Thank you for the information and insights. Alas, I am not up to speed with all the science and terminology. My interest stems from having to design and make my own sleeping bags, as I am only comfortable with a center (over the chest) zip, and I have not been able to find anything truly ultralight with syntehtic insulation. The bias toward synthetic inslation comes from several long hikes in damp, colder weather, where the moisture coming from my body dampened down insulation that then froze, rendering the bags useless. Not experiences I care to repeat.
Since I have to make my own bags anyway, I am interested in finding the most weight efficient layers to use.
Over the years, there have been a number of add-on products for synthetic layers. Early on, there was Texolite, a needlepunched foil sandwiched between scrim, as in the Yakworks Yaksack. At the time, there was a consensus that it was somewhat effective, albeit heavier than represented by the manufacturer.
Another approach has been to needle punch space blankets made of reflective mylar or polyehtylene materials, and this seems to be what is alluded to above, albeit with the conclusion that it is not effective.
So, what drew me to this thread is the possibility of using some other needle-punched reflective material like Insulbright, or a thin foam like Insultex, as an inner layer (sewed to the inner shell) with the more conventional synthetic insulation (choose your own - I don't want to get into that discussion, which can be endless) sewn to the outer shell.
What I understand you to be saying is that the Insultex, as thin as it is, nevertheless by taking a set when you lay on it, becomes ineffective when you roll over and need its full insulative value in the quilt or bag. Not encouraging, but the insight is very helpful, because I don't want to put a lot of effort into sewing it or Insulbright into a bag for nothing but a few ounces of worthless added weight.
Better to just add a little more thickness of the more conventional insulation of one's choice.
One other point is that the Insultex supposedly does breathe, in that it is represented by the manufacturer to allow water vapor to pass through it, but only in one direction. Credible? I don't know.
Thanks again. Am in the middle of a tent project this winter, so there is plenty of time to keep searching for the magic bullet to go into the next bag.
Sam Farrington, Chocorua NH