The trick to a warm UL jacket is the fit and sealing in the heat better from the bottom cuffs and neck.
fit is the most underrated issue on these forums i find
everyone oozes over the weight … but with an ill fitting insulating jacket all that weight savings means little, youre basically giving up “warmth” for a “paper weight” gain vs a slightly heavier but better fitting jacket
in fact a good fitting 650 fill jacket may be just as warm or even warmer than an ill fitting 850 fill jacket of roughly the same down fill
IME in order of importance for “warmth”
- down fill weight (as napolean said god is on the side of big batallions)
- FIT, FIT and FIT
- design and features to improve the fit and heat retention
- fill power (as long as its not a huge difference)
for example if you dont fit a MB exl well which doesnt have a bottom draw cord or elastic (mine doesnt) … it doesnt matter its 900 fill down … youre basically acting as a bellows pumping your warm air out of the bottom every time you move around. and heating up dead air space
which is why some folks have found the cheap uniqlo jackets to be as warm as other higher fill powered ones …. or folks find dead bird atoms with fleece panels warmer than patagucci nanopoofayz … FIT
paper weigh is irrelevant if it dun FIT
;)