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Michael Martin
(MikeMartin) - BPL Staff - MLife

Locale: North Idaho
Re: question for Richard... on 10/13/2008 22:54:53 MDT Print View

>> if you decrease the thickness of your typical UL sleeping bag by a factor of 2, you will not affect the warmth of the bag. Why? Because the density will be doubled, and so the clo/inch will double, but the thickness will be halved... so 2*0.5=1 and the warmth of the bag will stay the same.

>> Spot on!


I must be missing something here. The numbers aren't adding up for me.

If you start with down at 2.16 kg/m^3 and 2.556 clo/inch, and compress it to 18 kg/m^3 at the end of the linear region, it would need a 21.3 clo/inch value (at 18kg/m^3) to have the same clo it started with.

21.3 clo/in far exceeds even the maximum value of 6.562 clo/in which occurs well outside of the linear range.

What am I missing?

Edited by MikeMartin on 10/13/2008 22:56:55 MDT.

Ashley Brown
(ashleyb) - F
Re: Re: question for Richard... on 10/13/2008 23:17:42 MDT Print View

Good question Mike! It would appear that the linear region would have to tail off rapidly at about 5kg/m3 in order to not exceed 6.56 clo/in at 18kg/m3.

That would mean you could compress your bag by roughly 2.5 times before it started to become a problem.

I note that in Richard's example figures for an overstuffed mummy bag the amount of down in the bag is effectively doubled during the experiment whilst still retaining a linear increase in clo... so equally well you can can be fairly sure that squashing a bag by less than 50% won't affect it's performance.

Edited by ashleyb on 10/14/2008 00:39:51 MDT.

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Re: question for Richard... on 10/13/2008 23:35:10 MDT Print View

And if you decrease the thickness of the down by a factor of 100 you will increase the density by 100, meaning that a layer of down one thou of an inch thick will have an insulating value *per inch* exceeding anything on the planet.

Yeah, right. Beware extrapolations! Looking closely at what Richard wrote we find that the linear region only goes so far, then it fails. Yup.

The insulating value of the down will rise with density for a little while, until the ends of the down fibres are starting to touch everywhere. When that happens the Kozeny equation (I think that's the correct spelling) breaks down, because it is mainly the air around the tips of the down fibres that insulates. Continuous filament insulation (synthetic) is not as good as down precisely because it does not have the fibre ends, only continuous filaments, to trap the air.

The same thing explains why damp down is a poor insulator: the microscopic fibres start to stick together. Loss of ends => loss of trapped air => loss of insulation.


Edited by rcaffin on 10/13/2008 23:40:36 MDT.

Ashley Brown
(ashleyb) - F
Re: Re: Re: question for Richard... on 10/14/2008 00:46:17 MDT Print View

> Yeah, right. Beware extrapolations! Looking closely at what Richard wrote we find that the linear region only goes so far, then it fails. Yup.

I don't think anyone is in disagreement on that point! The interesting thing from Richard's data however is that there *is* a significant linear region and you don't have to extrapolate beyond the data points to show that 50% down compression on most UL bags should not be a problem. Losing loft because your bag is damp is another matter altogether!

Tad Englund
(bestbuilder) - F - MLife

Locale: Pacific Northwest
Re: Re: Re: question for Richard... on 10/14/2008 09:29:46 MDT Print View

Richard, it has been too long since I was in school and I think I went skiing the week/semester they discussed thermodynamics. Maybe it’s too early in the morning for me but what is clo (I probable know this but I’m having a brian cramp). I think I’m following what you are saying but the clo thing keeps me guessing.
I really appreciate you guys- It is not enough for me to just know I was cold but the why and how to fix it. It also helps in other areas to understand how not to let it happen with other equipment.

Michael Martin
(MikeMartin) - BPL Staff - MLife

Locale: North Idaho
Re: question for Richard... on 10/14/2008 09:46:20 MDT Print View

>> The interesting thing from Richard's data however is that there *is* a significant linear region and you don't have to extrapolate...

Indeed. We're all on the same page here. My original point was that according to Richard's numbers, the linear region extended out to 18kg/m^3 and "flat-ish" region extended from there to 75kg/m^3. So my last example was actually interpolating within the values Richard gave us, not extrapolating past the data points.

I suspect that the reason for the discrepancy is that the Y intercept of the clo/in vs density graph is above zero. If true, this would reconcile the data points we have been given. But, it also would mean that doubling the density would *not* double the clo/in -- even in the linear region.

EDIT: I just found this old post from RN that supports my suspicion:

(Look at the thermal conductivity vs density graph.)



