Moisture is no more or less a problem in your outbuilding than it is in a garage. Primarily happens when we come out of one of our winter cold spells in late winter/early spring and the temps rise--consequently the %RH (dew point, if you want) goes up--and the moisture condenses on anything cooler than the ambient air temperature, i.e., your bike surfaces sitting in a cold garage/shed. Exact same thing happens when you open your freezer on a warm humid day. If you have any kind of environmental control (heat/dehumidifier) to prevent this, not a problem, and you only have to either keep things just a bit warmer or a bit drier than ambient--doesn't have to be 70*F. Only reason to insulate is if you plan to heat. I usually drag my bikes into the basement where it's warmer and drier than my unheated garage.