basement when warm, moist summertime air hits the cool walls. Step #1 is to remove all the stuff stored in the room, run a dehumidifier, and promote air circulation with a fan. After several days of this, coat the interior surfaces of the storage room with a product called Drylok by UGL. It’s a concrete waterproofing treatment that came in head and shoulders above anything else in a recent Consumer Reports test. You can get it at Home Hardware, Lansing and Cashway stores. A reputable builder should help you solve your problem since it’s obvious that a design and construction problem is the root cause of the difficulty.