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Answer: there is, but I don't think it is all that important.
I was out early today, started out before the sun came up here in the DC suburbs. for no other reason than curiosity, I stuffed two overkill down jackets into my backpack: a mountain hardwear sub zero parka, with hood, and eddie bauer's first ascent peak xv, the new deep winter puffy jacket i picked up on sale. both of these are heavyweight jackets, intended for cold temperatures, but also with 'overbuilt' shells and features designed for using them in bad weather and rough surroundings - lots of storage, waterproof/breathable outer shells, heavier nylon at wear/abrasion points on the shoulders and sleeves. nothing at this level of warmth would be considered ultralight, but each of these somewhere between 8-16 ounces heavier than a comparable feathered friends down jacket.
'significant' difference? one has 650 fill power down, the other has 850 fill power down.
when i stopped, i pulled them out and put them on (it was in the low 30's). back at home, i laid them out on the floor side by side. what i observed from the process:
-both are absurdly warm. better for -30 than 30, for sure.
-the loft on both the sleeves and the body of the jackets was indistinguishable. interestingly, the loft on the detachable mountain hardwear hood is a fair bit higher than the first ascent. they are both fine, but mountain hardwear's hood is really outlandishly stuffed.
-both are clearly designed for use on a mountain - the heavier shell material, inner pockets that are large enough to hold a water bottle and spare hats/gloves/snacks, enough outer pockets to hold more stuff, zippers & hoods that, when fully sealed, protect your face, cinch down easily, have enough room for a helmet, and that put the hood pulls in a place where they won't slap your face in a strong wind.
the newer jacket , the first ascent, is a mildly more simple design in some respects. it doesn't have a two-way zipper that opens at the bottom as well as the top; whereas both have draft tubes under the zipper, the mountain hardwear has a flap, with velcro tabs and a bottom snap, over the zipper. the mountain hardwear hood is removable, attaches with snaps, whereas the first ascent hood is built in and cannot be removed. the inner nylon shell on the first ascent is notably lighter weight than the mountain hardwear.
-the older mountain hardwear jacket with the 650 fill weighs several ounces more. i don't have a hanging scale, but i lifted each by the hang loop with my index finger, and the difference is fairly easy to feel.
price, which is a little hard to compare for jackets purchased at least a few years apart, isn't really an issue. both of these were certainly not cheap, but moderately-priced compared to premium brands (western mountaineering, feathered friends, valandre, in my mind, are in that category).
[as an aside, i have an 800 fill down sweater and an 850 fill down vest. the vest is less than 10 ounces, like wearing practically nothing, but it also has among the lightest-weight nylon shells i have ever seen. the down sweater's warmth to weight ratio is awesome, but again, superlight nylon. paying more for the lighter/fluffier options is certainly nice in how light they feel]
my takeaway is that the real difference between 650 and 850 down is marketing. i suspect that at least half the difference in the weight of these jackets is in design and fabric improvements. for jackets that loft equally, i would be willing to bet the difference in the weight of the down fill is no more than five ounces. for many people and many applications, i suspect it would be better to focus on the features and fabric of a jacket rather than the fill.