The R value for IX that I came up with was calculated from the CLO value of 2.0 that the manufacture advertises. The R value of 2 layers of PE sheeting came from greenhouse studies and is based on the assumption there is an air gap between the two pieces (this is how cheep greenhouses are done, much like double pane windows but much cheaper). If you laid two pieces of 5 mil PE sheeting down and laid on top of it you would not have any where near an R value of 1.5 that was stated. Also something I forgot to mention in the other thread was PE sheeting has diminishing returns, so 4 layers would not necessarily get you an R value of 3
Thank you Yetee - I get it now regarding the PE sheeting with the spacing in between, makes much more sense. Sorry if I derailed the thread, but when I was young and dumb in the 70's I've slept on visqueen on packed snow - it was just like laying directly on packed snow, except you don't get wet. Getting that first ridgerest was like - WOW!.