Down Equivalence?

I think that 2 ounces of 400 fill power down would be equal in insulative value to 1 ounce of 800 fill power down. Not sure on that though. Also take into consideration the shell material adds some insulation.

Generally thickness per weight is the biggest factor. But there can be other variables theoretically, if not practically. For example, you could have an insulation that say is a certain thickness per weight, but the fibers are larger, then it would be less warm than another insulation which has much finer fibers but at a similar thickness per weight. Doesn't really apply to Down though in real world because the size of the barbs and barbules/fibrils don't tend to vary a lot even between duck and goose down. Might apply more to different kinds of synthetics. Edit: The rest wasn't really applicable to your post and questions (and may have a confusing effect).

the fill power is the oz per cubic inch. So 1 oz of 800fp down will cover 800 square inches to a thickness of 1 inch. As a previous poster mentioned, you'd need 2 oz of a theoretical 400fp down to cover the same area to the same thickness and yield the same "warmth." There are variations in the insulating value of different fill powers, but not enough to justify factoring them in as real world performance is always so inconsistent. A solid rule of thumb is that in the end, thickness is all that matters for down. Different fill powers just weigh more or less. How much thickness you need is a question only your personal experience can answer, but it gives you a a good way to compare items. So assuming design, size, cut and all other factors equivalent between two jackets….then to figure out the relative warmth between them you just compare the weight of down fill * the fill power. That gives you the cubic inches of each garment, the one with more is warmer. If you want to convert, just take the ratio. So for the above example, 800fp / 400fp gives us a 2x ratio. So you need 2x the fill weight of "400fp" down to be equivalent to an 800fp garment (all other garment design factors relatively equal).

2.46 oz of 650 fill down would fill the same volume as 2oz of 800 fill power down. 2(oz) x 800 (fill power) = 1600 (cubic inches) 1600 (cubic inches) / 650 (fill power) = 2.46(oz)

lower fill power has more fibers in the volume, which inhibit internal convective air flow, so maybe it's not quite proportional. Maybe for 400 fill you don't need quite twice the weight of down as 800. But nobody uses 400 fill. For 650 vs 800 it's close enough to proportional. Just theorizing. RJ once said something about studying lower fill down. Less affected by humidity or dampness because there are more feathers. I don't remember a conclusion though, so just another theory.

Appreciate the confirmation and additional details!

"lower fill power has more fibers in the volume, which inhibit internal convective air flow, so maybe it's not quite proportional." I know this is true for Eider Down (see the PDF that Rick M recently shared) which has lower FP than high quality Goose Down but has more fibrils per shaft/barb, but i wasn't aware this is true generally for regular Duck, and Goose Down?

like I said, theorizing : ) it just seems like if it weighs more per volume, there will be more fibers per volume if you compare synthetic to down, synthetic weighs twice as much, but has more clo/inch than down. At least by my measurements

I looked at MH stuff since the employee store is nearby and ended up getting the Kelvinator Jacket (271g Q.Shield 650) sewn through, weighs 17oz. They also had the Phantom which also weighs 17oz and uses 180g of Q.Shield 850 but with baffles but quite a bit more expensive and didn't seem worth it to me. Wore it today on a day hike with a T-Shirt in mid 50s with 20-30mph wind, quite toasty if not a very good layering plan.

Jerry, you're right on both accounts. No real studies done to quantify the effects of fp on warmth/thickness or resistance to humidity though. As such, I just use the rule of thumb that the higher fill power is slightly more insulating and collapses slightly more with humidity and assume it's about break even on any performance gain or loss :) Considering how variable and subjective perceived warmth gets, no need to over complicate the formulas. I'd say if you live in humid region that likes to hover around the freezing point, you probably don't need to focus on maximum fill power rating as it's not quite as effective as in the dry regions like the rockies. Other than that, get what you can afford/want based on your needs and always err on the side of warmer vs lighter until you understand how you personally tolerate the cold !

Ryan Jordan ( ryan – BPL STAFF – M) Locale: Greater Yellowstone NEW Re: Re: Re: Introduction to Outdoor Retailer Summer Market 2008 on 08/10/2008 08:04:56 MDT Bill et al., I spoke at length with IDFL yesterday about down testing. None of their tests stimulate real world testing. 900 fp in a test is going to be a pipe dream in the field, because they steam wash and dry the down to nearly zero humidity before doing the test. Ironically, this most recent iteration of test methods was designed to determine the maximum possible fill power for down rather than what it will look like in the field. Interestingly as a side note, we did some 900 fp testing of down a few years ago on two manufacturer's 900 bags. We cut the bags open and sent them to IDFL. Neither made the claimed 900 spec (they tested 830-870 using the steam method). What was more dramatic was that when each down (which clearly came from different sources as evidenced by visual inspection) was subjected to 50% humidity, the differences were pretty dramatic. One bag tested at 770 fp, the other at 680 fp. It seems that at least these two sources of 900 down had feathers in it that were not resilient in response to humidity. The kicker is that we ran the same test next to down taken from a manufacturer's 750 fp bag. at 50% humidity, the fp was 720. Why? It had more feathers that were stiff enough to preserve the loft in moist conditions. http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=39920