R values everywhere!

Who says loft values are not useful when comparing different insulation types? My understanding is that for all the standard insulations (down, polarguard, primaloft, cotton batting, loosely knitted wool, shredded paper even) it is primarily air which insulates and the purpose of the insulation is simply to keep the air from moving. Yes, some of these insulations might conduct heat along their fibers better than others, but my recollection is that these thermal conductivity differences pale in comparison to loft as a factor in determining the total insulation value of a sleeping bag/quilt. As I recall, the military did some testing long ago and this was the result they obtained: "thickness = warmth for all standard insulations"

Foam insulations are an exception to the rule that "thickness equals warmth". A thin layer of closed cell foam may insulate much better than a thick layer of down, for example.

My objection to loft comparisons is that loft deteriorates with time, especially for high-loft synthetics but also for down to a lesser degree. What is really needed is to adjust the manufacturer specified values to account for loss of loft. My experience is that Polarguard 3D, which is generally considered one of the more durable synthetics, loses from 30% to 40% of its loft after a month of use, and then loses no further loft thereafter. I imagine Climashield is similar and Primaloft and the other synethics are probably worse.

People say that down loses no loft but I am skeptical of this. Assuming the down is used in a quilt, I imagine it loses at least 5% of its loft over time during to breakage of fibers during stuffing and carrying in the backpack. If the down is put under the body, then I would imagine it will eventually lose at least 50% of the loft in the hip and shoulder areas due to pulverizing of the fibers as the body moves in the night while sleeping on a thin closed cell foam pad over hard ground. If used with a thicker pad on softer ground, then perhaps the loft can be retained much longer. I suspect much of the talk of how down lasts "forever" is by people who don't use their bags that often, and the story from people who camp 100+ nights each year is probably different.

For some information on how different materials have different thermal conductivity for the same loft, see Wikipedia: R-value (insulation).

However, it may be true that for all insulation types commonly used in sleeping bags, the loft->R multiplier is about the same. That had not occurred to me. Do you have a reference? Or anecdotal evidence? I haven’t used a really good synthetic sleeping bag…

I agree that loft deteriorates with time, and it would be lovely to see the R vs. time/wear function published! Based on a little experience and some more reading, down does not deteriorate a great deal in normal use, but it certainly deteriorates a great deal with, say, humidity. The decrease in insulative ability of a sleeping bag in a 3D graph (R vs. wear and moisture) would be fun.

A side note regarding your comment on closed cell foam (1cm is something like R 1.36 and I have NO IDEA how that compares to any sleeping bag, which was kind of my point): wasn’t there some talk a while back of a sleeping bag that was just an envelope of two evazote pads? Does anyone here have experience with such a beast?

There is a book by Hal Weiss, "Secrets of Warmth", which in turn references the research by the military (natick labs) which concluded that loft is what primarily determines sleeping bag or garment insulating values, and not the material that creates the loft, at least for the materials commonly used in sleeping bags and high-loft garments. I don't have this book but I read it long ago. Whenever you hear people assert that 2" of dry leaves or whatever is as good as 2" of down, they are basing their assertion on this military research, which they probably learned of via this book by Hal Weiss, which popularized the phrase "thickness = warmth".

The wikipedia article includes a few items that are mostly air and thus similar to the kinds of insulation used in sleeping bags: cardboard, fiberglass batts, loose cellulose. All of these items have R values for a single inch thickness of about 3, which is in accordance with the idea "thickness = warmth", regardless of material, for the materials commonly used in sleeping bags. Note that still air has R value of 5 and air with convective currents has R value of 1, and that 3 is right in the middle, which again is not surprising if trapped air is the true source of insulation in sleeping bags.