Edited by MikeMartin on 10/14/2008 22:25:19 MDT.

Richard Nisley
(richard295) - M

Locale: San Francisco Bay Area
Re: Re: Re: Re: question for Richard... on 10/14/2008 09:57:32 MDT Print View


As a builder you understand that R Value measures the resistance to heat flowing through an item. clo is the same thing as R Value. To convert R Value to clo, multiply the R Value by 1.13931529284938. To convert clo to R Value, multiply the clo by 0.8832059100018.

clo is generally easier for people to relate to when evaluating clothing or sleeping bags. 1 clo is the COMBINED insulation provided by a complete winter weight business suit ensemble. This ensemble includes the shoes, socks, underwear, shirt, tie, long sleeve shirt, winter weight pants, and winter weight jacket.

Edited by richard295 on 10/14/2008 09:58:23 MDT.

Tad Englund
(bestbuilder) - F - MLife

Locale: Pacific Northwest
Re: Re: Re: Re: Re: question for Richard... on 10/14/2008 10:45:23 MDT Print View

Richard, Thanks- BTW is that a "Hickey Freeman" wool suit or a "Men's warehouse" wool suit? There is a difference. Just kidding.
What does the initials "clo" stand for?

Ashley Brown
(ashleyb) - F
Re: Re: question for Richard... on 10/14/2008 21:30:39 MDT Print View

Mike, I see what you are getting at and am now rather puzzled! Reading the thread you linked to I found some comments by Richard similar to the point I was making above:

"You can increase the aerial density of 800 fill down about 2.5x before it is becomes less thermally efficient than fully lofted down. A compression sack will easily compress the down much more than this; so, it is important to let it loft up. This same phenomenon applies to an earlier thread dealing with the diamond quilting on a MB women's down jacket resulting in less loft than the rectangular quilting in the same man's jacket. Again, it doesn't make any difference."

Further down the page...

"If your Parmo raincoat doesn’t compress your Gilnet jacket such that the density increases more than 2.5 X, you should be fine. You can probably find hundreds, if not thousands, of Internet posts which make the statement that the loft of a down garment or bag determines its warmth. My contention is that this is true only if they are using the same fill power and density. You can make a down bag or clothing warmer by increasing its density even if the loft is less."

I'm not quite sure how these statements can be reconciled with Mike's point that the linear equation seems to have a significant y-intercept constant (meaning that doubling the x value won't double the y value).

Brad Groves
(4quietwoods) - MLife

Locale: Michigan
Density: Warmth on 10/17/2008 09:56:51 MDT Print View

I'm a writer. My eyes (and mind) glaze over when I see equations. A couple of "real world" examples on density, FWIW. We all know and respect Western Mountaineering bags because, along with being light, they're really warm. If you've had the experience of comparing not just loft, but density of loft, of similarly rated bags, you know that the Western bags have more down in each baffle. In my shop right now I have about 40 models of down sleeping bags hanging. If I compare the density of loft between models, Western bags basically always win. The down vest I recently made used much more down than some would expect; several people felt it was overfilled. But it didn't lose any warmth. In fact, it's the warmest garment I have.

To be as unprecise and speculative as possible, I'll also ask this: just how much loft does a goose have? The birds I've seen aren't several inches thick of loosely packed downy poofs. The down on the birds I've seen is pretty darn dense. Yet it seems to keep them warm...

Kathleen B

Locale: Pacific Northwest
Goose loft on 10/17/2008 10:48:37 MDT Print View

Maybe those webbed feet are good ground insulation? Windproof outer feathers?

I know of a PCT thru-hiker who used a WM Ultralite and was very cold on the last stretch of her trip. So she's thinking of replacing her bag with another brand. I would like to see her try better ground insulation. Just one 3/4 length closed cell foam pad, which probably smooshed over 5 months, probably had a lot to do with her being cold.

Plus, like the first poster, she may have been wearing too much in too confined a space, actually being counterproductive with efforts to be warm.

This has been an interesting thread. As I sift through the information I see the basic things to do are avoid tight or sweaty clothing, use effective ground insulation, and pay attention to where you sleep (under trees v. open, windy areas).

Personally, I've found putting quality closed cell foam (gossamergear) over my Prolite 3 insulates me from both the cold ground and any cold air coming from the Prolite and I am much warmer in the same conditions than when I had the foam and Prolite reversed.

Mary D
(hikinggranny) - MLife
Glazed-eye granny turns to practical experience on 10/17/2008 15:36:33 MDT Print View

I like to crunch numbers; I'm a retired accountant. But quite a few of the above posts were beyond me, and I will defer to the superior higher mathematical ability of those posters!

