It’s been a while since I thought about this, but at some point when we got our foil bags I had done some lookinga round and this is what I’m remembering…
One thing I didn’t know going into it was that foil bags are not just foil, they are a mix of layers of plastic and foil, often several layers of varying thickness, not just one of each. A lot of vendors will publish information about exactly what the layers are and even post how much exchange through the barriers there are (for both air and moisture, sometimes even breaking out Oxygen and CO2, I think). The upshot is that foil bags, in addition to providing light protection, also have increased exchange protection (i.e. they allow less exchange of air and moisture). My assumption has been that the better the resistance, the better the storage, and usually that means the thicker the bag (especially the foil layer), the better the resistance. But I’ve wondered if there is a certain thickness beyond which you are paying for more than what you really need.