Any attempt to be precise about the insulating value of a sleeping bag, beyond listing the average loft to at most 2 significant digits (e.g. "2.1 inches"), is pointless. Continuous fiber insulation has huge manufacturing tolerances as well as being subject to loss of loft with use, as I already pointed out. One section of a Polarguard roll might be 1" while another section just a few inches away might be just .75". The quilting used for continuous fiber or other synthetics introduces still more possibilities for manufacturing tolerances. Similar problems with down, where it is quite easy to put more down in one chamber than another so that the loft is different for each chamber, or to make the baffles slightly different in height, or to have the down drop off to the side during the night, etc, etc.

Any one remember military issue Mickey Mouse boots? These are one application of the theory that loft is most important for warmth, regardless of filler. They were just two walled balloon-type boots for arctic use. As a previous poster noted though, they would have only had an R value of 1/inch.

http://members.infinitecom.com/mmboots/index.html

As all UL'ers know, filler IS important, and is a strong determiner of insulation value, second to thickness maybe? If this were a taylor series approximation, I believe thickness would be the first term and material the second? pj help me out if you like..

Ben, here's something I found by AYCE in the thru-hiker.com message forum:

"Down is a loose insulation. The quality of the down and the density to which you stuff it all makes a difference, so you're better off sticking with inches of loft and a consistent way of stuffing your projects as a way to equate inches of loft to temperature.

The synthetics, though, are uniform. Clo is a measurement of the thermal resistance of a material. It's like R-Value, but specifically for apparel and gear.

If you didn't have clo values available, directly measuring loft isn't a bad idea. And even with clo values, if you're comparing two sample of the same insulation the one that's thicker is warmer.

Where you run into problems, though, is that a given thickness of one insulation might not keep you as warm as the same thickness of a different type of insulation. In other words, comparing inches of loft between insulations isn't a good comparison."

For the rest of it look for AYCE's post dated 2/15/2007 titled "re: working with clo values." AYCE provides the clearest explanations of clo that I've seen, easy to understand even for those of us who are not technically minded .

>>I agree that loft deteriorates with time, and it would be lovely to see the R vs. time/wear function published!

I have also been looking for something like this but without much success. All I've been finding is anecdotal.

One thing that caught my interest was this statement in an article about sleeping bag ratings:

"Indeed, tests conducted at Kansas State University indicated that a LiteLoft bag (no manufacturer listed) which had been abused until it had lost 27% of its original loft still managed to maintain 94% of its original insulating ability."

I'm not trying to stir the pot but just think that this is relevant to the discussion.

Actually, Mickey Mouse boots are an application of Vapor Barrier theory more than anything else.

As to the statement that "filler IS important", this boils to saying that the military research and Hal Weiss are wrong and "all UL's" are right. I would be interested in the evidence for this assertion.

The real reason filler matters is because of its influence on loft/oz. A typical 2" thick quilt made with high-quality down with standard 15% overfill weighs about 60% what a comparable quilt of continuous fiber synthetic insulation weighs, assuming we have adjusted for eventually loss degradation of the continuous fiber insulation. The huge difference in weight/loft between down and continuous fiber dwarfs any differences in thermal conductivity or R-values or whatever. If down is acceptable, then it is clearly the best choice. Otherwise, continuous fiber synthetics (polarguard, climashield) are better than short-stable synthetics because of the much greater durability, meaning much less loss of loft with use.

It is possible to boost the loft/oz of continous fiber sythethetics by making the fibers thinner and hollower, but this leads to more loft degradation since the fiber are then weaker and less resilient and tend to get compacted more easily, causing more loss of loft. Even the manufacturer admits that Polarguard delta is no better, in practice, than Polarguard 3D, because delta losses more initial loft than 3d and this offsets the greater initial loft/oz of delta versus 3d. Likewise, any differences between initial lofts of Climashield and Polarguard are likely to disappear quickly as the loft degrades faster for whichever of these insulations has the greater initial loft/oz, and hence the less resilient fibers.

