On the Design of Baffles, Part I REDUX: Shapes, Performance, and Design

[Bindle]

Hi all,

This is a remake of an earlier thread, incorporating some important new information and feedback from the community. If you read the previous post, some of this information is repeated from there, but do stick around: there is some really interesting new stuff, especially in Section 3. I hope you find it useful. Again, the focus is on tubular box baffles, not karo step or anything like that.

1. Basic Shape

Most baffles are designed and cut with a shape like this in mind:
badbaffle.png
That is, that they are rectangles capped with semicircles or arbitrary circular arcs, the exact shape of which left up to the designer. This shape ends up being pretty close to reality, depending on your goals, but it is not quite right. I'll explain the shape that is a closer fit in a moment, but in order to do so I must explain to you a very specific subset of baffle shapes, and that is the shapes that come about when the baffles are fully filled with down. Or, if you prefer, you can skip right to 1c.

1a. A Fully Filled Baffle

I know what you are thinking: But wait! I always fill my baffles completely. Sometimes I even overfill! While it may be true that you filled the shape you designed completely, the baffle itself still has plenty of room left. Think about what happens when you add overfill to your baffle. To make things especially clear, imagine overfilling it by a lot, just keep piling the down in there. Would it stay the same shape? Of course not. What happens is that the baffle gets rounder: it bulges up at the center to get a taller, and pulls in the edges a little closer to compensate.

To be more specific, what happens is that the baffle expands until it encloses the largest volume possible for the amount of fabric provided. Once it reaches this shape, the baffle cannot move anymore, so the down starts compressing. That means that, in a very important sense, the shape the baffle reaches before the down starts compressing is the shape of a fully filled baffle — and any other shape is an underfilled version of this shape. This isn't necessarily a bad thing per se. There are pros and cons to underfilling the baffles like this, and to a degree is already acknowledged when talking about baffle design (it's why we don't make baffles too wide). But acknowledging this is useful when designing baffles, as we'll see below.

So what is this mysterious fully-filled baffle shape? It is what I like to call a truncated circle.

1b. The Truncated Circle

I'm sure we all remember from grade school that the shape that encloses the most area for a given perimeter is a circle. Baffles are a little more complicated, as they have a pair of straight walls on either end (provided the baffles are equally filled). Fortunately, getting from a circle to our baffle shape is pretty easy: just cut off the ends. If anyone is curious about the proof of this, ask in the comments below, but for the sake of expedience I'll just use the graphic below to explain what I mean:
bafflewh.png
Exactly how much of the end you cut off is up to you, and depends on a number of factors addressed in later sections. There are two benefits to this shape, that go hand in hand: the shape is completely dimensionally stable, meaning that your quilt is more likely to come out the size you designed; and the down is stable, meaning it won't shift at all unless you really try, and even then it will return to normal over time. Unfortunately, this shape comes with a serious disadvantage: weight. Nobody really wants a quilt with baffles thinner than they are deep. A quilt like this will typically use 3x more baffle wall material than a normal quilt! Nobody really wants that. So what is one to do? Well, there are two solutions here, the normal solution and the clever solution. But we'll get more into that in Section 2. Before we do, we need to talk about the broader category of baffle shapes:

1c. Truncated Ellipses (Normal Baffles)

Look, we don't need down shifting to be zero. We just want it to be low enough that performance isn't impacted. So, we want an underfilled baffle, just not too underfilled. So if that doesn't look like a rectangle with elliptical arcs on top, what does that look like? Easy, it looks like a truncated ellipse:
ellipsebaffle.PNG
Like the truncated circle, you just cut the ends off an ellipse — again, how much you cut off is up to you. Now, in the real world, it will never be precisely this shape; things like gravity will distort it since the underfilled down is not able to provide as much resistance. But it is as close as you can get. To be honest, this shape is pretty close to the "capped rectangle" shape people tend to use — usually it's within a few percent. But that few percent is part of the reason that quilts tend to come out with slightly different dimensions than the maker intended. Using this truncated ellipse should help with that (though we are all human, so it'll never be exact). Another minor benefit is that it uses slightly less fabric than the capped rectangle, and so will cut down on weight.

Now, one thing there isn't much point in doing with these baffles is "overfilling" them. Since they are not completely filled to begin with, all overfill accomplishes is changing the shape so that it is more circular. Now, there is nothing wrong with changing the shape like that, and it will cut down on down shifting, as you intend. But if you are going to do that, use a different shape to begin with! Keep the perimeter of your baffle shape the same, but make it slightly taller and slightly less wide overall. That way it comes out the dimensions you expected, instead of the overfill throwing off your measurements.

