Calculating fill power from lofted down

[warbonnetguy]

one way might be to buy an oz or 2 of down of a known fill power and compare it to what you've got. i know that 850FP lofts to alot more than 850 cu. inches (possibly about double that) when just sitting loosely in a box

[warbonnetguy]

. But in the age of cuben and momentum 50, why do people over-stuff?

overstuffing increases the warmth of the item and keeps the down from shifting as easily, reducing the likelihood of cold/empty spots.
there is a point of diminishing returns though, too much overstuff is not helpful and just adds weight, at some point you're better off moving up to a taller baffle height.

"overstuff" is hard to define though. say an oz of 850 fp takes up 1500 cu.in sitting in a box, well nobody uses an oz per 1500 cu.in. as the fill ratio, so technically every down item is overstuffed to some degree or another. the exact ratio used varies by manufactuer

[Deep Thought]

AFAIK down creates warmth by lofting and creating dead-air space that provides insulation, is that right? So it's the air that does the insulating and not the down fibers themselves?
When there is enough down to fully loft the baffle, how does adding more down make it warmer?
If you have a 800 cubic inch chamber and add 1 oz of true 800 FP down, considering the down has already filled the chamber, won't more down just reduce the amount of dead-air space and compress the other down?
Funny, as I was writing this (and doing some Google searches) I found the Valandre website:
We do not advise overfilling down. Down is not warm by itself. What insulates is the exceptional capacity of down to trap body heat. In our products, we are using a certain density, which is the relation between compartment volume and injected quantity (which sounds like fill power to me). Overfilling changes this balancing point, and creates compression. And compressed down will not trap more body heat.

As for down shifting and cold spots, don't baffles and gates control the shifting of down due to gravity and movement? And wouldn't cold/empty spots have more to do with the distribution of the down and preparation/fluffing the bag than the amount of down? If I don't prep my bag after pulling it out of the stuff sack, I think some of the down, overstuffed or not, can stay compressed/wedged in a corner or one side.

I've got a continuous baffle bag and it's great because I can shake and force the down to the bottom (below me) when I want cold spots/empty spots on top when it's too warm (and do the opposite in winter when the down is otherwise compressed and useless underneath me).

Too much of anything is good for nothing gramps used to say.
I'm still not sold on the overstuffing. The only reason I can think of is that the advertised down fill power is over-rated and 1oz 800 fill doesn't effectively fill 800 cubic inches.
If adding overstuff or the same weight of 900 fill makes it warmer, then that tells me that 800 fill doesn't effectively fill 800 cubic inches. Additionally, if adding more down provides diminishing returns, then that means that there is an optimal density--which I assumed was the basis for the fill-power test...silly me.
So, if you need more than 1oz of 800 fill to effectively fill 800 cubic inches, wouldn't that mean that "fill power" would need to be re-examined? (new test, heavier weight, etc.)

I didn't realize I'd be so curious about overfill.

DT

[bear bag hanger]

The cheaper a quilt/sleeping bag is, the more likely I'd order it over stuffed. A few of my older/cheaper down quilts have gaps that, I'm guessing, wouldn't be there if I had ordered them over stuffed.