As you compress down, there is an increase in conduction from the materials contacting each other, but there is also an increase in the number of air pockets. This results in a substantial increase in the insulation value per thickness unit, but still a small decrease in overall efficiency (ie. fully lofted is best, but not by much).

When you say a down garment compressed to 50% might actually by only 40% as warm, you're double counting the same effect by saying it's thinner plus less warm (ie. 50% thick x 80% insulating = 40%). Actually, it's thinner but much warmer per thickness unit (ie. 50% x 180% = 90%). so it can be compressed by 50% yet only decreased in warmth by 10%.

Anyways, Will explains this much better than I in BPL's down jacket state of the market:
http://www.backpackinglight.com/cgi-bin/backpackinglight/ultralight_3-season_down_jackets_sotm_2010_part_1#.UTGScaVpuYU

And I agree that in truly windy conditions the bellows effect is probably a bigger deal anyways.

Edited by dandydan on 03/01/2013 23:03:23 MST.

While they were not involved with clothing, it might be useful to look at the R-value vs thickness results in Roger's and Will's 2011 tests and articles about many many insulated sleeping mats.

They definitely found a decrease in R-value as mats were compressed (pads compressed from full inflation thickness to quite thin in 5mm increments)

"When you say a down garment compressed to 50% might actually by only 40% as warm, you're double counting the same effect by saying it's thinner plus less warm (ie. 50% thick x 80% insulating = 40%). Actually, it's thinner but much warmer per thickness unit (ie. 50% x 180% = 90%). so it can be compressed by 50% yet only decreased in warmth by 10%."

The insulation is provided by air. The conductivity of the fibers is not a factor because they are a small part of the volume.

The problem is that there is convection inside the insulation. Air moves around carrying it's heat with it. The fibers of the insulation reduce that convection.

What's good about down is that the fibers are very fine and light weight. But the downside is they impede internal convection less than, for example, synthetics. My measurements:

clo/oz/yd2 - down = 1.29, apex = 0.81, apex is 0.63 of down

clo/inch - down = 2.28, apex = 3.2, apex is 1.4 of down, theoretically, air is 6.4 clo/inch without internal convection or radiation

If you compress down, the thickness of the air layer is less so there's more conduction heat loss. This is partially cancelled because the internal fibers are closer together, which impede internal convection better. My measurements: when I compressed the down 47% (e.g. the loft reduced from 1 inch to 0.53 inch) the clo/oz/yd2 was reduced by 26%, and the clo/inch increased by 39%. If there was no change in internal convection, then the clo/oz/yd2 would be reduced 47%.

Measuring this is tricky so you should take my numbers with a grain of salt, but that table of Richard's is consistent. I haven't seen anyone else measure this which is why I did.

This all ignores radiation which is rather non-intutive to me, so I don't know. Richard says radiation is why the compression of down doesn't reduce the insulation value. I don't know. It seems like that would show up in Richard and my measurements...

The only reason to talk about this, is, if you're making a down garment, make the baffles big enough so it's not over-stuffed by more than about 20 or 30% if you want to get the most warmth for the weight. But, even if you overstuffed by 100% or 150%, you would hardly notice. Maybe you'de need 1 pound of down compressed 20% or 30%, 1.5 pounds if it was compressed 150% to get the same warmth.

And anecdotally, they say to fluff the down up good when you unpack to get the most warmth. If the baffles aren't big enough, then they won't allow the down to do this.

Jim, yes virtually everything decreases in R-value as you compress it. However, there's a key distinction here between the total insulating ability (ie. R-value) and the insulating ability per thickness unit, which is what I'm trying to communicate.

Down is a better example, but lets look at Roger's NeoAir test data (2009). He found:
Pad at 2.5" thick = R-3 (R-value per inch = 1.2)
Pad at 2.0" thick = R-2.5 (R-value per inch = 1.3)

So we see a decrease in total insulating ability as the pad gets thinner, yet the pad is actually getting warmer per inch - there's just less inches. So the main point is that thickness is down by 20%, but R-value is only down by 13%.

A similar thing is true for down (and virtually any insulation). As they are compressed they get warmer per inch, but there's less inches so the overall result is a loss. With down this loss is really small. It can be 50% as thick yet still have total R-value at ~90%. This seems to be what Jerry is saying as well.

Edited by dandydan on 03/02/2013 09:09:10 MST.

Yes, that's what I'm saying

Except - with down, if you compress 50%, it's only about 25% less warm

So it's significant, but only half as bad as it could be

But measuring and interpreting is tricky so maybe I'm wrong

In strong winds, the shell is almost as important as the insulation.
Maybe that's why we Scots wear fleece whilst active under a decent shell, and keep down or primaloft type stuff for camp or belay use.

lets look at Roger's NeoAir test data (2009)

Neoair models relying only on layers of reflective material wouldn't be an apples-apples comparison to down or synthetic insulated clothing.

