Smaller sleeping bag with 100% overfill: I am confused

[James Marco]

Yeah, a constant sized cylinder will work. I worry that the square would still allow volume changes, though. In an overstuff test, this would be a critical measurement.

Simply doing an empty box first, then deducting that measurement from folowing test results, to calibrate the system, wouldn't be too hard to do.

[Jerry Adams]

You don't really care about theoretical over-stuffed, you want to know whether to over-stuff a sleeping bag.

So it makes sense to construct two test samples that are the same as actual sleeping bags. The only difference being the height of baffles and the amount of down. That is my test and Richard's chart (I think).

Or Richard's 5 sleeping bags test. But things can get complicated with sleeping bags as the evolution of testing leading up to EN13537 standard, but even that is inadequate because it doesn't include humidity, effect of person moving around,… Easier to just have square test sample. Easier to have just the difference between the two test samples be baffle height and amount of down.

[James Marco]

I understand that. My point remains, though. By simply testing a bags clo without regard for constant volume between baffles in 3 dimensions, then you are actually creating a different bag with different lofting by overstuffing.

[Michael Cheifetz]

@james

since richard didnt chime in – im not sure what he would say.

but what do you care what the theoretical CLO/oz/yd^2 is for a plexiglass cylinder filled with down? I dont think you could sleep with it :)

Now it MIGHT be that the changes you talk about (eg overstuff changes the effective baffle sizee even if the fabric size is same) have an effect..but that us the POINT IMO – there is a total warmth effect – which is what Richard measured:
5 same exact build shells with the only diff being amount of down in them – and the results he MEASURED were [totalCLO/totalOZ]= constant up to 75% overstuff

now i understand why it is INTERESTING to measure the "pure" effect of the down only in a glass cylinder – but then you would ALSO have to measure "expansion effects of fabric baffles constructions in the presence of varying amount of down and their impact on warmth"…….or something of that sort – cause at the end of the day this will ALSO affect total warmth per total weight

M

[Jerry Adams]

Richard's chart and my measurements are inconsistent with the 5 sleeping bags data

There is no conclusive answer at this time

So, if you over-fill by 100%, you may have the same weight for a particular warmth, or maybe it will be 30% heavier or maybe 15% heavier when you include the weight of the fabric

Maybe you're best having the baffles high enough to allow down to fully loft just in case that increases the warmth, because there's little down side.

On the other hand, if you over-fill 100%, you'll only pay a 15% weight penalty – maybe a sleeping bag will weigh 28 ounces instead of 32 ounces – you'll hardly notice

[James Marco]

Yeah, I agree. The point is that by changing the loft, you need to change the baffle position. Overstuffing simply results in a different bag, IMHO.

To give an example: a WM Summerlite and WM Ultralight. The differnce between the actual construction of the two bags is minimal with the bags girth and baffle positioning identcal. The only diference I could see on the web site was the collar. Subtracting 3oz for that, the two bags are nearly identical, except for the 30% difference in the fill (from 10oz to about 12-14oz, dicounting the collar.)

Similar examples can be made from Featherd Friends, Nanutak, Marmut, REI, EMS and others…often without the somewhat troublesome collar.

I question whether overfilling the Summerlite is actually compressing the down. I would suggest that it will compress to it's maximum given the limits of the material used to contain it, simply popping out, similar to a liquid under pressure, wherever resistance is less then the expansion. Thereby changing the loft between baffles given the baffle size remains the same.

I would therefore have to assume that the testing methodology is incorrect as described. I could be there is not enough difference to matter. In that case, simply buying the next size up bag will work. If it does matter, then it may be more effective to overfill the bag. But neither Richard nor Jerry has really satisfied me that there is a definitive difference to choose one methode over the other.

[Jerry Adams]

I agree, it's not clear whether you should over-fill 100%

On the other hand, some people complain that the down shifts in a baffle leaving empty/cold spots. To avoid this, I think you need to overfill 10% or 20%. Maybe some purchased bags don't require this.

If you calculate how much weight you add with 20% more fill than necesary, maybe it adds an ounce, but maybe that's good insurance to make sure you don't have shifted down in the baffles.

