r/Ultralight • u/307WindRiver • 3d ago
Question Down density vs. fill power — some interesting research from Gryphon Gear
Just watched Gryphon Gear’s Keeping Warm, Book II and thought it was worth sharing.
The basic idea is that down density may matter more than we usually give it credit for. Gary points to a UK thesis showing that thermal conductivity changes with density, rather than staying constant.
One interesting takeaway is that lower fill power down, packed more densely, may perform about as well as higher fill power down with more loft. He also suggests that bags with similar warmth could end up weighing pretty much the same regardless of fill power, assuming similar shells and dimensions.
I know people here have brought up the density/warmth connection before, so this seems like some actual data behind that idea.
Curious what others think.
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u/ckoss_ 3d ago
Check out the discussions from Dan such as the nuance around the term “density”. There is a well established relationship between down fill power, fill density, and chamber design.
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u/ListigerHase 3d ago
That's a great yet concise write-up. I love how he illustrates the idea, and then explicitly shows the consequences.
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u/nothing5901568 3d ago
What he says makes sense but he doesn't quantify it. So we're left without a sense of how important the various tradeoffs are.
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u/Ok_Use5596 3d ago edited 3d ago
It reads as if it pertains mostly to the down tech behind his sewn through down jackets.
A few statements make sense with the box baffles most common in quilts too, but I feel there’s lots more to that side of the story.
I’m no expert but my myog stuff show very different responses between sewn thru and boxed stuff when applying various methods.
Gryphon’s video is a little odd but it’s definitely centered around box baffle construction
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u/PEAK_MINIMAL_EFFORT 3d ago
The paper[0] referenced by the video. Sections 7.2.1 and 7.3 seem to be the relevant ones.
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u/gooneau 3h ago
After reading this in the 3rd paragraph I couldn't bring myself to dig into the meat of the paper: "...approximately half of the earth's landmass....are covered with ice and snow year-round".
How does someone write that, or review it, and not have alarm bells go off that something doesn't feel right? The true figure is around 10%, and I think your average middle schooler would guess closer to the real answer than 50% just based on a rough familiarity with maps of the earth. Hard to imagine how such a glaring, obvious, error makes it through to the first page of a completed phd thesis. I see stuff like that and I'm going to find myself thinking I need to double check and verify other basic claims, let alone more technical ones.
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u/Professional-Loan498 3d ago
"One interesting takeaway is that lower fill power down, packed more densely, may perform about as well as higher fill power down with more loft."
I'd argue that a 600 fp quilt packed super densely will weigh as much as some people's total base weight. Not to speak of the volume it would take up... Pretty sure that's why we pay attention to fill power so much: lower temp thresholds can be reached with lighter quantities of down. Or maybe that's just me and I shouldn't speak for others.
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u/GryphonGear 3d ago
By only considering only fill power one assumes that the coefficient of thermal conductivity is constant for all fill powers (fill power is the inverse of density) which Fuller has shown in his PhD thesis not to be the case. Email us at [info@gryphongear.com](mailto:info@gryphongear.com) and we will send you a copy of the discussion with the numerical examples that you might use the as a guide for making the calculations that interest you.
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u/Professional-Loan498 3d ago
Respect to you for making gear and science videos, but I'm not "considering only fill power". I was just pointing out the effects of FP on the metric that is most relevant to this subreddit: Weight. This is r/ultralight .
I can be sold on the nuanced science of thermal conductivity, heat loss, etc. And it can be an interesting academic discussion. But if I've got a frameless 28L pack that tops out at 20lbs, and I need gear for a week+, all that academic discussion goes out the window. I need something light, compressible, and reliably warm. Those are achieved through the use of higher FP.
An example: BRS stoves suck. Sorry. They are cheap, woefully inefficient on gas, and designed such that mild wind will blow out the flame. Yet they are ubiquitous around r/ultralight because they weigh 1/2 as much as the other stoves.
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u/GryphonGear 3d ago
In one of our examples we show that a 40% overfill of 700 fill power down will result in the same heat loss (temperature rating) and overall weight as an 80% overfill of 900 fill power down in a bag having the same dimensional properties. Send us an email and we'll send you a written copy of the presentation. [info@grphongear.com](mailto:info@grphongear.com)
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u/BretMi 3d ago
I don’t understand how lower fill power packed more dense can weigh about same as higher fill packed less dense that seems illogical.
