If you do not have anything overly valuable or delicate to store (such as a musical instrument or an expensive lace dress), you may be able to save yourself some money by opting for an outdoor storage unit rather than an indoor, climate-controlled one. You will, however, want to take some steps to keep moisture levels down in that outdoor storage unit. Here are some tips.
Check for leaks, first.
Most storage companies are quite careful about inspecting units before they rent them out – they don't want your items damaged, either, since it reflects poorly on them. Still, they are human and can miss things, so look over a unit before you agree to rent that particular unit. Look for any cracks in the walls, ceiling, or floor. Of course, if there are any pools or drips of water around, you should assume there is a leak and ask for another unit.
Make sure everything is dry before you store it.
If you put anything moist into a storage unit, such as a refrigerator you just wiped out or a shelving unit that's a bit wet from being rained on, that moisture will evaporate into the air and make the entire unit moist. So never put anything wet or damp into storage. If you clean appliances like refrigerators and stoves, leave them open for a day afterwards so they're fully dry before putting them in storage. Wipe down hard-surface furniture before you put it in storage, not as you're unloading it. If it is raining outside, you should wait for a dryer day to pack your unit rather than letting everything get rained on.
Put some desiccant boxes in the storage unit.
You can purchase boxes or canisters of silicone desiccant at most hardware stores. Typically, you just take the lid off and place them in a corner. They absorb moisture out of the air on humid days so the moisture doesn't accumulate in your items. Place a few of these around the storage unit, and remind yourself to replace them every month or so. (The back of the box or canister should tell you how often to replace them; this varies a bit depending on the brand.)
In addition to following these tips, make sure you also store your items on pallets rather than directly on the ground. This way, if there is a leak, your items won't get wet.