2. Shapes and Performance

Before I get into designing baffles for quilts, I need to clear up some points on how heat works, and how baffle shape affects performance. There are some pretty major misconceptions here, and it is important to clear them up before continuing.

2a. Major Misconception

People tend to use the amount of of down in a baffle of a given width as a proxy for how warm the baffle is. For example, take two baffles: one is a circular baffle, in a sewn-through construction. Since this baffle is a perfect circle, the width and (center-of-baffle) height are equal. For our sake, lets say that they are both equal to two inches. Now, compare that to a perfectly rectangular baffle. We'll keep the width of our rectangle the same as the circle, so 2 inches. But we'll make the height 2π inches, so that both shapes hold the same amount of down. This also means that both shapes have the same average height. Now, since the edges of the circle are so thin, those will be cold spots on the quilt. But on the other hand, the center of the circle is taller than the rectangle, so it will be warmer in the middle. Sure, the circle baffle would be less comfortable, but both would allow the same overall amount of heat to escape, right? Wrong. Heat is lazy, and will escape though the easiest route. That means that the edges of the circle allow more heat through, and makes the rectangular baffle 19% warmer overall! Before I get too much into the maths, there are two key points I want to cover.
- The more rectangular the shape, the warmer the baffle is relative to a curved baffle containing an equal amount of down. The trade-off is that rectangular baffles experience the most down shifting.
-The relationship between warmth and how rectangular or curved the baffle is is not linear, but inverse. Moving from a sewn-through baffle to really short baffle walls — even as small as 1/8 of the center-of-baffle height — has a huge impact on performance. But as that baffle wall height gets closer to the center of baffle height, there are rapidly diminishing returns.

2b. The Maths

Things get pretty dense here. If you believe me and have no interest in how it works out, feel free to skip to the next section. Otherwise, stick around for some fun! This is also a complete copy/paste from my previous thread, so skip ahead if you already read that one as well.

For those that remain: you are such a nerd! Anyway, some background on how thermal performance is measured. As consumers of quilts, usually all a manufacturer tells us is the "temperature rating", which is somewhat arbitrary. Where I most commonly encounter actual units expressing thermal resistance is home insulation, for which R-values are typically provided. This is also typical in sleeping pads. Since they are what I am most familiar with, I will use R-values throughout this post. Here is the US, R-values are expressed in (h*°F*ft2)/BTU, and are often provided per inch of material depth. The numbers themselves don't have much intuitive meaning, but they are easy to compare. Something that is R8 blocks twice as much heat as something that is R4, and if you put two R4 rectangles on top of each other you get an R8 rectangle. Using the National Institute of Standards and Technology Heat Transmission Properties of Insulating and Building Materials Database, I was able to determine that duck down has an R-value of approximately 3.93 per inch of down (note: this seems a little low to me; if anyone has better numbers I'm happy to take them).

To find the mean R-value for some assemblage of material, one needs to take the mean of the R-values weighted by there areas. But not the geometric mean, which is the kind of mean that we are used to (and would make the previously mentioned rectangular and sewn-through baffles equivalent). Since R-values represent a rate, we need to take the harmonic mean weighted by the areas instead, which is the inverse of the sum of the inverse R-values.

While the harmonic mean works great for discrete shapes, our baffles are curved. That isn't a problem though, all we need to do is take sum up the R-values from arbitrarily small slivers from under the curve to find an approximate area... wait a second, that's the definition of the antiderivative! That's right baby, it's calculus time, and we're breaking out the integral! I bet this isn't where you were expecting this to go. Don't run away yet though: I know not everyone is as excited about math as I am, but the good news is that we get a pretty simple equation at the end.

