A moisture absorber is a non-electric product that uses a desiccant, a material that draws water from the air, to lower the humidity in a small enclosed space. The two common kinds are silica gel packs, which hold the moisture inside the beads, and tub-type absorbers, where a calcium chloride tablet takes up moisture and turns into a liquid that collects in a tank below. Unlike an electric dehumidifier, it has no fan or power, so it works slowly and only on the air close to it.
At Wigwam
Our units aren't climate-controlled and there's no power for an electric dehumidifier (clause 22(e)), so a non-electric absorber is the only kind of moisture-removing kit you could add. Our own guide to climate control suggests them for small, sensitive things: "A silica gel pack in the box handles the practical risk" for electronics kept in long-term storage.
Tub-type absorbers need more thought. Yachting Monthly's review of the Unibond Aero 360 notes it "isn't a sealed unit though and should be protected from being knocked over to avoid spills". A spill of that liquid onto your goods, or through to a neighbour's unit, is your responsibility under our terms (clause 32), and our terms rule out chemicals and other potentially hazardous substances (clause 18(e)). Ask us before you put a tub-type absorber in a unit.
What to expect
A retail tub such as the Aero 360 is sold for rooms up to 20m² and lasts up to 3 months, so in a unit you rarely visit it can fill and stop working. Silica gel sachets are fine to use: they aren't a liquid-collecting tub, and they suit small closed containers of dry things, such as a box of papers or electronics. That's the exception to the airflow advice elsewhere, and only for items that are already completely dry. For tub-type absorbers, ask us first, as above. Neither kind dries out things that went in damp, so clean and completely dry packing still comes first.
Related terms: Dehumidifier · Relative humidity · Condensation
Related questions: How do I stop clothes going musty in a storage unit? · Do I need a dehumidifier in a container unit, and how do I stop condensation?