Fill weight and temp rating: Western Mountaineering vs. Montbell UL SS

[Richard Nisley]

Ronald,

Unlike down, each synthetic insulation type, including Primaloft, is manufactured at its optimal thermal arial density. Any significant compression will result in a decrease in insulation value.

[Richard Nisley]

Derek,

Yes I will supply two credible public scientific references and an excerpt from a proprietary study summarizing the relationship between down compression density versus insulation value.

The following information contains excerpts from three representative studies dealing with an early (public 1943 NIST), a mid (public 1990 Natick), and a recent study (proprietary 2004).

Reference NIST public data base of insulation density tests for 550 down that were published in 1943, http://srdata.nist.gov/insulation/:

Reference report Natick/TR90/030 for 550 down density published by the US Army Natick RD&E Center, published in May 1990:

Reference a 2004 proprietary research study for 800 down. In this study, the baffle size of a bag was fixed at 2.25” square but the amount of down in each baffle was progressively increased to change the density. The insulation value of the bag was measured for each density increment. The experiments demonstrated that the largest amount of down they chose for the bag was not high enough to cause leveling off or a decrease in product performance. Had they factored in any of the prior research on this topic they would have known the knee in the density / insulation value curve would be ~2.5x. None the less, it confirms the linear relationship between the 800 fill down density versus insulation value on the front part of the curve.

[Derek Goffin]

I am not claiming to be an expert and do not have references at this time. Before I look deeply into this answer me this. By your third reference in your name I take 24 ounces of down clo value about 5.3. If I take it out of its fixed baffle and let it loft to the same density as the 16 ounce reading of about 4.4, I in my innocence would expect a clo of 6.6. I would think at first glance that between those 2 examples you have lost insulation 6.6 versus 5.3 given the same down.
This is the thing I am interested in. If I wear my down gilet under a paramo raincoat the weight squashes it. I expect it to loose insulation. It would be nice if it did not.

[Woubeir (from Europe)]

I'm gonna try to explain this. I don't know if I am right but hopefully Richard can confirm my reasoning or indicate where I am wrong.

I you take an ounce of down and let it loft freely, you get a heterogeneous combination of down an air. It's the air that is responsible for the insulation. The down itself has a higher thermal conductance than air. So when you compress the down, you get higher density and since you associate higher density with higher thermal conductance, you would exspect lower insulation capacity. So the fact that compressing down would lead to a higher CLO value seems contradicting.

My guess would be that this has something to do with minimising losses by convection and radiation. More downclusters in a given volume means more reflecting surfaces, reflecting warmth back inside. More downclusters also means that convection losses are also minimised. So my guess is that it has something to do with a balance between minimising convection and radiation losses by compressing the down and not compressing the down that much that the increased conductance compensates for the minimised convection and radiation losses. If you look at the curves, I would exspect that at higher densities than shown in the graph, thermal conductance would rise again (sharply).

I haven't had the time to read the full Natick article but I wonder if what I wrote makes any sense.

Richard,
you mentioned that each synthetic fill is produced at its optimal bulk density. So I guess you mean that every synthetic fill is engineered and tested at different bulk densities untill the optimal density is found, right? If I look to the original figure in the Natick study, I notice that both Primaloft and Polarguard show similar curves as down. Doesn't that suggest that compressing primaloft and polarguard initially increases thair insulation value and isn't that contradicting to you what you said?

[Mike Nielsen]

I have the MB UL SS #3 like others and have found it to be very accurate in rating, if not a bit conserative.

On a recent 2 night trip there was isolated snow patches on night 1 and frozen precipitation on the tent night 2 plus some frost on the inside of the tent.

I wore light thermals and socks. Slept on on Insul Mat Max Thermal inflatable plus a full lenght "Ridge Rest" closed cell foam pad. Was totally warm both nights plus very comfortable.

When I got the bag I was very skeptical as it looked and felt so thin. Thinner than a 40 degree bag I returned as being too small and cold.

For whatever reason it looks like tissue paper, but really keeps me very toasty.

[Richard Nisley]

Derek,

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.

[Richard Nisley]

Tom,

You did an excellent job of correctly describing the offsetting roles of conduction, convection, and radiation as down is compressed.