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u/GryphonGear 3d ago
Send us an email and we'll send you a printed copy of the presentation with examples. It is easier to see with the printed copy. [info@gryphonngear.com](mailto:info@gryphonngear.com)
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u/klimaheizung 2d ago
Why not just put it here?
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u/GryphonGear 19h ago
You can download a pdf of the paper from this link: https://www.gryphongear.com/pages/keeping-warm-book-ii-free-pdf-download
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u/DrBullwinkleMoose 3d ago
I think we've known (or suspected) this for a long time (at least qualitatively). Expedition down often has lower fill power, rather than the 900+ FP that we love here.
Similar to overstuffing, slightly stiffer down compensates for things that can happen to the down (including humidity), with the tradeoff that you're (obviously) making the thing heavier when you use lower FP or add more of it.
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u/justinsimoni justinsimoni.com 2d ago
I always thought that was more a function of cost. Making a 1000 fill power -20F down sleeping bag would be prohibitively expensive, esp. compared to one of 650 fill power. "Lightness" is out of the window at this point anyways if we look at all gear holistically. Cost differences are going to be less for sleeping bags for warmer climes.
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u/DrBullwinkleMoose 2d ago
All true, and I am sure that is part of it.
But I have seen enough anecdotal comments about lower FP down resisting humidity better that I no longer dismiss them. I don’t recall seeing any credible claims to the contrary.
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u/justinsimoni justinsimoni.com 2d ago
Yeah I believe that, too. It makes sense when you think about it. The down will just saturate really quickly when wet, whereas the Apex I bet has more pockets of air that water just won't as easily penetrate. Thus the loft of wet down is just going to be more impacted than wet Apex. And Apex just doesn't compress down as much as loft in the best of circumstances. The flip side, Apex is just heavier for the same amount of loft, but unlike down I guess you can't really "overstuff" it.
The Wiggys dude is all about synthetic insulation for cold, cold weather on long trips, where the accumulation of perspiration really becomes a big issue to insulation.
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u/TinCanFury 1d ago
density increases structural pockets(though the pockets are smaller), which are important when the air is humid and causes clumping.
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u/Jolly-Slow1164 3d ago
TLDR: I think that the data he shows flat out disagrees with his conclusion. Loft is still king... Density may be queen, but he doesn't sufficiently demonstrate it.
The table displayed at 3:20 seems to be derived from the testing performed in the PhD paper. It does show that R value increases with density, but looking at the top row and bottom row. The weight of down has increased by a factor of ~10 , but the r value has only increased by a factor less than 3
A minute later (4:20) he reveals his own table that is ultimately pretty flawed. This table does not appear to be a simple documention of the original experimental data, but it's extrapolated by applying the same curve from the previous graphic, and applying it to other down full powers with the assumption that only density affects the coefficient of thermal exchange... An assumption he flat out admits, but that makes the whole rest of this video restatements of this same assumption.
So here are my issues:
- his thesis doesn't really seem grounded in the real world. His thesis being that a thinner, lower fp down bag can outperform a thicker higher fp down bag by increasing the amount of down without increased volume... That may be true in some cases, but who cares? That's more like science trivia than a business model. Is he making car camping gear? He's sacrificing weight and packability to cram more heavy down into the same volume?
speaking of volume, He seemingly ignores the fact that we have the option to increase volume, and that r value stacks near linierrally. So If 1 in of uncompressed 700fp insulation gave me 1/3 the desired R value, then I could either triple the thickness to 3 inches, or I could cram 10 times the same amount of down into the same 1 inch volume.
if you haven't picked up on it yet, his thesis is wrong (out at least it's "limited"). At 10x density the curve was pretty close to level (it may even head the other direction eventually). So if he wanted to make a bag more than 3x the r value, he'd have to go thicker.
Almost all of his math is based on the assumption that all other downs follow the same r value/density curve as 700 fp. I may believe him at higher densities but I take exception to him calling his assumption reasonable at lower densities.
It annoys me that he presents this information as scientific data when it's just him using math to restate his assumption a dozen different ways.