There was a long discussion of clo values for synthetics (climashield vs polarguard) here on BPL during the summer. I didn't participate because I was traveling. My impression is that the discussion shows a complete lack of practical real-world experience with these insulations. No mention whatsoever in this discussion of how much loft these various insulations lose with use. In my experience, that loft loss is on the order of 30% to 40% for Polarguard. Now the difference between 30% and 40% is so enormous that it means you can't even get 2 significant digits of precision with respect to clo values, and yet you had people taking these clo calculations out to 3 significant digits. There's a difference between theory and practice.

Using R values would add enormous potential for misleading the public. Some people talk of 3oz Polarguard 3D as being 1" thick initial loft, others say .9", still others say .75"-1". The issue is manufacturing tolerances. 1" is best case, .75" is worst case, .9" is a good middle figure. Note that all of these figures are for initial loft, which will rapidly degrade with use. With loft, an ordinary user can more or less test the manufacturer claims. Not so with R-values, clo's and whatnot.

Similar issue with down. When a manufacturer specifies fill power of the down used, ounces of down used, baffle height and thus loft, and overfill, then it possible to get an objective idea of how well the bag/quilt is filled. Not so with R-values, clo's and whatnot.

Regarding AYCE: he is essentially asserting that the military and Hal Weiss are wrong. Also, note that he doesn't mention loft degradation of climashield and polarguard. My impression is that AYCE, who is normally a sensible person, has been drinking manufacturer koolaid. He blithely quotes these figures of .77 clo/oz for climashield or .84 clo/oz for primaloft, as if it were actually possible in practice to be so precise as this. Even rounding off to .8 clo/oz for both insulation would still probably be misleadingly precise. In any case, given that primaloft is thinner than climashield, these figures might seem, to some people, to be proof that the military and Hal Weiss are wrong. To me, these figures seem more like evidence that the manufacturers of primarloft and climashield are using different methods for measuring clo values, and at least one manufacturer is giving misleading information.

Regarding the Kansis State research: they are essentially asserting that the military and Hal Weiss are wrong and that "thickness = warmth" is not true.

Frank, thank you for the detailed post. Due to the time difference I will have to reply to you on the morrow. Briefly though, I disagree with your statement that my assertion 'filler does matter', contradicts the military research that thickness=warmth. They can both be true. Differences in thermal resistance for equal thicknesses of differing materials would be evidence of this. Ill look for that evidence tomorrow.

However, this is making me think more about why certain fillers lead to higher insulation values; is it just that they loft better (down for example?) Thanks for the thought provoking post.

There is a weak and strong form of the "thickness = warmth" concept. The weak form says thickness is far more important than other factors, but the other factors are not negligible. The strong form says thickness is all that matters, in practice, and the other factors ARE negligible. The military research, as I recall, supported the strong form of the concept. Hence statements like "2 inches of shredded paper insulates as well 2 inches of the most expensive down". (I don't recall Hal Weiss's exact phrasing, but it was something like that.) You, and AYCE, and the Kansas City research seem to be supporting the weak form of the concept.

Frank, I would never argue that the military is ever wrong–about anything.

Regarding Hal Weiss, I happen to have “Secrets of Warmth” in front of me. In general, I agree with his mantra: thicker is warmer. Up to a point. On page 54, Weiss states,”Again it’s thickness that determines warmth(if other things are equal. This rule changes when you introduce the micro-fibers into the insulation. The micro-fibers(Thernolite, Thinsulate, Eizac, etc.)allow jacket or pants to be about about half the usual thickness for an average amount of warmth.”

Keep in mind that “Secrets of Warmth” was written almost twenty years ago. My guess is that if Weiss were writing today he would have a thorough discussion of clo in his book.

Clo is objectively measured, just as R-value is. There IS a difference between Polarguard 3-D and Primaloft One with in regard to clo.

Regarding the Kansas State research. Dr. Elizabeth McCullough is in charge there . Here is her resume and a list of publications.

On page 54, Weiss states,"Again it's thickness that determines warmth(if other things are equal. This rule changes when you introduce the micro-fibers into the insulation. The micro-fibers(Thernolite, Thinsulate, Eizac, etc.)allow jacket or pants to be about about half the usual thickness for an average amount of warmth."

