Storing Coins In The Basement

To be honest, I think the basement is the worst place for coins. I'm not sure where you live, but if there's ever a possibility that the basement could flood, I think I would reconsider. As well, near the hot water heater is probably the most humid place I can imagine, but perhaps a de-humidifier could take care of this problem.

You should really purchase a Hygrometer to measure the humidity. Although you think a basement is dry, it may be horribly damp and you do not know. For one thing concrete is porus and absorbs and emits humidity constantly. Even concrete that is many years old may still be wet if no really great air circulation. And then too the humidity in your basement may well depend on the water table of your area. Although most don't realize but there is a waster table height that may well effect your home. For example many construction projects are dependant on this for possible frost lines that could well effect piping. Again, your concrete basement may well be absorbing moisture from the outside water table and emitting it on the inside of the basement. In addition to all that, moisture in the air is one of the heaviest gasses so it settles down to the lowest place in your home, the basement. Also, note that in some areas of the country Sulfur rich coal is still burned in power stations or steel mills. This Sulfur in the air is now SO2 and mixed with moiture forms an Acid which most know as acid rain when it rains. If your hot water heater and/or funace is a gas burner, not the flame is open. Most of the gasses go out but not all. Natural gasses by products are CO2 and Moisture making another source of humidity in your basement. Now also, where is your cloths dryer and is it too gas?
If a dehumidfier runs constantly, it is either to small a unit or the thermostat may not be functioning properly and/or may not actually be working at all. If it does produce a lot of water, then your basement is a real damp spot.
As you said there is some type of stuff on the walls. This stuff may well be an absorber of moisture also. A findished floor does not stop moisture completely but does slow it down.
To sum it up, a basement is not usually the best place for coins or anything of value.

Think the only problem with the basement is humidity and air circulation. If you can get that under control and always keep it under control there should be no problem. Large sealed plastic containers and a desiccant bag in each will work great as an option and/or additional protection. Get the kind you can re-cycle in your oven or microwave. Re-think the water heater location, unless enclosed? Mainly due to the one in a million pipe break.

I had a coin collection in a basement in Long Island for a while. I was stuck living there shortly after marriage because LI is incredibly overpriced and it is nearly impossible for students to find a place to live. I rented out an older couples "finished" basement. I didn't have any problems while there, but...who knows. I also stored my collection in a basement in Missouri for a few years. I guess my biggest advice is not to store anything even remotely close to the floor because of the humidity in the concrete and because of the possibility of flooding.

Of course you have to eventually open any bag, box, safe, etc. As soon as you do, any humid, gassous air rushes in and you start all over again. As I said an electrical dehumidifier for a basement is the only real method if you must keep coins in a basement. Other methods work but are less effecient and more troublesome.
Actually that attic was a much better location than you think. Most attics are leaky, airated and that flow of air makes the formation of moisture less of a problem than a stagnent basement.