Of the down mats Roger measured, we see the following approx r-value reduction when compressed to 50% of full thickness:
Exped DownMat Pump 7 50%
Kookabay goose pad 40%
Warmlite DAM 58%
That's a lot of variability but also only three data points

Of the synthetic insulated mats (not foam)
BA insul ac 60%
exped synmat 7 36%
kookabat syn 26%
POE ether elite 49%
vaude norrsken 40%
Also quite variable.

There's clearly no single story in these measurements. I suspect that is due to differing interior construction details.

i see we are again at the "compressed down under a sleeping bag does not insulate", or such as that ...
i have a bit of an issue with this, as exampled by using a sheet of newspaper to assist in picking up a hot pot.
the thin (very) sheet(s) of paper, made of throughly compressed fibers will absolutely retard the heat transfer into your hand.
there is vastly more to insulation than air. loft be good, for sure. but beyond loft, there remains another world of insulative values.
let's look at that stuff used by Manzella to make thin, and nicely warm gloves. i have 2 pr of them and the older of those gets used on cold weather (for sacramento) bicycle rides. i suspect it's made by goretex, has a bit of stretch to it, and for what it's thickness is, supports an astonishingly high temperature differential between the nippy nippy cold air on the outside, and my reasonably toasty hands on the inside.
like any good bpl member, i keep a wide surface micrometer next to my computer and well within reach. it shows us that Manzella glove stuff (goretex N2S ? ) is 0.029" thick, call it .75mm. that is not really much of a loft if you think about it, and yet, it is reasonably windproof and insulates quite well.
i have an entire top made of N2s, and it's so warm i have never been able to deploy it in the field. great stuff for gloves though !

thickness of insulation and it's effectiveness "may" be related, but it seems not by a lot.
a 2" plate of aluminium alloy will insulate quite a bit more poorly than a similar looking 2" plate of stainless steel.
there are no "perfect" insulators, just as there are no perfect conductors.
air is a good insulator, and if you smoosh enough of it together, you'll get it as a liquid, where it conducts heat pretty darn well !
even used at lesser pressures such as underwater research, it's insulation goes in the pot, and they have to heat underwater stations all out of what might seem reasonable on the surface.

slowing down is a fine concept, and keeping the same state of exertion all day when in challenging environments is vastly tougher than it sounds on a forum. it can be done, but expect some delay before optimal performance is achieved.

nice article, and it gives us food for thought.
those guys on McKinley .. you can easily drive a truck into places you can't back out of. on any real job or adventure, there can be .. consequences.
most of the time when something like that happens , it's just a matter of playing out the end-game .. they were quite effectively dead a long time before they physically expired.
there seem to be a lot of "webs" available, in that one can go down many different roads at many different junctions, the all of them seem defensible choices at the time, and the majority of them just being dead ends, but some of them having a bit of a beast at the end. even just an excess of dead ends will run you out of food, then you are in a hurry, then you do stupid, then you are "unfortunate".

i recently sent back a pair of the lofted wool hoodies, which although looked forward to with much expectation and glee, were found to be so poorly tailored (poor sleeve design) that even at 60% off, i had no use for something like that in my kit. Ibex knows better than to produce something that fits that bad. has all the appearances of a management failure in my op.

a pair of my friends once commented on brown bear dangers on my walks. one, being a mechanic said "well, if they eat 2 people a year in that area, you just wait a bit, and then call up there ...".
the other (and these are my Friends mind you ! ) an actuarial type of accountant said " well peter, as an american citizen, your chances of being consumed by a bear are staggeringly small, although you do raise them quite a bit by Actually Going There... "

that's all i got to say for today. (we hope )

cheers,
v.

"there is vastly more to insulation than air"

First, maybe looking at the physics is more of a geek exercise. If you are warm wearing something, more power to you, and that's really all that's important.

But, in outdoor gear, insulation is almost totally about air.

Sure, if you sat on a block of nylon or whatever, it would have insulation value but

Consider Apex insulation. I measure 2.62 oz/yd2 and 0.66 inch loft. That's at least close. 2.62 oz/yd2 / 0.66 in * 28.5 g/oz * 0.028 yd/in * 0.028 yd/in / 2.54 cm/in / 2.54 cm/in / 2.54 cm/in = the density of apex is 0.005 g/cm3.

The density of polyester is 1.4 g/cm3, so in a batt of Apex, the polyester makes up 0.4% of the volume and the rest is air.