Yes, and closed cell foam is even better. So what? None of these materials is suitable for use in a sleeping bag. Closed cell foam doesn't breathe and the micro-fibers have poor loft/oz ratings. Thinsulate is great for things like boots, precisely because it is so heavyweight. Something like polarguard wouldn't last very long in a boot.

My guess is that if Weiss were writing today he would have a thorough discussion of clo in his book.

The concept of Clo is older than Weiss's book.

Clo is objectively measured, just as R-value is. There IS a difference between Polarguard 3-D and Primaloft One with in regard to clo.

The military research and Hal Weiss's book, if I recall correctly, both indicate such differences are of no practical importance and that the only thing that matters is thickness. I don't know what the clo differences between Polarguard and Primaloft are, but I'm sure they are small and dwarfed by the issue of loss of loft with use. Also, Primaloft is fragile compared with Polarguard and a poor choice for a sleeping bag/quilt that won't be sturdily quilted. Buy a yard of each insulation and experiment with pulling them apart and you'll immediately see what I mean.

Regarding the Kansas State research

Anyone who says 27% loss of loft constitutes heavy abuse of LiteLoft loses all credibility. First, too many digits of precision. Second, LiteLoft loses at least 50% of loft with true abuse. In particular, like all short-fiber insulations, it shreds easily unless you quilt it like crazy, in which case the quilting will ruin the loft. And I don't care what Dr McCullough's resume looks like. I spent enough years in high-tech to know that academics can be really dumb when it comes to practice as opposed to theory.

>>And I don't care what Dr McCullough's resume looks like. I spent enough years in high-tech to know that academics can be really dumb when it comes to practice as opposed to theory.

If we discount everything that takes place in a lab (under controlled conditions) then what we are left with is –mere opinion.

Note that Dr. McCullough has received a number of research grants from THE MILITARY. Allow me to salute.

The military is never wrong. Anyone who claims it could be is anti-American ;)

Seriously: yes, some fillers are lighter per loft than others. However, that would be completely irrelevant if there weren't a _correlation_ between loft and warmth. What matters is the weight per warmth. That table at Wikipedia clearly demonstrated that different materials at the same loft insulate differently. End of story, no matter how you interpret the military figures. Hell, Wikipedia mentioned that the relationship between thermal conductivity and thickness can even be nonlinear!

"The huge difference in weight/loft between down and continuous fiber dwarfs any differences in thermal conductivity or R-values or whatever."

That statement only makes any sense if down and synthetic have about the same warmth per loft. Here's some evidence that supports your claim: the Western Mountaineering VersaLite, a good -12C down bag, is reported to have 15cm of loft. So does the -12C Hyperloft MEC Habanero. This pair also roughly support your claim that down weighs about half as much, although WM uses 20D shell material vs. the Habanero's "30 to 40 Denier", so there are some fudge factors…

Also, from Dondo's post, "[munch] lost 27% of its original loft still managed to maintain 94% of its original insulating ability": that's very interesting! So in that case, at least, degradation doesn't really matter. On a similar note, I have never heard that felting wool reduces its warmth much.

But back to the topic at hand: whether R values (or some new system that does the same thing but that no-one understands) would be helpful in sleeping bag selection. You raise an excellent point: loft _is_ verifiable with common household equipment (a ruler, a level or another ruler, and a flat floor), whereas thermal conductivity isn't. I would hope for consumer reviews to verify R claims but I guess that's asking a lot. Still, consumer understanding of loft is so murky (as evidenced by this thread) that I'm not convinced yet. How about… both?

ps. I tried to include links in my post (to the relevant product pages at http://www.mec.ca), but they end up getting eaten. Is there a way?

Cheers!

Whoa, you guys are leaving me at the trailhead.
Another interesting place to read more about loft, r values, materials, and conduction/convection heat loss is Wiggys. Ol' Jerry can get a little far gone on some subjects but he does do a decent job with heat loss,compression,convection/conductance, complete pictures with circles and arrows and exclamation points.

