Dane of www.coldthistle.blogspot.com says: “Primaloft 1 (PL1) and Arcteryx's Coreloft are the only two synthetic insulations I would buy for climbing use.” Bearbreeder says, “known PL1 belay parkas and sweater ... DAS, Micro/nano puff, FA igniter/serano, cloudveil enclosure, etc ...” MY QUESTION: How do the Mont-Bell thermawrap parkas fit in?
DANE: “My idea of a "belay" anything is an additional layer with a intergal hood to add at the end of a pitch when you are wet from sweat and will cool down rapidly while belaying. So you add the "belay" sweater, jacket or bivy weight outer. What I want that layer to do is keep me warm while stopped and most importantly dry me out when stopped or while climbing.
For the use as a "belay" jacket at any weight/insulation thickness down insulation is obviously limited to very cold temps and limited physical levels.
So to keep them all straight in my mind I have used the three terms, "sweater", "jacket" and "bivy" to define levels of warmth and amount of insulation. Obviously there are some pieces that will overlap in utility and warmth in each category and each person is different. Fatigue and your physical condition will change as well and require different levels of insulation at different times. Belay "sweaters and jackets" I will climb in when cold as a second or when conditions dictate that it is cold enough I have to leading.
The "Bivy" level are the thickest belay jackets generally not all that fun to climb in as a second and worse yet while leading except in really, really cold conditions (Alaska or high up in Canada winter) and on moderate terrain where you aren't working too hard but make a perfect additon to a light bag or even alone on a sparce bivy.
I like the Atom and a buddy has been using his a lot as a belay "sweater" in moderate conditions like low level waterfall cragging in the desert. Maybe the perfect "sweater" there. But I think it is better used as a primary cold weather climbing piece than a belay specific piece. I like it in that position while saving a Compressor hoody as the belay jacket to go over it. Makes a pretty warm system for cold weather climbing. More than the Compressor Hoody could do alone for sure. And 1/2 the weight of the soft shell it replaces.
I used that combo (Atom/Compressor) at 0F as a bivy set up with a lwt bag (Vireo) and made it through the night and actually got some sleep while in terrible physical condition, starting wet and dehydrated. So the system works
But in general I would suggest a pile/shell (or soft shell) combo with a dedicated "belay" jacket. Compressor Hoody (lwt weight) or DAS (warmer level and a cross over from "belay" to "bivy") as a good outer depending on the level of warmth and compressibility when stuffed that you require.
Steve House's Patagonia video discussing his and Vince Anderson's clothing system on Nanga Parbat is still "cutting edge" and a proven clothing combo.”