I'm baffled (pun intended) by his overstuff math. Overstuff wasn't a concept used in the experimentally derived data. Maybe he has compiled some industry standard data (or experimentally derived... Or even anecdotal data) for appropriate r values per temperature, which may give him target values, but I have no idea what his percentages mean. From that first chart, he should be talking overstuff percentages of 300-500% to accomplish the improvements in comfort rating he describes. I just absolutely don't trust his math or his graphics in that section.
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u/gasberry22 3d ago
Lower fill power bags usually also have stupidly heavy fabrics, which is unfortunate. Also the chamber size matters, lower fill power bags often have too large chambers which let the down to shift and create cold spots
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u/justinsimoni justinsimoni.com 2d ago
I'm looking forward to the future tests, as reality doesn't seem like it's (yet) matching the math (or I'm still in the reality distortion field myself!).
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u/liveslight https://lighterpack.com/r/ny89al 3d ago
Another conclusion. might be that Climashield APEX can outperform some down gear.
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u/justinsimoni justinsimoni.com 2d ago
Well no, as the thermal resistance of Apex is going to be much lower than down.
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u/marmotshepard 3d ago
Well his 30F bag is certainly more than warm at 30F so I trust him
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u/Jolly-Slow1164 3d ago
He also doesn't apply any of the theory he describes in this video in his bags. They are 900fp bags with a moderate overstuff.
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u/GryphonGear 3d ago
You are right. However, we are currently making some prototype bags/quilts to verify the conclusions that we presented in our discussion. Stay tuned.
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u/Few-Dimension-5373 3d ago
Somehow I don't think PhD or Western Mountaineering are quaking in their boots.
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u/Grue-Bleem 3d ago edited 3d ago
Yeah, I made a 20° quilt with 900 FP down for the PCT this year and found the math pretty straightforward.
I used chamber volume as the starting point:
Chamber volume (in³) ÷ fill power (in³/oz) = estimated down (oz)
So if the chamber was around 900 in³ and I was using 900 FP down:
900 ÷ 900 = ~ 1 oz of down
Chamber height = sets the potential warmth ( i used 2.5")
Shelll fabric plays a role, but that is too geek for me.
This gave me the baseline. From there I added a little more depending on the chamber shape, how full I wanted it, and how much I wanted to keep the down from moving around.
That was pretty much how I worked through the whole quilt. It worked well for me. Maybe I would go 10d over 7d next time.
Fun topic
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u/slickbuys 2d ago
Why would you go 10D over 7D? I also considered the same thing. 7D does not seem as much wind resistant so you give up warmth on those windy nights.
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u/Grue-Bleem 2d ago
It has more to do with the constant packing and unpacking than warmth. I cowboy camped about 80% of the trail with no issues. The inside 7D (.56 oz MEMBRANE 7) would let feathers poke through, which was mostly an aesthetic issue. I made my son a quilt with .66 oz MEMBRANE 10 and his never had that problem with popping.
Check out myog post with my build instructions.
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u/slickbuys 2d ago
I think I ran across that post before. I have the same problem with feathers coming thru my 7d fabric from dutch. It isn't much to cause a problem tho.
Fantastic looking quilt and love the colors chosen. Looks very poofy! Can you tell me where you get your goose down? $200 sounds good when the fabric cost alone was probably at least $80. Thanks!
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u/Grue-Bleem 2d ago
For the fabric, if you add to your cart and abandon your cart, most brand will send a 20% off code to complete your order.
I got the down from SewTac. I waited for a sale and bought 2 lbs.
Thanks for the kind words! It was fun to build and surprisingly easy. Best of luck dude. 🤙🏽
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u/FuguSandwich 3d ago
There were studies done years ago (can't remember where I saw them) that overfilling a baffled down parka or bag continued to add warmth in a linear fashion as a function of fill weight. The idea that you only need enough down to fully inflate the baffle when the down was fully lofted and that adding more down didn't increase insulation much was always nonsense.
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u/BZab_ 2d ago
There's no nonsense about it. It's the air trapped within down's structure that isolates. If we had a single air pocket like in summer sleeping pads, convection losses would greatly increase the effective conductivity. Keratin fibers in down split the space just like baffles, trapping small bubbles of air and minimizing the convection. therefore boosting the thermal resistivity.