[Richard Nisley]

Jerry,

You said in part, "Richard's chart and my measurements are inconsistent with the 5 sleeping bags data. There is no conclusive answer at this time." I will provide you with three hints to help you understand the inconsistency. You can find the first hint in the BPL Preamble to your article on insulation testing:

One of Backpacking Light's aims is to present authoritative technical articles on the performance of outdoors gear. Doing so normally requires both calibrated measurement equipment and scientific skills. However, there is always room for research done in a less-than-perfect environment by a non-scientist, if only because that sometimes leads to insights that the more technical approach can miss.

We present here some research done by Jerry Adams into the performance of synthetic and down insulation. While doing the research and preparing this article, Jerry had many discussions with me about the methods used. While I offered lots of counsel, the decision as to how to proceed and the writing of the article was Jerry's alone.

The article was subsequently reviewed by another scientist experienced in this area, who pointed out many problems with the equipment, the methods and the results. While all of these problems are real, it was agreed that we should still publish this article here, while appending a short summary of the third-party review with editorial notes. We made this decision for two reasons. First, we want to encourage anyone with the will and time to do this sort of research – and we will assist if we can. Second, Jerry presents some insights that do not come from the strictly scientific approach.

Roger Caffin (PhD)
Senior Editor for Technology
Backpacking Light

The second hint is that as down is compressed there is interplay between the increase in conduction heat transfer and a decrease in the radiation heat transfer. One of the many flaws in your tester design is that it negates the ability to measure the radiation component.

The third hint is that a blackbody is a perfect emitter, meaning that it has an emissivity of 1.0. Everything else emits less than perfect and we say it has an emissivity factor. Human skin has a .99 emissivity factor and your diffuser must possess similar emissivity.

[Jerry Adams]

And my measurements were done in my office which was about 72 F which effects radiation loss.

So is your chart and the 5 sleeping bags data inconsistent about whether compressing down 100% reduces the clo/oz/yd2?

[Richard Nisley]

Jerry,

The emissivity design flaw in your tester will always yield erroneous compression test results; it is independent of your ambient test temperature.

Theoretically a 100% overfilled bag would have similar STATIC warmth/weight performance as a 0% overfilled bag. The problem would be that in DYNAMIC use (different movements, positions, and clothing worn inside) you would create a larger area of down densities outside the linear range.

Low to moderate over-stuff options, typically offered by vendors, will reduce down migration, provide a valuable safety tolerance, and not reduce the warmth / weight ratio.

[Jerry Adams]

"Low to moderate over-stuff options, typically offered by vendors, will reduce down migration, provide a valuable safety tolerance, and not reduce the warmth / weight ratio."

I agree, that's what I said, some people complain of down shifting creating places in a baffle with no down and this would fix that problem

In your chart "the effect of down density, in a fixed 3.25" baffle size, on the insulation value of sleeping bags"

On the left side is 4.5 clo for 16 ounces of down = 0.28 clo/ounce
On the right side is 5.8 clo for 28 ounces of down = 0.21 clo/ounce

That's not quite 100% overfill but close

That says you lose a little warmth/weight when you way over-fill

Or am I mis-interpreting?

[Richard Nisley]

Jerry,

Each peer reviewed research paper test shows MINOR variations in the aerial density/insulation curve shape. I wouldn't place undo importance in a .007 clo variance for one test.

[Jerry Adams]

There are 5 points in the plot that show the same relationship

The difference between 0.28 and 0.21 is 33% or 25% depending on which you make the numerator/denominator. So the weight of down in a sleeping bag will be 25% or 33% less or more. That could be something like 4 ounces.

If your sleeping bag has 4 ounces more down to get the same warmth, I agree that isn't a big deal, but to a gram weanie BPL person it's a little bit important.

[James Marco]

It seems quite logical to me that compressing more and more down into a black box, would result in decreasing clo performance per fill volume. I would expect to see a slight curviture to the graph at close to 100% overfill. I would expect this to trend to accilerate with higher stuffing amounts, 200%, 300%, 400% right on up to about a 1000% or 10x. (Theory says you should be able to stuff MUCH more, but given the expansive nature of down, it becomes impractical to consider for anything except a rigid container, hence of no value to a hiker.) More compression should yeild smaller air pockets, hence, more effective insulation up to a point. At the point heat is removed as quickly as the increase in insulating value, the curve will flatten and perhaps turn down. But this will be different than the maximum insulation per weight curve. And, exactly how much overfill should be added, in terms of percentages, to have practical value to a hiker, ie, a formula that will let everyone calculate his needed overfill for brand X bag. What constitutes a saftey factor: 10%, 20%, 30%, 40%?

