I went through this a few months ago on my boat and studied the issue to death. I wrote it up (kind of a free from blog) with lots of little humidity calculators if you are interested Humidity
. The bottom line is that if you seal something up, it will go to 100% humidity. If you keep it open, you can keep it to the humidity of the outside. To do better than that, you need some kind of smart vents that let dry air in and moist air out (nobody does that on a boat) or an electric dehumidifier (air conditioner like contraption). The little bags you buy that fill with water help very little. Everyone is right, ventilation is the key. As an example, my cockpit lazarette cover was always dripping water when I opened it and everything in it was always wet. Since I started leaving it cracked 1 inch, it is always dry. My article has some hints on getting an accurate hydrometer cheap and a few other things but like has been said, it comes down to vents. I run a low grade heater with a fan not to heat the place but to facilitate air circulation.