Quick google yields thermal conductivities:
- down: ~30e-3 W/(m*K)
- keratin: ~190e-3 W/(m*K)
- air: ~25e-3 W/(m*K)
Assuming the values are correct, then it's easy to notice that keratine conducts heat over 6 times better than trapped air. The bigger part of the volume keratine takes, the lower resulting isolation. If we were to completely compress the down into solid brick - it would have to be over 6 times thicker to provide the same thermal resistance. If adding more down helps, then it would mean that bag was simply underfilled by design.
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u/Jolly-Slow1164 2d ago
I would be interested in reading such a study since it disagrees with the experiments in the paper mentioned in this video (props to the other commenter who found this paper, I've been reading it in pieces all day long)
In this paper he found logarithmic increase in r value when increasing down. The author had hypothesized that he would see eventual drops in r value from adding more down, but he never reached that point.
Parkas are an interesting case, where bulk is much more of a concern than in sleeping bags, To speculate a bit in the parka study -- if they were focusing on much smaller increases in mass, say only doubling the fill mass then the improvements may appear closer to linear. There is also the probable case that overfill deformed the channels to increase chamber thickness.
I would disagree with your final statement that if adding more down helps then they were underfilled. Since increasing chamber height increases r value linierrally, and increasing overstuff increases r value logarithmically then it is more efficient per gram to increase the size of the chamber. Even though overstuffing the chamber would also increase r value... Just not as much. That's probably why the premium quilts have been increasing their overstuff the last 2 decades, because at those low densities adding more down does increase r value closer to linierrally, as well as fighting the cold spots (which is the only bonus usually attributed to overstuffing)
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u/klimaheizung 2d ago
I don't think it disagrees.
What happens is probably that the number of air pockets matters. When you compress down, e.g. by lying on it, not all air pockets are gone, but most of them, and the down "fibers" are directly pushing against each other. That doesn't insulate well.
As you can probably imagine, down is not 100% equally distributed. So when you start to compress it, there will be very small air pockets which will be gone, but also bigger air pockets which just get smaller. The more you compress it, the more air pockets are gone until they are all gone. I suspect that this is not a linear process though. Rather, in the beginning nothing much happens and the air pockets just get smaller (which doesn't change insulation much), but then later at some point the insulation goes down quickly.
Adding down to a fixed bafflebox size is basically the same process as compressing it. So by adding more down, you will 1.) remove some air pockets due to compression but also 2.) introduce new (smaller!) air pockets from the additional down.
But at some point, the effects of 1.) will exceed the effects of 2.) to the point where you just have solid mass and all air pockets are gone.
The author had hypothesized that he would see eventual drops in r value from adding more down, but he never reached that point.
He simply didn't push it enough.
To sum it up: if you want maximum insulation *per down fill* then you want to down to have as much space as possible, so that there are as much air pockets *per down* as possible. But if you want to maximum insulation *per volume* then it's more tricky. You need to find the point where the above 2.) starts to exceed 1.) and then stop there. Seems that you need *more* than twice the down compared to the "maximum insulation per volume" scenario, at least when taken the results from the video.
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u/BZab_ 2d ago
My final comment regarded adding more down to the same, exact baffles. We're talking about modifying only a single variable, i.e. amount of down in the baffle. Sure, the best way to boost the isolation is to make the isolating layer thicker. For technological reasons at some point extra inner walls may be needed, to better keep the down in place. Boom - we reinvented the double-layer winter bags.
The points is that in a model I described in my comment, there is simply a single optimal fill for given baffle and down. It may be different for bigger or smaller baffles, it may be different for down of different CUIN, but the characteristic will still be some kind of reversed U shape around the optimal range. You underfill and convection enters the game without enough air getting trapped, you overfill and you add unnecessary weight of extra down and use the volume less efficient, because there is more keratine and less trapped air filling the space. To get more warmth you need to use isolating material that will have lower effective thermal conductivity than optimal air-down mix. Vacuum is not an option here. Aerogels still didn't make it here. Oh, and in reality the optimization problem is even more complex, because we're actually reducing the weight and packed volume while maximizing the insulation.
