PE sheeting - this is basically visqueen were talking about right? its essentially incompressible. And while it was what I used for years for a ground sheet (as recommended by colin fletcher) until I switched to tyvek, it did essentially nothing as far as insulation - its only 5 mils thick ( I think I used 6 mil, but whatever). I still think there is something going on regarding units or something because I find it hard to believe 4 sheets of visqueen = 3/4" foam pad (of course I've been wrong before, just saying)