Active insulation breathability - exposed Polartec Allpha “nap” more breathable than sandwiched between 2 nylon-ish fabrics?

The OR Ascendant and Rab’s Alpha Flux have an interior where just the polartec alpha fibers are exposed.  Other active insulation pieces tend to have some kind of slick inner nylon - type fabric (eg Patagonia nano-air, Arcteryx Proton).   Does the lack of this inner shell-type fabric result in more breathability?  Or does the existence of an outer shell fabric make the issue moot (maybe the vapor permeability is not linear / additive in nylon layers)?  

I run hot and am trying to talk myself into one of these pieces but based on experience I feel the lighter one(s) will work best for my needs in the Cascades where it is relatively less cold.  As such, the exposed alpha fibers seems like a feature.  But maybe I’m not thinking about it right (or overthinking)?

I just bought an Ascendant but haven't used it yet. Probably get out in the next few days and will reply with some feedback.

Originally Polartec didn't want their Alpha fabric exposed, until Rab convinced them to allow it for their pieces (and later OR). From what I can tell looking at the two pieces I own (Ascendant Hoody and Nano-Air Light Vest) Polartec Alpha is more of a fleece-style synthetic (with a patterned structure similar to a grid-style fleece), while the Patagonia insulation (and I believe Dead Bird as well) use a continuous filament insulation that is less durable and more likely to snag, hence the inner layer. I don't really want to cut my vest open to confirm that though...

I’ve used pretty much all the active insulation jackets and regardless of which one it is-once you get exercising enough and working hard enough they all need to be shed.

To me they are great for activities like ice climbing and rock climbing where your working but have rests etc at belay stations.  The ascendant jacket seems to be the most breathable of the bunch but even while skinning or running for a sustained period in temps 0-15 degrees it needs to be removed.

Best jacket imo for activity with high breathability is the arc’teryx acto.  I have 3 of them and have sold all
My other active insulation jackets except a photon and ventrix which are reserved for more relatively stagnant activities.  

The acto with a Squamish will get you through most cold weather conditions fwiw 

Gavin is right about the difference.

Alpha is basically an evolution of fleece, think of it as extra hard face fleece (Uberlayer, Alpha flux). The alpha fibers need to have a higher amount of abrasion resistance in the "direct" version than the insulation fibers of continuous filament insulation.
VS
Nano Air, Ventrix, Atom (don't know of the others that are using the same tech) are a "continuous filament insulation" ie a sheet insulation that usually has a breathable stretch woven on both sides of it. Since the insulation is protected by the woven on both sides they can be a finer fiber. 

Would be interesting to see a scientific testing of some of the stand out pieces for breathability vs weight (wet and dry) vs insulation. 

I recently got an Alpha Freak for the winter. It does seem much warmer than a normal fleece (R1) when used with a shell. I haven't had a chance to use it in really cold weather yet, but my impression is that for high aerobic output it's too warm for all but the coldest days (single digits at least, and I get cold easy). But it's completely air permeable, and without a shell is much colder than my R1. For stop/start or moving slowly at altitude, obviously it would work well on warmer days. To me the big plus is being able to pair it with whatever shell is appropriate, but I'm not sure how the warmth to weight of Alpha compares to whatever Patagonia is using and whether the lack of an integrated liner saves enough weight make up the difference (if there is one). 

akafaultline wrote: I’ve used pretty much all the active insulation jackets and regardless of which one it is-once you get exercising enough and working hard enough they all need to be shed.

To me they are great for activities like ice climbing and rock climbing where your working but have rests etc at belay stations.  The ascendant jacket seems to be the most breathable of the bunch but even while skinning or running for a sustained period in temps 0-15 degrees it needs to be removed.

Best jacket imo for activity with high breathability is the arc’teryx acto.  I have 3 of them and have sold all
My other active insulation jackets except a photon and ventrix which are reserved for more relatively stagnant activities.  

The acto with a Squamish will get you through most cold weather conditions fwiw 

$350 for a grid fleece lined softshell, ouch. More than a Proton LT. 

Looks like it breathes well, nothing really beats fleece. Patagucci had a somewhat similar piece called a Dual Aspect a couple of years ago, they discontinued it suddenly like they do every good piece they make.

When a Proton/Nano etc.. is too much I'm usually ok in thermal base layer + windshirt. But this piece looks like a good compromise.