My first lightweight down bag was a Marmot Hydrogen (30*). I started getting cold in it, and having to add outer clothing, at 40 degrees. The night it got down to 24*, I had my base layer (Capilene 4) and all my outer clothing on inside it, including rain gear, on a Thermarest LE (2" thick) pad. I lay awake shivering all night. The places that got the coldest were my hips (I'm a side sleeper) and feet, even though to the best of my knowledge I was not stretching the insulation. I made my dog sleep at my feet, which took care of that problem, but not my hips. It was a three-dog night with only one dog! Two conclusions: I am obviously a cold sleeper, and the Marmot Hydrogen, at least the 2005 model, is a bit over-rated.

I sold the Hydrogen and bought a 20* Western Mountaineering Ultralight Super. There wasn't any weight penalty because WM bags come in "short" for us folks under 5'6". I also changed my pad to a 3/4 length POE InsulMat Max Thermo (now replaced by the Ether Thermo) insulated air mattress. The Max Thermo, like the BA, is rated, ostensibly, to 15 degrees. I didn't have a chance to test either bag or pad to close to their limit until a week of frosty nights in Wyoming's Wind Rivers last August. I didn't have a thermometer along, but on at least two nights my dog's water (in a nylon bowl, out in the open) froze completely solid (that was one puzzled dog in the morning!), and there was ice in my Platypus water bottles which were left under a small fir tree with thick low branches.

One thing I found out after my first below-freezing night was that my body's moisture was evidently condensing, freezing and later melting on the outer shell of my sleeping bag, part of it inside the shell. Since I had non-breathable rain gear (the Brawny Gear rain jacket and pants now sold by BPL), I tried wearing that over my base layer as a vapor barrier. It worked--no more problems with moisture in the bag, and I was comfortable, with no excess sweating and no other additional clothing except a fleece balaclava and fleece socks. There were a couple of nights (the same ones in which the dog's water turned solid) during which I woke up cold. However, once I'd made the inevitable (at my age) excursion outside, I moved around enough to get my circulation going and, once back in bed, was able to warm up and go back to sleep. The main problem was when I woke up, very groggy still, and lay there asking myself if I really had to get up! The draft collar on the WM Ultralight Super really helped--tightening that up often solved the problem. There were a couple of times I wanted to grab my jacket, but I never quite got that far--by the time I was alert enough to go that far, I had to get up anyway. I had no problems with my hips getting cold; generally it was my upper body. My last night out, the cold was underneath me, meaning, of course, that the pad was marginal. If it had been cold enough to keep me awake, I'd have grabbed my sit pad, cut from a GG Thinlight 1/8" pad (my dog uses the rest of the pad) and put it under me on top of the air mattress. I will put it there to start with on potentially frosty nights from now on.

I do recommend using the BA Pumphouse rather than blowing up your pad, to keep moisture out. However, while it's a great pump, I found the Pumphouse to be a failure both as a pillow and as a dry bag--it leaks both air and water. I use it as a stuff sack, with a turkey roasting bag inside to keep my sleeping bag dry, and the combo is a little lighter than a dry bag.

I've read, also, that when enough air is let out of an insulated air pad to make it comfortable--I pump mine only about half full--that it degrades the insulation a bit. I don't know if this was the problem or whether the pad would have been as marginal if inflated rock-hard. But I wasn't lying awake shivering for long, so it didn't affect my sleep. Next time, I'll take a thermometer!

It does help to be able to air out a down bag every few days. I was able to do it daily in the Winds. If it's really wet, with lots of condensation in the tent, for days on end, the insulation is going to degrade. Kathleen, I also suspect that after 5-6 months your friend's sleeping bag insulation was a bit dirty, which also degrades the insulation. But you're right; it's lots less expensive to upgrade the pad first!

Edited by hikinggranny on 10/17/2008 15:55:29 MDT.

Brad Groves
(4quietwoods) - MLife

Locale: Michigan
Cleanliness on 10/17/2008 16:27:48 MDT Print View

Hey, hikinggranny- Spot on with the dirty bag. For a while I had a thing about not wanting to wash a bag, being afraid of stripping the natural oils off the down. Well, some odd years later I was distraught because my bag wasn't as warm as it used to be. Turns out it didn't loft nearly as well, either, and there were several clumpy spots. I had a summer bag that I had quit using because there was so much clumping, etc.

Bottom line, I washed the bags. Still bad loft. I ended up washing them three times, drying thoroughly, and they were literally good as new. The weird thing to me was that I never really noticed the loss of loft along the way.

