Relative humidity (RH) is how close the air is to being saturated with water vapour at its current temperature, given as a percentage. At 100% the air can hold no more, and water starts to condense. Because warm air can hold more vapour than cool air, the same air shows a higher relative humidity as it cools, even though no water has been added.
At Wigwam
We don't regulate humidity. Our units are clean, dry and secure, and we don't have units with thermostats or humidity regulators at any site. You can't add your own control either: there's no power for your use, because our terms don't allow you to connect any utilities or services to a unit unless we've authorised it (clause 22(e)). We don't publish humidity readings for any site.
Why it matters
Relative humidity, more than temperature, decides whether stored things go mouldy. The Canadian Conservation Institute says "the practical boundary for damp is given as 75% RH", and that damp causes mould, which discolours or breaks down leather, textiles and paper. Temperature moves humidity with it: by the same institute's figures, a 1°C swing shifts relative humidity by about 3%, and a 5°C swing by about 15%.
So what you do before the door closes counts. Moisture you pack in raises the humidity in a box or under a cover, so everything goes in clean and completely dry. Air around your things lets that moisture move away, so keep them off the floor, leave a gap at the walls, and avoid sealed plastic for fabric. Our clothes and musty smells guide works through it for textiles.
Related terms: Condensation · Damp · Moisture absorber · Dehumidifier
Related questions: How do I stop clothes going musty in a storage unit? · Do storage units get cold in winter or hot in summer?