What about how they respond to washing? Are they both easy to launder or does the UP have to be treated more like down?
Apex is a sheet, and UP is loose fill, so construction methods are quite different. I happen to use both in some of my hybrid insulated hammocks: the bottom layer of fabric is shaped to fit the hammock by piecing together irregularly shaped panels to which the sheet insulation has been quilted ahead of assembly. I double the thickness of the synthetic sheet insulation along the edges of the hammock. Then I sew that layer to the hammock, sizing it so the insulation along the edges is not compressed and there is an inch or two of space in the middle. Into that space I put a couple of ounces of lofting insulation - either down or UP. It lofts to fill the space. This uses the properties of both types of insulation to advantage in different areas.
More often than not, my choice of synthetic sheet insulation is Primaloft, but Apex works the same, and I've used that, too. For lofting insulation, at first I used down and found it worked well to eliminate air gaps under my back without compressing the insulation next to my shoulders. Based on that success, I tried UP; it does, indeed, act like 650 down. (I have also used 1" squares of Primaloft, cut from the edge scraps of my irregularly shaped panels. This works, but down or UP work better.)
How does this relate to material choice for an underquilt? You can use Apex quilted to one or both layers of fabric or you can use UP placed in tubes formed by baffles between the toip and bottom fabric layers. Karo step type baffles work, too. Leigh-lo quilts sometimes use lofting insulation (down) in tubes along the edges, with a section of Karo step construction (and more down) in the middle.)
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