I had 4 days of testing this weekend in pretty cold conditions in the adirondacks and have narrowed it down to what I think will probably be the perfect baselayer for my needs.
This baselayer is going below a vapor shirt from RBH.
it needs to be:
-as lightweight as possible, in other words not thick/warm but also gram light too!
-a design that encourages small pockets of air to be trapped/warmed
-have a snug fitting hood
-ideally a chest pocket to warm my snacks but this is not a deal breaker
I tried a patagonia R1 hoody, a sugoi shirt that is windproof on the front and has a roubaix fabric on the rear (it's from my cycling collection), an old EMS bergelene midweight baselayer, and finally a climatize EMS powerdry/merino blend.
my favorite was the r1 but it's heavy and gets even heavier when soaked with sweat. a lighterweight version of it would be ideal but i don't see one on their site.