2-4oz of down can mean more than the weight. It means a larger volume bag, also. Neither person really considered the volume of the stuffed bag. We know that larger volumes tend to multiply outwards, perhaps increasing the size of the pack needed to carry it, potentially increasing a packs weight. Or, if we cram it down into a small compression bag, what would be the long term effects on durability on two month hike? More damaged down due to increased compression and loss of loft more quickly? And the compression bag adds weight. Then why overstuff at all? Is it possible to rely on the bag being an "overstuffed 32F bag" or a "normal 20F bag?"

Both Richard and Jerry did an excellent job on presenting data on a very complicated subject. Thanks, guys! As any scientist will tell you, often there is no answer, just more questions.

[Jerry Adams]

Good point James, not always possible to get definitive answer

[Amy Lauterbach]

FWIW,

I recently made a double down quilt. So I tried to figure out how much fill some of the well respected bags use. I'll share my findings. But I won't weigh in on why one value would be better or worse than another.

I could only calculate for two dimensional quilts, so my sample is limited. I believe my calculations are correct, and will correct this if somebody finds otherwise.

I did my calculations by yard-inch (one square yard, one inch high) because that was the easiest way for me to think about my project.

When calculating volume, I figure height = baffle height + 1/2"

Jacks-R-Better (Mt Rogers, Katahdin, Hudson River) uses "800+ fill down" at 1.52-1.66 oz per yard-inch. (Values differ for the different models)
Jacks-R-Better with 2 oz overstuff (same models, same fill power) is 1.75-1.81 oz per yard-inch.
Nunatak (Backcountry Blanket) uses "800+ fill down" at 1.18-1.25 oz per yard-inch. (Values differ for the different sizes)
Enlightened Equipment uses "850 fill" at 1.52 oz per yard-inch

For reference, it takes 1.62 oz of fully lofted 800 fill down to fill one yard-inch.

Since I calculated volume by adding 1/2" to the baffle height, I would say that Jacks-R-Better with no overstuff is exactly that – not overstuffed. Here's one example of my calculation:
Kathadin is 84×61 with 3 inch baffles. In terms of volume, that's 84x61x3.5 =17934 cubic inches = 13.84 yard inches. It is filled with 23 oz of down. 1.66 oz down per yard-inch. So the basic product is essentially not overfilled, unless you believe that using enough down to puff out beyond the baffle height is overfill. I don't consider it overfill to puff out between baffles, as if you don't have that space puffed you'll get noticeable shifting.

When I've bought Nunatak products in the past I have always ordered extra down. But I had not done the calculation that shows that their default product (at least the Backcountry Blanket, and in my experience the Skaha pullover) are pretty seriously understuffed.

FWIW, I decided to use 2.0 oz of 900 fill thru-hiker down per yard-inch. It takes 1.44 oz 900-fill fully lofted down to fill a yard-inch, so I think one would say that I'm 35 or 40% "overstuffed". That's more down per volume than three above-mentioned companies use, and it's (in theory) 900 fill instead of 800+ or 850 fill. In addition to being influenced by the studies that RN had referenced, I was tired of the down shifting in the Nunatak BackCountry Blanket that we've been using. I'm about to make a second quilt (1.5" baffle instead of 2.25" baffle) and I'm planning to use 2.0 oz 900-fill again – in other words I'm pretty thrilled with the results of the first bag. BUT – I'm not pretending that I know whether my Love Bird Quilt is warmer with 25 oz down than it would have been with 20 oz; no way for me to figure that out!

Amy

[Jerry Adams]

I quickly looked at some of your calculations and I think they're right on.

Theoretically, un-definitively, if you overstuffed 20% rather than your 35% to 40% you'de have more warmth per weight but it's so little difference you'de never notice.

On the other hand, you won't have to worry about empty baffles and that's worse.

Your logic makes a lot of sense to me, and thanks for posting this.

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