Modern technology has offered some wonderful and useful tools when regulating humidity inside your humidor. Depending upon the size of your humidor it is not necessary to purchase expensive electronics to do the work for you.
Ideally for perfect aging conditions, your humidor internal Relative Humidity should be about 68% - 72%. Any less than 65% and your cigar wrappers will likely begin to crack. Any more than 75% and you could run the risk of mold on your cigars.
We at Vanderburgh like using the passive Humidity Bead or the RH Crystal system. We find these solutions control humidity more accurately than the typical sponge system you find in cheap humidors. Crystal systems require 'Humidor Solution', which in most cases this solution consists of a solution of 50% Propylene Glycol and 50% Distilled Water. This balance of PG to Water usually keeps the humidor interior to about 70% RH give or take a percentage. If you decide to only use distilled water in the crystal system you run the risk of having mold appear inside the humidifier. The bead system works very well and only requires distilled water to charge the beads.
Propylene Glycol Removes Humidity.
If your humidor is consistently running too high in RH (Higher than 75%) - try adding a bit of Propylene Glycol into your cigar solution. PG will draw moisture out of the atmosphere inside your humidor and will lower the RH. Initially try adding an additional 5% PG solution and play with it from there. Since every humidor can be different in size this process is trial and error.
Alternatively if you need to raise the RH in your humidor, try adding more water to the humidifier and less of the propylene glycol solution.