Yes, the synthetics are tested to find their optimal bulk density and then consistently manufactured at only this density. For example, if you look at the current Primaloft specs you should see they are manufactured at ONLY the optimal density determined by subsequent joint Natick and Albany testing. The resultant manufacturing density factors in the requirement to quilt Primaloft One every 6 inches and Primaloft Sport every 2 feet.

[James D Buch]

You may want to read the data more carefully before making conclusions.

There are seven data points in the alleged Natick figure, but if you go to the data table at the Natic site and click on the different materials in the data table, only one of those seven data points is actually for Duck Down. The others are for chichken or turkey or turkey/chicken blends and they are compressed into latex containers (like balloons).

The alleged Natick figure is probably an Excel plot of the data in the table, mixing duck down and turkey and chicken feathers — AKA Apples and Oranges.

It is probably best to forget that the "Natick" curve exists and to deal with more controlled data on known substances.

[JW]

If one can make a down item warmer by increasing density, and we know it can also be done by adding loft — which is the most efficient or desirable for ul hiking? Or is there a most efficient density and loft combo for each temp rating? Do manufactures know this already and that's why they stuff baffles to a certain ratio?

Also, we have continually searched for construction techniques and fabrics that allow the least amount of down to achieve the most loft and to save weight, under the impression that LOFT is the main factor in determining insulation value.

Should we instead make a 32* bag with 1" baffles instead of 2" and cram them with down, achieving the same finished weight as before due to less fabric used, and more down used?

[James D Buch]

The ultimate in increasing down density would be vacuum packing. In fact, at one time, down sleeping gear was alleged to be handled and shipped by the military in such a form.

So, "if a little (compression) is good, a lot (of compression) is better".

I think we all need real public or verifiable data.

Enough of mixing goose down, chicken and turkey feathers.

Without clear data, we just end up making hot air, not warmer bodies.

[Ron D]

James – While Richard's position is certainly different from what I have always believed about loft I consider him to be one of the most credible posters on this forum. He has backed up his position with data and I know from having read his posts over the last few years he has extensive field experience as well as technical knowledge. Your objections seem based on things like the study "probably" combined apples and oranges without presenting any real basis for that claim. Your post on vacuum packing presents a mock position that Richard absolutely never made "if a little (compression) is good, a lot (of compression) is better".

[David Lewis]

Here's my experience with my SS#3 on a trip about a week ago up here in Canada…

– 15 degrees
– Camped on about 6" of snow
– 4 season 4 person MEC tent w/ groundsheet (and 4 people inside)
– 3/8" full length foamie
– 3/4 length Thermarest Prolite 3
– Two pairs of thick wool socks
– Silk long johns
– Lightweight loose fitting synthetic running pants
– Midweight long sleeve running shirt
– Lightweight fleece jacket
– Polartec hat

Result? Toasty warm.

[Richard Nisley]

James,

In your Amazon book review of Strictly Speaking by Reid Buckley, you said "He gives you the good advice for a speaker not to make unneeded personal attacks." Aside from your tone, I thought that much of your message was valuable.

Since I looked at the Natick study first, maybe this had an effect on my judgment that NIST was a relevant historical artifact. I found it interesting that the old 1943 NIST data plot for down and feathers exhibited the same general density versus insulation curve shown in the contemporary independent Natick study and Journal of Engineered Fibers and Fabrics study.

I will withdraw the NIST data set for discussion purposes and replace it with a 2007 data set published in the Journal of Engineered Fibers and Fabrics Volume 2, Issue 2 – 2007. Please note the linear portion of the down density versus thermal conductivity (inverse of insulation value).

[Derek Goffin]

Richard you did not answer the first bit of my question. Using your data I see lofting 24 ounce of down to the same density as the 16 ounce mark as giving more insulation than the 24 ounce compressed. This is the crux of the issue. 4.4 clo plus another 50% thickness gives 6.6clo which is more than the 5.3 shown for your compressed 24 ounce of down.
Again go back to the data for 800 fill down which has a point" max loft" marked. This point is at 3.1 BTUinch/hr.ftsq.F with a density of 2.2 lb/ftcube. If I imagine compressing this down to 4.4 lb/ftcube (near the max efficiency mark) your graph gives 2.6 Btuinch/hrftsq.F but there will be half the thickness. I would have thought that was equivalent to 2.6×2=5.2 Btu lost, more loss than the 3.1 of the max loft position?

If you are correct "2.5 times compression from fully lofted does not loose warmth" but as I pointed out any containment will stop full loft. So where to begin for my down gilet.