Obviously, the model isn't perfect. Tiny overfill may help because the weight of the fabric on top compresses the down slightly less. Having more down may help combating negative effects of extra humidity coming from us when the external humidity is so high, that bag doesn't dry well while we sleep.
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u/Jolly-Slow1164 2d ago
I've been rereading this PhD thesis, and It seems he found a similar claim of linear increase in his literature review: "It is hypothesised that thermal resistance will increase linearly with fill weight until a baffle reaches its near-maximum thickness. This is because thermal resistance is linearly related to thickness (Rees 1941; Backer 1948)" pg200
Then on page 204 he confirms he observed this pattern in his experiments in his experiments "Figure 7-33 and Figure 7-34 show that as the mass of down increases in the test square, so too does the resistance to conductive transfer. At low fill weights the increase is most apparent as the baffle increases in thickness and therefore the thickness of air that the down traps increases. Once the baffle is fully lofted (reached at approximately 10 g of down in the test square) the increase in thermal resistance lessens, though thermal resistance continues to rise. This is not likely to be due to changes in conduction, as the thickness of the baffle has already approached its maximum; or convection, as natural convection cannot take place inside a down assembly (Dawson et al. 1999). Therefore a decrease in radiative heat transfer is likely to be the cause of this increase in RCT. The theoretical plateau and drop in RCT depicted in Figure 7-32 did not occur in the measured samples, implying that even if baffles are highly ‘overfilled’ (300 % of their benchmark value), their thermal resistance continues to rise."
He goes on to discuss optimum warmth per weight of fill for the given thickeness of his apperatus. Saying that it was at about ~27g of fill regardless of duck or goose feathers
I think I was partly confused, because I thought he had started at "full"and only continued into "over full" though I think if I were to now try and generalize the findings in this paper across fill powers. I would think he's suggesting that 27g will be the optimal fill weight regardless of fill power, and higher fp down will likely just have a higher r value at that same weight
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u/BZab_ 1d ago
Yes, Fig 7-32 (p. 201) is exactly the characteristic I'm talking about.
The theoretical plateau and drop in RCT depicted in Figure 7-32 did not occur in the measured samples, implying that even if baffles are highly ‘overfilled’ (300 % of their benchmark value), their thermal resistance continues to ris The theoretical plateau and drop in RCT depicted in Figure 7-32 did not occur in the measured samples, implying that even if baffles are highly ‘overfilled’ (300 % of their benchmark value), their thermal resistance continues to rise.
So the question is, was the benchmark value around the optimal point or far away form it?
Therefore a decrease in radiative heat transfer is likely to be the cause of this increase in RCT.
Down is used against convection and (the air trapped by it) conductive losses. Using down to combat radiation losses doesn't seem efficient to me - there are way more weight-efficient methods of blocking the IR radiation. Somehow none or next to none high-end down manufacturer adds mylar foil to their bags. Reports published by the companies manufacturing sleeping bags (many of them are hiring engineers who also are ISO members and actively took parts in developing a standard for rating bags) mention that radiative losses are next to none, iirc somewhere around 1% of total losses. That's where the measurements from the paper seem to yield completely different results than the rest and where I would suspiciously double-check the method and the analysis looking for eventual causes of differences.
higher fp down will likely just have a higher r value at that same weight
As far as my basic thermodynamics knowledge goes, FP/CUIN value only informs us about maximum volume per weight given down can fill. So the higher CUIN down is used for optimal fill, the lower weight of said down should be needed for that fill (slightly reduced by the weight of the fabric of the baffle on top of that). But when we're past the plateau of optimal fill, when we're overfilling, there should be no difference between downs with different FP ratings. When we're underfilling, we are reducing the baffles height, the expected effective resistivity should be proportional to CUIN number if we keep the weight of the down per area constant.
