Insultex is an incredibly thin, micro-porous, closed cell foam. While it does not compress, it is so thin and light that the pack size is only slightly larger than down (when comparing for the same temperature rating).
This three layer design will get most users to 35-40° F when wearing warm sleeping clothes.
For most people, each additional layer of Insultex will give another 10-13degF of comfort.
Another great use for Insultex is as an outer shell for a lofted insulation (like PrimaLoft or Down). As a closed-cell foam, Insultex will cut heat loss significantly that the wind would normally steal from lofted insulations. Just a layer or two will drastically boost the temperature rating of your current insulation system.