Mould is a type of fungus that grows in moist conditions, and mildew is the everyday name for the black, green or whitish patches a fungus makes on things like paper, cloth and walls, usually where it's warm and wet. In a home or a storage unit the two words describe the same problem: fungal growth fed by damp, which stains, smells musty and weakens what it grows on.
At Wigwam
Under our terms your goods are stored at your own risk, including "deterioration of the Goods caused by any reason" (clause 30). We're liable for loss or damage only where it results from our negligence or breach of contract, and then our liability is limited to £100 in total (clause 30(b)). Your goods must stay insured for their full replacement value (clause 31). Our Contents Protection (an RSA Self Storage Customers' Goods policy) excludes atmospheric or climatic damage, so check the wording of whichever policy you use before relying on it for mould.
That leaves prevention as the real protection. Our units are clean and dry when you move in, but they aren't climate-controlled, so the humidity inside isn't regulated.
What to do about it
The Canadian Conservation Institute puts the practical boundary for damp at 75% relative humidity and says mould disintegrates or discolours leather, textiles and paper. Keep moisture out: everything clean and bone dry going in, packed so air can reach it, off the floor and away from the walls. Check in from time to time; our terms advise it (clause 26). If you find mould, photograph it, keep the damaged items, and tell us straight away on 0800 2545585 (clause 24). Don't pack a mouldy item back in with the rest. For leather, the Canadian Conservation Institute advises isolating it in a sealed bag, wearing protective equipment to handle it, keeping anyone with allergies or breathing problems away from it, and checking with a conservator if in doubt.
Related terms: Damp · Relative humidity · Breathable cover · Exclusions
Related questions: Who is responsible if mould appears in my storage unit? · How do I stop clothes going musty in a storage unit?