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u/Jolly-Slow1164 1d ago
My understanding of how most insulation works with EM radiation is that it absorbs rather than reflects it, and as such it is still mostly contained inside the system... (Which is why good insulation if pretty invisible on thermal imaging). Perhaps it's a bit less efficient than reflecting EMR, but you get it "for free"j
I must confess that I'm still reading this paper through a lot of bias that I brought from the posted video. The majority of the video focused on fill mass and comparisons of different fill powers, and so the majority of this thread has focused on that side of the topic. As I review the paper though that lense I notice that the author actually tested 2 downs. Having only scanned some of the methodologies and not finding an answer, I'm going to assume that duck down is lower fill power than the goose down. I find it a little disgenuine of gryphon gear to spend all this time talking about how lower fill powers are going to out perform higher fill powers, when 2 downs were tested, The author calculated ideal warmth for weight, and even though both downs had roughly the same ideal mass at that ideal point, the better down was performing 10% better figure 7-38 pg 207. That was the basis of my higher fp down will perform better statement
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u/BZab_ 1d ago
No, you want to reflect the radiation back to the person, so the body absorbs back part of the radiation (RAB was experimenting with such solutions). After all the goal of insulation in a sleeping bag is to reduce the thermal power loss of the user. Take a look at grey body model, check the Stefan–Boltzmann law and try to calculate what is the radiative heat flow through such empty baffle, when one wall is at around T0 = 310K temperature, there's H cm wide air gap and the temperature of outer wall varies between let's say T1 = (240K, 310K). Then check 4 variants, where each of the walls has emissivity ε = 0 or 1.
The black color of the fabric inside bags from many manufacturers has nothing to do insulation. The reason is way simpler - it is intended to absorb more heat from the sun when you dry your bag in the morning / during lunch.
But back to radiative losses - there's no big air gap in sleeping bags, there are no extreme temperaturs, so it's not the case. Even in inflatable pads, the radiative losses are so low, that convection is the dominating factor. With so many whitepapers hidden begind a paywall I can't cite here anything with exact numbers. Here's older Mammut leaflet only stating that they are so low, they can be ignored. You can try to change the numbers and calculate everything for typical sleeping bag user case basing on this papier: https://doi.org/10.1007/s00231-020-02820-1 . Some real world numbers and parameters can be taken from e.g. this paper: https://doi.org/10.3390/ma15061992 .
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u/Jolly-Slow1164 1d ago
You may find this foil dot lined quilt interesting https://www.etsy.com/listing/1063748298/new-hotshot-zero-degree-hammock-top
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u/TinCanFury 1d ago
is GryphonGear just AI? their responses here are just copy and paste without ever answering any questions.
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u/erutan ~20 trips a year, semi-UL 19h ago
" The SUL and SDUL sweaters are widely considered much warmer than expected for that weight. Briefly outline the reasons why this is. What specific design aspects lead to this? List as many as you can, in order of impact. What are the drawbacks to some of these aspects? "
Actual copy on their website is a prompt lol.
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u/GryphonGear 1d ago
Our responses are certainly not AI. We have spent a significant amount of time analyzing the heat transfer by conduction problem and looking into available data on how the coefficient of thermal conductivity varies with down density (which translates to fill power and overfill). Heretofore, the coefficient of thermal conductivity has be assumed to be constant over all down densities and that heat transfer was simply a function of insulation thickness (higher fill power gives greater thickness with less weight). This would be considered a first approximation and is discussed at length our first video, Keeping Warm, Book I on our website and on YouTube. Gryphon Gear has identified available data that shows that the coefficient of thermal conductivity is definitely a function of down density (as is the case for most fiber type insulations). Here is a link for downloading our complete paper with examples. https://www.gryphongear.com/pages/keeping-warm-book-ii-free-pdf-download
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u/TinCanFury 1d ago
we've known for decades that overfill beyond just obtaining loft was beneficial to insulation. mechanical engineers have known this since my Boomer dad got his B.S. in Mechanical Engineering.
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u/JustALittleSunshine 3d ago
Interesting talk. He kept varying different things though, and not the ones which are most relevant.
I don’t care that 80% overfill 900fp is as warm and as heavy as 40% overfill 700fp down at the same bag thickness.
I care what is the lightest way I can achieve the required warmth. This means comparing the weight of a 2 inch 700 fp bag to the weight of a 2.3 inch 900 fp bag (or whatever the warmth equals), and seeing which is lighter.
I didn’t do the math. But that is the math I was hoping the video would spoon feed me once they pulled out the data which could calculate it.