[James D Buch]

Journal of Engineered Fibers and Fabrics Volume 2, Issue 2 – 2007

As I'm retired from materials research, I'll have to have the local library secure me a copy of this article. Would you have the page numbers and author, as they will be surely asking me for this at the library.

Oh, never mind. It is online. Downloadable as a PDF

http://www.jeffjournal.org/papers/Volume2/issue2.html

"Application of Nanofiber Technology to Nonwoven Thermal Insulation"
Phillip W. Gibson, Ph.D.1, Calvin Lee, Ph.D. 1, Frank Ko, Ph.D.2, Darrell Reneker, Ph.D.3

I think that anyone interested in insulation technology might benefit from glancing through this paper.

I'll print it out and make some calculations before continuing.

This is an excellent example of the published and open to readership nature of the data useful for this discussion.

Thanks.

[Don Selesky]

I just took my MontBell SS 30F bag down to about 10F wearing VBL clothing and the BPL synthetic parka and pants, with a bivy sack under a tarp, although I was starting to get cool at that temperature [wouldn't have wanted to push it any farther, but that was why I had the 180 quilt as a backup]. Still, I went out with a 12 pound pack, including everything but my water. Not bad for a winter overnight.

For me, it's been a quality piece of gear.

[David Lewis]

Yup… I took my 30F bag (#3) down to 15F a couple of weekends ago and I was still toasty warm. That was in a 4 season tent with 3 other bodies and some clothing on… but still. I LOVE my SS3. As you've probably all gathered… LOL. I'll stop now :)

[Chris Jackson]

Richard's graph titled "The effect of down density…on the insulation value of sleeping bags" has a line of best fit,

CLO = 0.12 * weight_of_down + 2.5

This suggests that an empty sleeping-bag shell has a CLO of 2.5 . Surely that's not the case. But then why is the constant term in the equation so large ? (Derek Goffin asked essentially the same question.)

[Richard Nisley]

Chris,

Your best fit calculation is a linear regression for only a segment of the 800 fill curve. The complete 800 fill curve's best fit is a multiple order polynomial regression that I did not show.

I didn't understand Derek's question when I read it. For example, he said, "800 fill down has a point marked "max loft". I clearly stated that this was a 550 fill chart, etc. If indeed it is was the same as yours, then both questions have now been answered.

[Chris Jackson]

Richard,

It's interesting that the graph of CLO v's density-of-down is nonlinear when the density is low.

For the sake of discussion, I'm going to play devil's advocate and argue that the graph shows that low-density down is more efficient that high-density down.

The slope of the graph is a measure of thermal efficiency (the slope has units of CLO/oz). The higher the slope, the better the efficiency.

Between 16 and 28 oz's the slope is 0.12 . Between 0 and 16 oz's the average slope is 4.4/16 = 0.28, assuming that the CLO of the empty sleeping-bag shell is negligible.

So low-density down is more than twice as efficient as high-density down.

Presumably this argument is flawed. Can anyone spot the error?

[Richard Nisley]

Mr Devils Advocate aka Chris,

You said, "Presumably this argument is flawed. Can anyone spot the error?" The first error is the premise "…assuming that the CLO of the empty sleeping-bag shell is negligible." The clo of a lofted, but empty of down, sleeping bag shell is not negligible. The still air in it provides ~.8 clo of insulation. The still air layer on the outside of the bag, if kept out of the wind, provides ~.8 clo of additional insulation. So the total before you add the down is ~ 1.6 clo.

The second error is making the assumption that the oz of down per clo is a linear relationship. The curves I provided clearly show the relationship is a multiple order polynomial relationship.

Rather than theory, let’s focus on some real world tests. Five identically built; except for the fill density, test bags were used to derive the data in my chart labeled "The Effect of Down Density, in a fixed 3.25…". Clo/kg is the most relevant measurement as it pertains to an ultra-light backpacker. Over those 5 bags the amount of down progressively increased up to 75% using the same fixed baffle size which obviously progressively increased the density. The clo/kg for the respective bags was 4.1, 4.0, 4.0, 4.0, and 4.0. Yes, there was a ~ approximate .1 clo/kg benefit to the least dense bag versus the other four but does it really matter?

