Down fill power relation to packability

[crollsurf]

I'm trying to understand down fill power(450, 750, 800 etc) and the advantage in the size it packs down to.
At first I scratched my head, why would you pay $100's more for a sleeping bag to save just a few grams. Made no sense unless you had money to burn and then I thought, maybe the higher rated down takes up less space in your pack.
Does higher fill power mean less litres in size when packed?
If so, is there a rough ratio I can use to calculate this i.e: 800 will pack down 20% less than 700

[Mark F]

The fill power rating of down is a measure of how much volume a certain weight of down will fill. The two main standards are EU and US with the US standard being pumped artificially to provide a bigger number by about 10% (900 loft down US is about 800 loft EU). This is done by specifying an artificially lower humidity level for the test. The tests also specify a small weight on top of the down being measured.

The insulation provided by down is all based on the amount of air trapped in the lofted down, so high fill power down provides more insulation than the same weight of lower fill power down. Less down when compacted takes up less volume and weighs less. So 600 fill power down will need 50% more down by weight compared to 900 fill power down and would take up a similar increase in volume when compressed. Considering that a sleeping bag contains about 500-700 grams of down then a 600 fill power bag will weight 250-350 grams more than an equivalent 900 fill power bag. The volume of the packed bag won't increase by the same ratio as the shell, zip etc don't change.

The level of down to feathers in the down will also have some effect. Lower fill power down usually has more feather content and fewer mature down plumules. You can feel the feather content by pinching a bit of down inside a bag and rubbing it between your fingers. The quills on the feather are noticeable.

[Moondog55]

Nice precis Mark
I look at it slightly differently.
I simply assume that a kilo of compressed down will take up a certain amount of space because as Mark says most of the compressed volume will be the fabrics and zippers. I have 2 versions of the same sleeping bag, both rated to the same temperature, -4C; one with 650 FP down and the other with 900FP down; they both use the same carry bag and compress down to approximately the same volume, there is a weight advantage to the higher FP down but if difference is only small, in the case of mine about 220 grams

[crollsurf]

Thanks Mark, very insightful.

A rough guide would be based on the fill weight. If the same bag with 600 is 1kl fill, total 2kl and the 900 is 500g fill, total 1.5kl, the bag construction = 1 kl. So the 900 fill would pack down 50% smaller but the rest of the bag would not, so you could assume the 900 will pack down ~25% smaller.

But like Moondog says, they quite often sell them in the same stuff sack anyway, so you would also need compression straps (added weight) or a smaller stuff sack to make use of the 25% saving in volume.

So the colder the conditions you are planning for, the more this calculation comes into play but for me, trips to the Snowy Mountains and hopefully Tas one day, the 800 strikes a good balance of cost, weight and packability.

Thanks

[undercling-mike]

Sounds like you've got the general concepts covered so I'll just make a comment with regards to quilts. Generally with quilts the shell makes up a smaller fraction of the weight and bulk than sleeping bags (for the ones I make the shell weights are generally in the range of 150g-210g) and so the contribution of the down to the weight and bulk is larger in percentage terms.

Another thing is that the higher rated down tend to feel a bit nicer than lesser rated down, it's just a bit more fluffy and lighter on top of you, though this is a subjective factor and unrelated to warmth.

[Nuts]

I'd not get too concerned, save the $

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Keep in mind too that the highest & loftiest may not perform at it's best after multiple damp nights. Iv'e had this thought in comparing various fills of our Marmot bags, and read similar recently from an experienced source. That the cheaper 650+ fill bags are appreciably loftier at the same designated temp range. I haven't seen the reasoning examined, feathers content?, while relatively heavy, providing some 'spring'/ less effected by moisture?

I'd probably choose (if a good choice was available) a synthetic bag layer with down garments or a down liner at weights adding up to sufficient for three seasons walking.. which is probably initially as or more expensive.. and may not be welcome advice here.

[Moondog55]

It is a valid POV Nuts, after all One Planet use 550 FP down in their Antarctic sleeping bags and offer the synthetic outer bag to go with them for this reason