The conductivity of air is 0.0257 W/mK. The conductivity of polyester is 0.2 W/mK, about 8 times that of air.

Calculating the conductivity of the Apex is tricky. Worst case, if the polyester was totally aligned in channels perpindicular to the surfaces going from front to back, then the conductivity would be 0.3% * 0.2 W/mK + 99.7% * 0.0257 W/mK = about 2.4% worse than just air. Best case, if the polyester was totally aligned in sheets parallel to the surfaces, then the conductivity of the apex would be insignificantly more than just air. The actual alignment of polyester fibers, if you just look at it, are fibers that mostly go parallel to the surfaces so it's closer to the best case so I would be surprised if the conductivity of the polyester fibers adds 1% to the conductivity - you can't measure it. There's probably also some binder material but it's thermal conductivity is probably similar to polyester.

Polyester numbers were from wikipedia. There is a range of vlaues so I just chose something in the middle.

Or, if you want to think about it qualitatively, take a piece of insulation and squeeze it. It squeezes to practically nothing - consistent with my 0.3% calculation. You aren't compressing the polyester fibers, you're just compressing the air. Like, try compressing a block of nylon - it is very stiff. And since the conductivity of polyester is only 8 times air, it's qualitatively not a factor.

And with down, the conductivity of the fibers is probably even less of a factor, but I don't know what the density or conductivity is of the fibers


i see we are again at the "compressed down under a sleeping bag does not insulate", or such as that ...

sorry, I'll try not to talk about it any more... someone else brought it up

... I've said it many times on this forum:

Lower fill weight down such as 500 or 600 weight compresses less than higher fill weights and will likely be a better jacket and pants down for that reason.

>Lower fill weight down such as 500 or 600 weight compresses less than higher fill weights and will likely be a better jacket and pants down for that reason.

It may compress less, but I can think of many reasons why higher fill down is better.

*it compresses more when you want it to
*less feathers
*warmer for weight
*less loss of down because the soft down plumes poke through the shell less often than feathers.
*down is more durable than feathers

Edited by T.L. on 03/03/2013 19:37:41 MST.

Travis,

You're assuming lower fill weight down always has a content of some % of feathers.

Actually good lower fill weight down can be ALL down, just from more immature geese or from ducks.

Now if I could only afford a Russian eider down filled winter bag...
(Excuse me, I must run out to buy more lottery tickets.)

A quality article followed by evidence based discussion and descriptions - one of the things that sets BPL apart.

+1 on Part 2.

Great article. Please do a part 2. I wish there was a clothing rating system like good sleeping bags use to give some idea of what you are getting.

Very interesting article, but Part 2 seems essential!

I don't understand the claim that stiff fabric protects the insulation from compression.

In the wind, in use, the force on the garment for a given wind speed and direction depends mostly on the shape/size of the garment. The fabric is supported away from the body by the insulation. Thus a given amount of force on the garment must be supported by the insulation, which means that the insulation will be compressed, on average, a constant amount for a given windspeed, independent of the fabric's stiffness. So your explanation makes no sense to me. If the observation is that stiffness increases loft, then there must be another explanation. But I'd like to see an experiment to verify the observation, first!

(In fact, a stiffer fabric will generally be heavier, increasing the downward force on the insulation.)

One might propose a garment held away from the body by, e.g., tent poles. Hmmm, ultralight backpackers' anti-corsets!

Has anyone tried a jacket constructed like a sleeping pad? Blue foam would make a formidable stormjacket, and Exped might be able to do some real magic, assuming you slow down enough to minimise sweat... I also wonder if something like the Ventisit seat cushion (a minimally compressible but lightweight construction used for recumbent bicycle seat pads) might be filled with down, which would be incompressible, breathable, and lightish-weight. Of course any good storm jacket will be nigh impossible to pack unless it's inflatable. Oooooh--how about a jacket with inflatable ribs like those soft tents use, supporting the outer fabric away from the body?

Edited by fugue137 on 03/09/2013 10:44:41 MST.

Klymit has an inflatable vest insulated with argon. Wonder how that compares to down?

Great report, I've gone to thin wool, medium fleece, then puffy insulation as my layers for mountaineering. (Here in California the puffy stuff is only when resting).

Alpine skiing I generally wear fleece or wool instead of puffy insulation because the compression from sitting on cold chairlifts is no problem with the knits, but ice cold when wearing puffies.


Ray Jardine talks about frostnip and frostbite on the front of the thighs from walking into a strong cold wind all day even though all parties were wearing thick down pants. I think a wool or fleece midlayer would have been a good choice.
http://www.rayjardine.com/adventures/2006-SouthPole/index.htm