/ Breathability of down vs synthetic
If they're broadly similar, is the 'controlling resistance' the face/backing fabric?
If its a midlayer then I would have thought that the outer layer will dictate what happens. Sweat is not going to be able to get through an insulated mid layer and then a waterproof outer layer, no matter what marketing nonsense the manufacturers may say. The mid layer will just absorb it all first whether its down or primaloft. Its just that down will feel worse as damp clumps.
That's basically the point of Polartec Alpha, isn't it?
The face fabric of down jackets has a very tight weave to prevent the down leaking out as much as possible, this has some impact on the breathability. Fabrics for synthetic insulation don't need to be quite as tightly woven, so are slightly better for breathability.
As Shearwater says, Polartec Alpha is a new synthetic insulation that can cope with much more breathable fabrics. See: http://polartec.com/warmth/polartec-alpha/ How much better is it? No idea!
All I know and care about is that it keeps me warm and if I'm sweating wearing it: it's working fine. I just unzip it a little or remove it if it's too warm!
I've been testing the Marmot Alpha hoody thingy - I'm not totally convinced so far, the inside is mesh so ultra breathable but the outer shell is pertex which is as breathable as pertex is - pretty good to my mind still the same as any other pertex thing. So it's nice but I'm not convinced its a replacement for a say a micro fleece and windshell.
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