Relative humidity (RH) is the amount of water vapour in the air expressed as a percentage of the maximum the air can hold at its current temperature. Because cooler air holds less moisture, RH rises as air cools, and when it reaches 100% at a cold surface such as a window, the vapour condenses into water.
Why it matters to you
Lower Mainland winters are cool and damp, so indoor moisture from daily living often shows up as condensation on windows and mould in cold corners. Sustained indoor humidity above roughly 60% supports mould and dust mites.
Practical guidance
Use an inexpensive hygrometer to track indoor RH; many homes aim for roughly 30-50% in winter, lower when windows show condensation. Fix moisture sources first, such as bathroom fans and range hoods that do not vent outside, before buying equipment.