So what is the advantage of the rubbery ponchos over the nylon ponchos? ... other than providing super cool science projects on a hot summer day.

About the use of closed cell foam to insulate one's butt on a camp chair or bench ... Years back, when I was more involved in my son's Scout troop (he'll turn 18 in May so I've faded away from troop activity - planned it that way) it was really cold out and I found and used a small sheet of corrugated cardboard as insulation on the seat of my camp chair. As usual, while I was up doing something someone else sat on my chair and was shocked by how warm it was because of the cardboard. Ever since then winter trips will find the adults hunting for corrugated cardboard for their chairs. Pretty funny.