Vacuum insulated panels (VIP’s) are made out of a composite of a few layers of plastic film with an aluminum foil sandwiched in between the plastic layers. Then there is a core, spacing, insulative material placed within, a vacuum is pulled to remove most of the air and during that it is sealed while under vacuum (there are also Getters placed in there to help absorb and/or adsorb moisture, oxygen, off gassed molecules of the plastic, etc). The highest quality VIP’s are rated for 50 years of useful life. This means that they can successfully almost completely limit the huge majority of gaseous exchange from within to without for a very long time (and under strong atmospheric pressure, pressure strong enough to break/splinter thinner, S glass reinforced plywood. Which I’ve done while doing some VIP making experiments). If they didn’t, they wouldn’t work. Vacuum insulation requires well, a vacuum, to work optimally.
The aluminum foil is doing the lion’s share of the heavy lifting in these systems. This is what prevents the huge majority of gaseous and moisture exchange. If there is no, or very little gaseous exchange, then there is also little to no scent being exchanged (scents need to be convected with and by gases to spread). The plastics on the other hand, well most plastics do allow some gaseous exchange over time (via molecular diffusion), as well as offgas some (there are a couple plastics that can hold vacuum much better, but they aren’t typically used in VIP construction for other reasons, like sealing/bonding issues. For example, PTFE).
With VIP’s, they do ultimately fail because there are micro to nano gaps between the plastic and aluminum foil layers, but I would say up to 50 years of holding a significant vacuum is pretty impressive.
What this means for bears and scents: If you use vacuum sealing bags with an aluminum liner, or mylar bags with an aluminum liner, and you properly use them, these would be much better than nylofume bags and the like for limiting scent diffusion.
The best practice would be to heat seal them, but that is not really feasible or practical in the backwoods. Instead, when at home, you use a little heat and a hard flat surface (like an iron), to fold over a few times, and then secure the last fold (with reusable tape, hook & loop/velcro, clips, or the like) this is the second best way of limiting odor diffusion. As long as the aluminum foil is tightly folded over itself a few times, it will be very hard for inside air to exchange with the outside air.
Again, by limiting gaseous exchange, you are also limiting scent exchange. A bear’s sense of smell is remarkable, yes, but operates within the bounds of physics. If a barrier is good enough for holding vacuum long term, then it is also very good for severely limiting scent exchange. It is really that simple.
Of course handling food and touching the outside of the bag will leave food scent on the bag. Pretty hard to eliminate that completely. However I wouldn’t worry about that. A bear’s sense of smell is so acute and their intelligence high enough, that they can differentiate between relative amounts–differences between residues (like on a used candy wrapper, or the outside of a touched odor reduction bag) and outright full meals/larger caches, and then run risk assessment. Why would a bear go investigate residual odors alone, unless it was starving/desperate? If they are under the latter condition, I wouldn’t worry too much about light, residual food smells, because then you’re potentially on the menu.
