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Down fill power and warmth

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 kingborris 26 Jan 2019

Can anyone give me an idea of how much difference the various fill powers of down make in real world use?

eg how much warmer will a jacket / sleeping bag with 800FP be than a 600FP jacket / bag if all else is equal, such that they have the same weight of down, same construction etc?

I was hoping to be able to find something that comes in two different fill powers (everything else the same) with a suggested temperature rating to give an indication of the above, but i cant find anything? 

 nufkin 26 Jan 2019
In reply to kingborris:

PHD do some of their stuff in a variety of fill options.

Probably the subjectivity makes it difficult to give specific assurances of heat retention at a given temperature - it's all very well having a kilo of the finest kakapo down, if you're hypothermic you're not going to be as warm as you would be after polishing off a round of brie before bed

OP kingborris 27 Jan 2019
In reply to nufkin:

The PHD website was my first port of call before posting here, but I couldn't find the info I'm after.  

I appreciate how warm something feels is subjective but it should be fairly easy to actually test heat transmission through a particular FP of down? 

??????The lack of this info makes comparing different jackets very difficult  Would a jacket with 150grm 800FP down be warmer than one with 180grm of 700FP? Who knows!? It'll be thinner and lighter for sure but the warmth is a mystery. 

 

 Marek 27 Jan 2019
In reply to kingborris:

As a first order flat approximation, thermal conductivity is linear with thickness, so (for example) if you are comfortable with 20 degree difference between internal and external temperature with 750 down then 800 will gain you about another 4 degrees. In practice however, a bag is better modelled by a half pipe with different ground insulation, so probably 2-3 degrees is more realistic unless your mat is really good. 

 Dave Cundy 27 Jan 2019
In reply to Marek:

4 degs?  I would have thought it would be nearer  to 1 deg, going from 750 FP to 800.  The fixed mass of down will have 6% more volume  (and hence thickness), so should support a temperature difference which is 6% larger.

 Ciro 27 Jan 2019
In reply to kingborris:

> The PHD website was my first port of call before posting here, but I couldn't find the info I'm after.  

> ??????The lack of this info makes comparing different jackets very difficult  Would a jacket with 150grm 800FP down be warmer than one with 180grm of 700FP? Who knows!? It'll be thinner and lighter for sure but the warmth is a mystery. 

If it's thinner it won't be warmer - higher fill power means you need less weight of down to create the same volume of jacket; there will be *slightly* more air to down ratio but not enough to make a significant difference, total volume of air trapped will be pretty similar in two jackets that look identical size when puffed up. The higher fill power will just pack down a little bit better.

If we wanted a way to compare jackets across brands for warmth, an industry standard volume rating would be useful - but even then fit and features (such as integrated hood, high neckline/low bum line, baffle type etc.) make such a difference to heat loss in the real world that it would be of limited value compared to putting on a jacket and feeling for yourself.

 

 Marek 27 Jan 2019
In reply to Dave Cundy:

Yes, you're right. Brain not working on Sunday morning! 

 tom 27 Jan 2019
In reply to kingborris:

https://triplefatgoose.com/blogs/down-time/a-guide-to-down-jacket-warmth-do...

These guys suggest -- if correct (I have no idea if it is or not) -- Clo value can be used...

…550 fill power with 1 oz. of down has a Clo of approximately 0.7,

…625 fill power with 1 oz. of down has a Clo of approximately 0.92,

…800 fill power with 1 oz. of down has a Clo of approximately 1.68.

 

In reality I have found fit, construction, and energy/ activity levels to be much more important in choosing...  Rab's micorlight didn't fit very well, perhaps why I found the montane equiv. warmer? 

OP kingborris 27 Jan 2019
In reply to tom:

Interesting. So looks like a non linear relationship between FP  and Clo. 

Thanks

 AlanLittle 27 Jan 2019
In reply to kingborris:

> non linear relationship

It's as close to a straight a line as you'll ever see given three data points.

Post edited at 12:52

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