It seems like I just posted on the subject of bag vapor permeability. Don's poly-Nylon bags are definitely superior in preventing leaf from changing its case.
Polyethylene (Ziplocks of every kind) allow water vapor to transpire. The thicker the bag, the slower the transpiration.
Doubling the bags introduces a time lag between when the changes in ambient humidity (and temp) affect internal humidity (and temp). So, doubling a standard Ziplock is a dramatic improvement. It basically subjects the internal contents to the average ambient humidity (and temp), rather than to minute-by-minute fluctuations. Doubling a freezer Ziplock does provide some serious protection. But alas, if the average humidity is always quite low, the contents will dry fairly quickly. Doubled freezer Ziplocks plus some attention to case every few weeks works well.
If the humidity is always too high, then you will need to play with heating and evaporation. If you warm the air within the bag when the bag is vented, then it will tend to drive off moisture. Sealing it then slows the re-entry of moisture into the bag. This is all trial and error. Warm tobacco holds less moisture than cold tobacco, while warm air holds more moisture than cold air.
The greatest drawback of Ziplocks for whole leaf is the size of the bags that are typically available. Many varieties of whole cigar leaf will not fit a 1 gallon bag. Two gallon bags are not as easy to find, and are starting to get pricey, especially if you intend to double them.
With the perfectly sized WLT poly-Nylon bags, folding the cut edge a few times (then anchoring it with tape or a couple of clothespins) will maintain the internal humidity at whatever it is for many months, if the ambient humidity does not differ too dramatically. If you intend to store the leaf unused for more than a year, then thermal sealing will halt nearly all transpiration.
The bottom line is that any barrier will slow the exchange of humidity. How much of a barrier is determined by the vapor transpiration properties of the material. Even a paper bag works for a short time.
For a tropical setting, a tripled Ziplock, with a packet of desiccant crystals between the two outer bags might work.
Bob