The idea that insulation can lose loft (over time as a result of wear) but not warmth is probably a bit ludicrous and violates pretty basic principles of physics and heat transfer.
This is the problem with anecdotal evidence, and what happens when youtubers/thru-hikers start defining the conversation (sorry, couldn’t resist! but I don’t discount their testimonies either…)
I have a textile microscope that I use to look at fiber-level structure for fibers down to about 3 microns in diameter. A few things I look for when it comes to loft degradation are fiber damage and batting porosity (% volume of air). I also use thermal imaging (on real people) to see how insulation degrades over time, to view cold spots.
The problem with short-staple insulations made up of cylindrical fibers (especially solid ones that are traditionally extruded) is that batting porosity goes way down in response to repeated compression cycles. The reason is that individual fibers start getting crimped (damaged), and (what we call in textile research) “resiliency” or resistance to compression, or “bounce-back” goes down. Larger *effective diameter* (not necessarily denier, which is a density measurement) fibers do better here.
That’s why Teijin Octa is such a nice development. You have a hollow fiber (so it’s light for its diameter) and a bunch of (8) fins (so it’s effectively thick-ish and resistant to damage). Wrap that fiber in a bundle, and transform that bundle into a batting, and you (potentially) have something that’s a “standard of some type” in the synthetic insulation category.
That’s part of the premise of the Proton FL’s performance:weight ratio – but I’d probably look at Octa blends for more creative uses as well…2021 will see some advances in synthetic insulation, certainly, as these blends hit the market (we now have tricot-like mesh blends of bicomponent Octa (FL), and it’s now being developed into merino blends…hmmm…)
And don’t discount Patagonia Plumafill. They focused less on fiber, more on batting structure, and created a winner there in terms of longevity (due to macro-structures and macro-porosity built into the batting). I see this type of insulation structure gaining momentum in the next few years.
Bottom line:
1. Apex is sorta durable but you still have to baby it (don’t compress, launder gently) if you want longevity.
2. And stay away from short-staple cylindrical fiber insulations if you want longevity.
3. And if you want any semblance of real longevity, go with down. There’s material physics with extruded petroleum polymers at play that just isn’t going to make synthetics any kind of holy grail right now.