Cigars stored above a relative humidity (RH) of 75% or greater will mold. Cigars rolled with overly damp filler may mold, even if the storage RH is below 75%. Below about 60% RH, cigars tend to become too dry, and the foot may split.
b. Temperature stability
Cigars should be stored at a relatively stable temperature, or at least an ambient temperature that does not swing rapidly. The manner in which rapidly changing temperature affects stored cigars is by its effect on RH. An increase in an ambient temperature of 20°F will drop the RH by half. Stored cigars within any container are not uniformly heated or cooled. One part of the container heats more rapidly than other parts. This creates warm spots and cool spots, creating a heat pump that will drive moisture from the warmer area, and condense it in the cooler areas. Cigars in the cooler area then become subject to mold.
Cigars can be stored within bags or boxes of various materials. And these may or may not be themselves contained within a humidor or larger box or tub. Wood containers provide better temperature insulation, as well as a better stability of the internal humidity. Bags and plastic boxes or tubs do not have those advantages. If a plastic container can be made vapor-proof, with a tight-fitting, gasket lid, or a bag zip-closed or rolled and clamped at the open end, then humidity can be maintained for much longer. Thin polyethylene bags, like sandwich bags, are not vapor-proof. Thicker polyethylene bags, such as freezer bags, are more vapor resistant. Laminated poly-nylon bags are completely vapored proof-the material used for vacuum-sealed foods.
The greater the total mass of cigars (or whole leaf) stored within a container, the more slowly it changes in both humidity and temperature. So a full humidor is more stable than a nearly empty one.