Not absolutely sure on this, but you could try some plants in the room. There is water in the soil which evaporates also. Think conservatory/plants/humidity. They do absorb water, but do give it out. Also a water container - bucket or such and keep filled. You may need to heat the room in low temperatures and provide a water source for the humidity to rise.
If you have that humidity sensor you would know exactly what is happening. One in the case is good, but one in the room means you know what is happening there too. If the automatic ones for the case do work, then you do not have to have a sensor in there as such. Put it in the room. This means you do not have to worry about it when practicing (it being humid enough and similar to the case.)
You can look up DAMPIT as well.
http://classicalguitarbuilder.com/march ... etter.html