How much loft for 15F? (Down)

[thepikey]

Given the formula above, the ratings would be different for example the quilt above 60*40*3/550 shows 13.1 oz of 550fp down needed, but if you were to change it to 800fp
only 9oz would be needed. So if you used the same amount, I would expect you to reach really low temps.

[BillyBob58]

http://www.jacksrbetter.com/Mt%20Washington.htm

The Jacks ‘R’ Better philosophy in creating our 900 Fill Power option.

First of all, we have not redesigned the quilt shells. Whether the insulation is 800 fill power down or 900 fill power down they are the same width and length. A three season model has a two inch baffle and a winter model has a three inch baffle. That means for a given quilt model, the internal volume is the same. The difference is that it requires less 900 fill power down by weight to fill that volume than it does 800 fill power down. Thus 900 fill power three season quilt will weigh about 1.5 ounces less than the same quilt with 800 fill power down. Similarly, a 900 fill power winter model will weigh about 2.25 ounces less than the same winter quilt with 800 fill power and a 900 fill power summer quilt will weigh about 0.75 ounces less than a summer quilt with 800 fill power down. The basic performance of the quilts is established by the quilt dimension/design not by the insulation material. We provide a 15-22% overstuff factor in all our quilts. This over stuff compensates for deviations from certified standards and it provides for field variations in humidity from laboratory rating and testing standards. It also reduces down shifting beyond normal expectations.
Again, the insulating performance of an 800 fill power quilt and a 900 fill power quilt of the same model are identical. The 900 fill power version is simply lighter.

[TinaLouise]

Given the formula above, the ratings would be different for example the quilt above 60*40*3/550 shows 13.1 oz of 550fp down needed, but if you were to change it to 800fp
only 9oz would be needed. So if you used the same amount, I would expect you to reach really low temps.

If you used the same quilt shell and didn't change the height of the baffles, you'd end up with a quilt that was way over stuffed (using your example of 13.1 oz of 800fp). That would mean that the higher rated down wouldn't have the room to fluff out right and would not give you the ablility to reach lower temps. It'd be even worse if you used the 900fp rated down.