It seems like the go to materials for making an ultralight bivy are momentum or pertex top and silnylon or cuben…
The go to material for sleeping bags is also momentum or pertex…
And the go to material for a ground cloth is cuben or silnylon…
So my thinking is that the momentum top in a bivy is a bit redundant, it will add water resistance but under a tarp it's not paramount.
So what it someone made a momentum sleeping bag with wings that attached to a cuben or silnylon bathtub groundsheet creating an enclosed sleeping bivy