[Chris Jackson]

Richard,

Could you guide me through the calculation of the clo/kg results in your previous post? When I try to calculate it I get the wrong result:

For 16 oz's of down,

clo of down + bag shell = 4.4

clo of down only = 4.4 – 1.6 = 2.8 .

clo/oz = 2.8/16 = 0.18

clo/kg = 0.18/0.0284 = 6.3 (correct result is 4.1)

[Richard Nisley]

Chris,

First, the clo value of the base bag was 4.5, not 4.4. Second, the sleeping bag's clo/kg calculation uses the total sleeping bag weight, not just the down weight.

Bag1: Down kg = .454, bag weight kg = 1.115, clo/kg = 4.0
Bag2: Down kg = .539, bag weight kg = 1.178, clo/kg = 4.0
Bag3: Down kg = .624, bag weight kg = 1.260, clo/kg = 4.0
Bag4: Down kg = .709, bag weight kg = 1.347, clo/kg = 4.0
Bag5: Down kg = .794, bag weight kg = 1.440, clo/kg = 4.0

[JULIO GOMEZ]

First I would like to say I am not an ultralight backpacker. I am always willing to carry a bit more just to be very comfortable. I am though looking for the best technology and if it is ultralight the better it is.

I had an Antelope MF from WM for 8 years or more. Used it many times and had no doubts it was the best bag in the market for me… until my girlfriend and I went to the everest base camp for christmas and she used a Valandre expedition bag (heavier off course than my antelope)… I felt envious and I had to say I realized it was time to upgrade my bag to a warmer and more updated one. Around 10 degrees more I wanted from my new bag so I did not have to worry about how cold it could get at the places where I usually go trekking.

That was a year ago and REI was carrying Valandre so I ordered the Shocking Blue model wich is described here:

http://www.valandre.com/us/bags/shock.htm

When I received the bag I was really happy to see the attention to detail and the quality of the materials Valandre uses for their sleeping bags (the collar was very nice!).
I opened it and let it loft on the bed by the side of my older WM.
To my surprise, the same thing that you see in the pictures comparing the MB with the WM happened… my old bag was so lofted and looked to my eyes better. (off course I did not know anything about what Richard explained in this forum).

I really liked the Valandre bag so I decided just to upgrade to a warmer bag from them. I thought maybe just the temp ratings were different for both companies an I returned the Shocking Blue and ordered the Freja sleeping bag wich had an extreme rating of -22 F (-30 celcius).

http://www.valandre.com/us/bags/freja.htm

When it arrived I was, as before, very happy and really wanted to keep the bag.
I decided this bag looked as lofted as my old WM and probably was going to be warmer. I tried the bag one night and had my WM close by… I have to say this and I really dont want to… my WM felt warmer to me…
It was very difficult for me to return the Freja since I really loved the bag…The color, the quality of the materials, the cut… everything felt nice about this bag… but I found it was not warmer than my old antelope.

Then I ended up ordering a WM Dakota that I got for a good price and is described here:

https://www.tenttrails.com/show_sleepingbags.asp

I can not describe how much better this bag feels compared to the other bags… again, just the loft was so different! so much better. It is warmer than my old antelope. Even though this bag is not a new model it weights the same as my antelope but it is certainly warmer. This is the bag I am using now. I just used this bag in Lantang (northern Nepal) at 5 F and everything was fine there!
I met a british guide who saw my bag and kept looking for the fill weight on the tags!!! He was using a Mountain equipment -20 F bag and could not believe my bag was so much lighter than his and that the loft was so different!!!

I really think WM makes top quality sleeping bags and that they are conservative about their temp ratings.

Happy trails

[Charles Kukla]

I have owned a MB SS#2 for about a year. I've had it in temps from 50 to 18 degrees F for nightly lows measured inside my tent. For me the bag is comfortable with a thin base layer to about 30F. Add a solid base layer of Patagonia R1 bottom and Capilene 3 top and you get 20-25 degrees out of it. Wear a down puffy on top and you can make it work to 15 degrees. I got it because I have trouble with mummy bags as I sleep very restless. The stretch feature solves that very well and it's still a high quality, very light bag. I love mine and I'd like to add that mine looks much loftier than the bag in the photo. If you grab each baffle and stretch it out (20 seconds for the whole bag) it lofts up much higher and stays that way. Don't shy away from the MB Super Stretch series if you are a restless sleeper or want more room. They are less expensive than WM also – I got mine for $255 online. All this being said if I could sleep on my back at all I'd probably get a trimmer mummy by Feathered Friends or WM.

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