IMHO, Wiggy is a bit,um, wiggy.

As I understand Ben Pearre's original point, he was suggesting the use of r values for comparing sleeping bags would be a small step towards objective comparison. True, but there is already a much larger step in Europe with the EN 13537 standard, which tests the whole bag rather than just the material used. Two bags tested to EN13537 with the same ratings from different manufacturers will have the same ability to keep you warm.

The bad news for Americans is that the US is not part of the scheme. The good news is that there is one US laboratory approved to test to En13537 – it is
The Institute for Environmental Research, Kansas State University. Co-director of the institute and Professor of textiles Elizabeth McCullough (sounds familiar, where have I heard that name before?).

Anyone wanting a background on the history of sleeping bag testing, and with 20 minutes to spare for reading should see http://www.mammut.ch/mammut/uploadedFiles/Sleep%20Well_Pt1_E.pdf

(Edited to correct my mis-spelling of Ben Pearre's name. Sorry Ben.

David, The Sleep Well pdf is fascinating reading.

>>The bad news for Americans is that the US is not part of the scheme.

Yeah, that is a bummer for US consumers. So far, I've only been able to find ratings for TNF and Marmot in a a roundabout way by going to their European sites and translating the C to F ratings. My experience with one of the rated bags indicates that the EN 13537 ratings are dead on, at least for me.

I guess that we can always buy European bags, if we can find them locally.

>>The good news is that there is one US laboratory approved to test to En13537 – it is
The Institute for Environmental Research, Kansas State University. Co-director of the institute and Professor of textiles Elizabeth McCullough (sounds familiar, where have I heard that name before?).

I've noticed that her name seems to pop up everywhere when researching these things.

Wiggy is wiggy, but he does say a lot of true things.

Let's get one thing straight. I'm assuming we are talking about what works in the field, not in a laboratory. Yes, the R values in the wikipedia article are different for the same loft. For some of those materials, such as closed cell foam, I have already conceded that loft doesn't tell the whole story. But consider something like loose cellulose (more or less shredded paper). The R value measured in the laboratory will be the same R value if used for building insulation (which is its purpose), because the insulation is never abused, meaning never stuffed into a stuff sack, carried around in a backack, unstuffed, etc, etc. But how long do you think the R value of loose cellulose would be retained in field conditions if used in a sleeping bag? Not very long.

So, yes, you can goose polyester in various ways to get some added R values either per oz or per inch. Either you fluff the stuff up real lofty or else you make sure to cut the fibers real fine so they block convection currents. And the final result is something really flimsy which loses loft really fast in the field. There is the problem.

Everyone who has used synthetics extensively in the field has come to the conclusion that continuous fiber works best, because it is most durable. The other synthetics tend to break apart and then you get huge cold spot. Wiggy has lots to say about this, though he exaggerates and lies at times. Most of the people on forums who criticize synthetics are talking about the non-continous versions.

As for continuous fiber polyester (Polarguard, Climashield), there really isn't that much you can do, given that the underlying technology is simply strands of polyester. This is almost 50 year old technology. Yes, you can make the strands thinner and thinner and hollow them out and make them funny shapes. And the result is something that has great initial loft and then loses loft rapidly because the thin fibers lack resilience. Wiggy has stuck with thicker fibers, which have much less loft per oz initially, but which don't lose loft so rapidly because they are more resilient. My impression is that Polarguard 3D is better than what Wiggy uses, at least for quilts, but you do have to take into account the loss of loft with 3D. That is, if you want a 3-season quilt roughly 2" thick, you need to start with 3" or 9oz/sqyd of 3D, or 11.2 oz/sqyd including 1.1oz shell fabric. Compare with 800 fill-power down, which requires about 6.5 oz/sqyd to give 2" loft (assuming 20% overfill, no-see-um baffling, 1.1oz shell fabric).

