I personally have been conducting tests on waterlogged EPS floats. So far they are still not giving up the absorbtion. It has only been 7 years in the Nevada desert, however.

It's very likely the quality of the blow. Bad does exist. As a boy we used to buy EPS surfboard blanks and use them without further skinning. We often left them in the water and some would waterlog very quickly, others not at all. Leave it out of the water, however and it would dry out in a week or so. Maybe this early experience is part of the pre-disposition that you refer to?

good point.
I did cut one float log into thin sheets and it did dry out.
so, my experience and perception of my own observations ( "reality is perception and visa, versa.") is that yes, it does absorb a very small amount. It does not--to me at least--seem to want to give that back up in some conditions. (underground, applications where it is easier to absorb than de-sorb)
Density is a factor.
Quality control during manufacture is an issue.
In the old days (my ICF experience goes back to 1974 so that skews my perception as well) the quality and consistency was no where near what it is today.
It is still my perception, and thus my chosen reality, that the power of vapour transmission through a wall assembly is an under emphasized dynamic.
Lucky for me, considering my "sensitivities" (also a factor in one's perceptions/realities), I know of alternatives to plastic foams and feel that the research and development is in that direction.

Sorry about that, not been here in a while. I think you will get more bang for your buck with ICF then any one thing you can do.