3 layers of Insultex in an UnderQuilt constructed using a differential cut will get most people to 40*F, some (including me and Thing1) to freezing. InsulTubes help (learn more with HF search). It is the easiest, lightest, cheapest way to insulate a hammock above freezing.
I don't recommend using Insultex as your primary insulation below freezing because it is a vapor barrier, and because the amount of Insultex required takes it beyond the realm of practicality. If you use it to supplement down below freezing, be sure it is between the hammock and the down.
Below freezing, I switch to natural fibers and down.