IMO, the only real improvement possible with continuous fiber at this point is to improve the manufacturing tolerances, so that there aren't all these variations. That doesn't improve R values/oz or loft/oz, but it does make for a better product. Obviously, this would raise costs as well.

"[munch] lost 27% of its original loft still managed to maintain 94% of its original insulating ability":

What this suggests is that the insulation had lots of air space but wasn't blocking convection currents well initially, and then it got compacted and had less air spaces but was blocking convection currents better. A sort of meaningless result, IMO. You really have to have very little field experience to believe that a bag which has lost a signficant amount of its initial loft will keep you as warm as one which hasn't lost loft. Once that LiftLoft bag loses 50% of its initial loft, as it will eventually, you can be pretty sure it won't retain 94% of its initial insulating capability.

One last thing: all reputable down manufacturers list loft, overfill%, fill wt, etc. None of the synthetic manufactuers list this sort of information, which suggests they are all dishonest. Furthermore, even though the manufacturer of Polarguard admits their insulation loses loft with abuse, none of the manufacturers of sleeping bags made with Polarguard point this out. The European standard was created because of all the dishonest manufacturers of synthetic bags. There was never much confusion with respect to the manufactuers of high-end down bags. The European standard is an improvement, but it still doesn't take into account loss of loft in synthetic insulation with use. You should probably add 20°F to the manufacturers spec for any synethetic bag to account for this.

Oh yeah, with regards to the military, I'm not defending the military per se by any means. The Natick Labs research was done long ago and it is confirmed by every intelligent study of insulations I have ever read, as well as by all my own field experience and what I have learned from other backpackers. I am not surprised that the military is continuing to fund new studies when previous research already gave the answers. Nor would I be surprised if the researchers contracted to perform studies for the military take the money and perform the studies, rather than rejecting the money because there were previous conclusive studies on the subject.

I don't quite get this discussion.

AYCE's article states that warmth is basically how insulating the material is (Clo) * weight (e.g. amount of material). The weight will be directly related to loft by some constant. This make sense.

I believe, as Frank noted, that the claims that loft is all that matters was the weaker form. They were making the assumption that the Clo ratings of the possible insulation materials was somewhat similar. This makes sense since this has been historically been the case. My experience in the field has largely found that an extra 1 inch of any material more than made up for some variance of the materials Clo. There was just one exception to this. I found that give equiv loft of thermomite and any other material I have used, thermolite was notably warmer (though it wasn't significantly lighter and was kind of stiff).

AYCE point is easily seen when when we pick two very different materials. Lets say 1 in of steel -vs- 1 in of aerogel… actually 1 inch of anything -vs- aerogel. Aerogel would be significantly more insulating. Look as the POE pad with a few millimeters of aerogel. A 24oz pad that's rated at R24. Simular thickness foam pads without aerogel are more like R4 or R5.

–Mark

–Mark

MarkVerber: steel, aerogel and thinsulite (I assume you meant that rather than thermolite) do not fall into the category of standard sleeping bag insulations. With regards to things like down, cotton batting, primaloft, polarguard, climashield, thermolite, liteloft, etc, there are no doubt some minor R value differences. But I maintain that these are dwarfed by the factor of loss of loft in the field, as in the range of 30-40% for Polarguard, which is the synthetics I have the most experience with. The number which AYCE quotes on his website, .77 or .84, are meaningless when you consider the huge loss of loft in the field (up to 40%) and the disparities in the actual loss of loft (30% versus 40% is a big difference). At best, he should round off to .8, and even that is misleadingly precise. Also, I mention AYCE only because someone else did. He is by no means the only or the worst offender in blitely repeating these manufacturer specs without questioning them.

Frank, I think Mark may have been correctly referring to Thermolilte. I have an ancient Moonstone Cirrus pullover that contains Thermolite insulation and my experience is that despite the amount of loft it has lost over the years, it's still plenty warm. I don't think anyone is questioning that, in general, loft= warmth. It's a good rule of thumb. But there are indications, both anecdotal and the study cited above that there is a bit more to it than that.

