative" because warm air can hold more moisture than cold air at the same absolute moisture content, so RH changes with temperature even if the actual amount of water in the air doesn't.

This is why a room can feel humid even when it "shouldn't" — a small temperature drop (evening cooling, air conditioning) raises RH without adding any moisture, simply because cooler air holds less before it's saturated.

Most comfort and mould-prevention guidance targets a 40–60% RH band indoors, though the right number depends on the specific space — see our sizing guide and the 60% line guide for the detail.