I have a problem every year as buy seeds. I buy more that I can possibly fit in my garden, so this thread is pretty timely. In addition, I was also thinking about making certain that I had a good number of heirloom varieties that would allow me to grow and collect seeds for the following year. (If need be!) That, or leave a collection of heirloom seeds to my one daughter who does attempt gardening.
So, as either you or Ellendra mentioned, moisture is big problem. I wondered about oxygen and how that affects seed longevity and found this.
"Experiments performed by Wageningen University and Research showed that next to low humidity and temperature, anoxia is a third important factor that can be used to prolong the shelf life of seeds, also called seed longevity. The reason is that the type of damage accumulating in the stored seeds is mainly oxidation, stimulated by oxygen in the environment. Early experiments had shown that seeds stored in an oxygen-free environment could survive much better in certain occasions. Seed companies were initially hesitating about applying anoxia, concerned about whether – as living organisms – seeds could survive without oxygen." ..snip....
...Gene banks store seeds from old varieties and wild crop relatives as genetic heritage for future generations. Since regeneration is costly and runs the risk of reducing genetic variation, it is preferred that the seeds can be stored for decades without loss of viability. Unfortunately many gene banks have to regenerate more often due to overly fast deterioration. One reason for this might be that many gene banks use glass storage jars from which seeds are taken from time to time (the Dutch gene bank does not use this method).
This creates a dilemma for the gene banks: On the one hand, they want to be able to supply seeds on demand, but on the other they prefer to open storage jars as infrequently as possible in order to minimize any deterioration of seed quality. If the oxygen could be removed from seed jars after opening, the seeds would maintain quality much longer. This can be easily and inexpensively done by including a package of oxygen absorbing iron powder in the jar next to the desiccant. An alternative is removing most of the oxygen through vacuum packaging in hermetically sealed foil bags, as performed by for instance the gene banks in the Netherlands and the International Rice Research Institute in the Philippines."
Amazon sells sealed oxygen absorbers (You want the individual vacuum packed absorbers.) and dessicants. I think that I will store my unused heirloom seeds in a 1/2 gallon Mason jar and include these:
https://www.amazon.com/s?k=oxegen+absorbers&crid=93P0Y0GAXNAY&sprefix=oxegen+absorbers%2Caps%2C125&ref=nb_sb_noss_2
https://www.amazon.com/Premium-Silica-Packets-Desiccant-Dehumidifiers/dp/B00DYKTS9C/ref=as_li_ss_tl?s=books&ie=UTF8&qid=1508524538&sr=8-1&keywords=silica%2Bgel%2Bpackets&linkCode=sl1&tag=tenthacrefarm-20&linkId=dd69a53891d8ee302e9064db03d146e2&th=1