A couple of thru-hikers I know had the same problem as your friend--just gotta find a laundromat!

Richard Nisley
(richard295) - M

Locale: San Francisco Bay Area
Re: Re: question for Richard... on 10/18/2008 23:19:26 MDT Print View

Mike and Jim,

Of course the Y intercept is above zero. Primarily the conductivity of still air and to a much lesser extent, the conductivity of the fiber itself defines the Y intercept. In other words, no fiber fill insulation based on still air can have lower conductivity than still air.

I suspect we are boring the majority of the forum participants with the detail this topic has drifted to. Although I am on a month long trip and won't get to a computer for many days at a time, please email me via the forum if you want to discuss this topic further off line.

Jim Sweeney
(swimjay) - MLife

Locale: Northern California
Post to track on 11/23/2008 10:31:15 MST Print View

Posting to be able to return to this great discussion in the future. (This post will be listed in "My Account--Change My Forum Profile". Is there another way to do this? "Watch this" doesn't quite work, if nobody posts again.)

Edited by swimjay on 11/23/2008 19:59:23 MST.

Tim Cheek
(hikerfan4sure) - MLife
Always ask for overfill? on 11/23/2008 19:46:18 MST Print View

So this doesn't get too academic, is the bottom line that we should have Nunatak's Tom overfill an ounce or two or three?

Density makes sense for warmth, but I've always been focused on loft. This has been a helpful string.

Greg Mihalik
(greg23) - M

Locale: Colorado
Re: Post to track on 11/23/2008 21:04:29 MST Print View

James -

'Bookmark' it.
And give it a meaningful name.


Andrew Dolman
Loft and density on 11/25/2008 19:17:09 MST Print View

I've thought for a while that some UL bags are under stuffed. With really good down, and very light shell fabric, they can loft so well, with so little down, that there are essentially big air gaps running through the baffles allowing convection.

On a different point. I need a warmer mat when using a down bag than when using a synthetic because the down compresses underneath more than synthetic insulation. This caught me out when I first switched to down.

Everett Vinzant
(wn7ant) - MLife

Locale: CDT
Forgive me for ressurecting an old thread... on 09/25/2010 21:45:00 MDT Print View

I asked in another thread:
about multiple sleeping bag layers. I'll give a few more specifics to see if I can really muddy things up. Let's say I'm looking at a Marmot Lithium. This is a 0 degree bag, and weighs 3 pounds.

I'm also considering a WM Highlite (1 pound, 35 degrees) and a Marmot Helium (2 pounds, 15 degrees). I should be able to get to ~ -3 F just off of the insulation if I understand correctly (better than the Lithium alone at the same weight). Is there also a benefit to the fact that I have two surfaces next to each other (the outside of one sleeping bag inside the other)? If so is this significant enough to matter?

I hear lots of statements that a silk liner will improve the functionality of the sleep system by ten degrees (approx). What kind of R value (or col) would that make silk have?

I've also seen people say that bivies (I have an Integral Salathe) will give you 10 degrees more warmth as well... Is this actually science?

If anyone reads this, and answers, I thank you in advance for any answers provided, or heckling for my obvious lack of understanding of thermodynamics (although I do like the math formulas, and, if this is a function, wouldn't a calculus formula be better suited to identifying it)?

Edited by wn7ant on 09/25/2010 21:46:55 MDT.

Travis Leanna
(T.L.) - MLife

Locale: Wisconsin
Re: Forgive me for ressurecting an old thread... on 09/25/2010 23:31:20 MDT Print View

I understand the concept behind the down compression thing, but I'm not really too much of a numbers guy to steadfastly work the numbers out.

However, I can comment on silk liners. IMO, a 10 degree boost is VERY liberal.

Many manufactures of any product like to inflate their numbers. Especially when it comes to the issue of warmth. The worst offender is the Sea to Summit Reactor liner that claims a 15 degree boost. I had the liner and promptly returned it. I have NO clue how they came up with a 15F boost. 3 degrees at most.

From my own personal experience, liners made from silk are going to be your best option considering weight, bulk, and warmth. Really though, do not rely on any liner for warmth. What I mean is, if you expect temps of 10F, the get a bag conservatively rated for that. Do NOT rely on a 20F bag and hope to get a boost from a liner. That's asking for trouble. Yes, liners will add some warmth, but only like 2-5 degrees. Their main purpose is to help keep your bag clean, not keep you warm.

I don't use bivies, but I can be pretty sure they'll help "boost" the temp rating of your system much more than a liner will.