Fill power and weight, how warm?

[SLIMTIM]

Hi, hoping someone can give me a simple answer to this question please.

Assuming that the down is the same in each jacket, will a jacket that has 650g of down with a fill power of 800 be as warm as a jacket with 800g of 650 fill power down? I understand that the fill power will have a relevance to size of the jacket, in this case, I am just trying to understand warmth.

I have made the mistake of starting to be interested in this type of question before sinking some money into a new jacket. I have three options which all claim to be warm, one has 970g of 650FP, one 500g of 800FP, and the last 660g of 800 FP (although this one is 'waterproof' which is good due to being waterproof, and bad due to condensation issues).

If I just multiply the weight by the fill power to create a 'warmth rating' then there is a clear winner but I suspect it can't be that simple.....or can it?

[whitestone]

No

Warmth isn't determined by either the weight of down (or any insulating material) or its quality/performance but by loft. It's just you get to that value by varying routes.

I can't remember where I saw it, probably a rather geeky discussion about this very subject on a US hiking site, but the figure given was that if you had X grammes of 600fp down in a jacket or bag then you needed about half that amount of 900fp down. So it wasn't double the fill power to need half the down but 50% increase in fill power to require that reduction.

I'll see if I can find a link to the discussion.

[Bearbonesnorm]

You must also remember that the weight of the shell material and cut of the jacket will have a bearing on how well it can loft - regardless of fill.

Also, are all the downs 90/10 or are the higher fp 95/5?

And ... what about construction? Are they all constructed with proper baffles or just sewn through?


Probably easier to simply pick the one you like the colour of

[SLIMTIM]

I am slowly answering my own question and becoming (maybe) an expert on down filled jackets. And can see how my original post was utter rubbish when quoting weights!

My 'how warm will it be' question is now posed between two Rab options - the Electron and the Valiance . Both use 90/10 RDS hydrophobic goose down rated at 800.

The Electron is a sewn through jacket with 190g of down and a Pertex Quantum outer.

The Valiance has box baffles, 170g of down and a Pertex Shield 2.5 outer which should make it a lot more waterproof and windproof.

I'm close to opting for the Valiance but hesitating as I'm wary about the marginally lower level of down - does the box construction and more robust outer fabric compensate so making the two jackets much of a muchness from a warmth PoV?

PS: I am quite liking the suggestion of just choosing the one in my favourite colour........

[Bearbonesnorm]

Proper baffles can make quite a difference to warmth as they generally remove the potential for cold spots that sometimes develop at the seams of stitch through garments. Unless weight is absolutely critical then I tend to avoid stitch through when possible.

20g isn't much down when taken as a whole over an entire jacket.

[SLIMTIM]

BBN, that's good advice and chimes with my own view. Thank you.