Could it be due to a less porous material? I've seen the thermolite, they seem more permeable, like a car cover type of material. Never seen the heatsheets. Are the heatsheets more of a plastic feeling like the mylar spaceblankets? I don't see any temperature ratings an them except for the thermolite.

I've been following this topic closely because I'm looking to replace the Outoodr Research bivy sack that I have with something a lot smaller and lighter. Does the bivy have any type of drawstring to close up the head opening?