Thermolite is an okay insulation for garments. It is short-fiber and thus needs to be quilted tightly to avoid tearing, which is not a problem with garments, though it is a problem with quilts/bags. Some really cheap European bags used to have themolite insulation. These weighed in a 500g or a little over a pound, had perhaps .25" of loft, were rated to 20°F, wouldn't keep the average person warm at lower than perhaps 55°F, and sold for maybe $20 in the stores. Fine for sleeping in a hostel in the summer, but woefully inadequate for outdoors in the spring or early autumn. Bags like this were the motivation for the European testing standard.

Loft=warmth is not merely a good rule of thumb. It is essentially ALL that matters in deciding on what type bag you need, and I have explained why this is so at length. In denying this, you are not being scientific. Rather you are needlessly complicating some aspects of the problem while ignoring the things that really matter, like how much loft can we expect to lose from the various insulations in the field, and such overcomplication mixed with oversimplification is hardly the mark of good science.

For anyone who really want to know how to use all the information given in this thread, the procedure is as follows:

1) Determine how many inches of loft you need for the conditions you expect to encounter. Answering this question is hardly an exact science, I might note.
2) Find sleeping bags/quilts which give you that much loft AFTER taking into account any loss of loft due to use, which is primarily a factor with synthetics.
3) Compare weights/prices/features within those sleeping bags/quilts that meet the loft requirement.

Down will always be the winner in terms of weight and space, for a given loft, but might be ruled out due to other considerations. If down is unacceptable, then use continuous fiber polyester. It is not clear which of the continuous fiber polyesters is best (polarguard 3D, polarguard delta, climashield, wiggy's lamilite). The reason it is not clear is because no one has ever studied the loss of loft among these insulations in a scientific fashion. The manufacturer quoted clo values are for initial loft only.

Years ago The North Face, I believe, developed the mannekin (sp?) "Copper Man" (a copper, human-shaped container filled with warm water) to test its sleeping bag insulation effectiveness.

Later several other companies and the army Nautic Lab began using this method.
It appears to be the only scientific BASELINE means so far used to measure sleeping bag insulative properties.

Anyone else know if this is still used? If so then THIS is the best basis for realistic comparisons. It, nicely, also measues insulative properties of backpacking sleeping mattresses because of its weight.

Eric

Forgive me if you already know about this, but try a search for "13537" here at BPL. It is a standardization of copper man tests used in Europe. There is none required in the US as far as I know.

Wow, this discussion has really taken off.

Frank and anyone else who says "x" is the only factor for "y" is simplifying the analysis greatly. That type of theory can be internally valid if you accept the underlying assumptions; hold certain variables as constants, drop terms from equations, etc… So, yes, Frank is correct when x=loft and y=conductive heat transfer, and all other variables and methods of heat transfer are ignored.

However, a more detailed analysis of other contributors to what Frank calls warmth (actually thermal resistance- bags dont make warmth) would reveal other terms in the heat transfer equation, namely convection, radiation, evaporation, condensation, etc.. most of these are secondary to material thickness (loft), true, but they can be significant.

Additionally, in the formuala for conductive heat transfer there IS a term for the insulating materials R value, not just material thickness. Frank can suggest dropping the R value term from the equation, but that limits the accuracy of the resulting analysis. This value of R can vary from 1 for air to 3 for fiberglass insulation(probably same order of magnitude as down, I dont know). R values can be found all over the web so I won't quote them exactly; see wikipedia for example.

Frank, it sounds like you do have quite a bit of scientific experience, and you correctly limit your significant digits to be equal to the minimum number found in any input to an equation. So by holding certain variables as constants in your discussion of heat transfer you are making a valid argument for simplicity when a consumer goes to buy a bag, but there is actually much more to the subject; agreed?

http://en.wikipedia.org/wiki/Heat_conduction