Since we are dealing with circles, the maths are actually pretty easy. The equation for a semicircle in the plane centered at 0,0 is:
CodeCogsEqn.gif
For our baffles, we can determine the R-value by multiplying that equation by our R-value per inch value (3.93), doubling it (as we have a full circle instead of a semicircle), inverting it, taking the antiderivative over the length of our baffle, dividing by the length of the baffle, and then inverting again. Using our w and h values, we get the integral:
CodeCogsEqn(1).gif
Thankfully, that works out to be equivalent to the much simpler
CodeCogsEqn(3).gif
Now we can use this equation we can use to determine the performance of our baffles. Plugging in w=h=2 for our sewn through baffle, we get that the R value is almost exactly 5, compared to the rectangle which would have an R value of 3.93*π/2, or about 6.2. That's where we get the 19% difference in performance! Of course reality would never work out quite so precisely, as even the fabric has enough of an R-value to cut that percentage to 15% or so. Even considering that, that is enough of a difference that a blanket with smaller box baffles can match the performance using less fabric and down, and thus being lighter than the stitch-through baffle (still more time consuming though).

3. Designing Baffles

Now that we covered the basic shape of baffles, I can talk about how to design them for use in your quilt. I'll start with normal baffle design, before getting into my clever solution I'm sure you are all waiting for.

3a. Normal Baffles

We talked before about wanting to keep down shifting to reasonable levels, if not exactly zero. That means we don't want our shape to be too much wider than it is tall, because the wider it is, the more shifting there will be. Of course, you don't want it to be too narrow either, because then your quilt will be too heavy. This is common knowledge, and we can rely on common knowledge too for a range of height to width ratios that provide acceptable levels of down shifting that minimize weight. In general, people seem to like baffles about 2x wider than they are tall, sometimes as much 2.5x or 3x. Now, this is the height to width ratio of the final baffle — since your baffle comes from cutting the ends off an ellipse, the ellipse itself must be wider. Exactly how much wider is a delicate balance: if you make it too wide, then the baffle becomes more rectangular and has better performance, but also is unstable and allows more down shifting; conversely, narrower ellipses have worse performance.

I'd like to say there is a right answer here, but there is not. We can measure the amount of down shifting by the percentage underfilled the shape is relative to the truncated circle with the same perimeter (there is a little trigonometry, but its not that hard). And we can measure how warm the baffle is. But exactly how warm it needs to be, how much down shifting is acceptable, and how light the quilt must be will vary by person. Most people should just follow the rules of thumb that people generally follow for baffle wall heights and baffle widths and whatnot, just with a slightly different shape. Now, if you use those rules of thumb you can fix some variables, and then you can do some very mathematical optimization if you want (see Part II), but you need to make those somewhat arbitrary choices first. This is not the case in my clever solution though.

And again, as a quick reminder, there is not much point in overfill when using these shapes.

3b. Clever Baffles

Alright, you stuck with the post so far, so here is the really game changing stuff. A quick reminder though: this is still just theory, and I hope to build a demonstration quilt using the baffle outlined here later this month.

Let us go back to our truncated circle shape. The problem with that shape is that the baffle walls weigh too much. But what if there was a way to reduce the baffle wall weight? A way that you cannot use on underfilled shapes? Lucky for us, there is. Let us look at the functions of a baffle wall. In general, it serves two functions:

-To hold the shell material in place, so that you... have baffles.
-To prevent the down from shifting between the baffles.

Here is the key insight: with a truncated circle shape, down does not want to shift between the baffles. That entire second function is moot. But why is that, and how does that help us?

First, the why. With a typical, underfilled baffle, down shifting is a fact of life. Since the shape isn't completely filled, the down offers no resistance. Imagine if you have two baffles, side by side, both about 80% full relative to a truncated circle. Also imagine that the baffle wall material, whatever it is, allows down to pass through it easily. Now, if you grab the baffle from the side and give it a shake, it is easy enough to make it so that one baffle that is 100% full, while the other is 60% full. With time and effort, you can shift the down back so that they are both approximately equal again, though without a scale you'll never get it exactly right. Even if the spread becomes 75% in one and 85% in the other, that is a pretty major difference in both shape and performance, and entirely likely to happen when in use. This tells us it is important that elliptical baffles need baffle walls that not allow down to pass through.

On the other hand, if both baffles are at 100%, they want to stay that way. It is possible to force one baffle to become overfilled at the expense of the other, but that would put the overfilled baffle under pressure. The down would want to shift back so that it is even, and some random jostling of the quilt will allow that to happen. To increase this tendency, you can add some down so both baffles are overfilled. This fact allows the use of down-permeable materials for truncated circle baffles. This is a big deal, as it allows us to save a significant amount of weight.

One thing we can do is take our regular baffle material, like noseeum netting, and cut some pretty big holes in it. How big, and what kind of holes? The exact limits are anyone's guess, but I think it will be very easy to make them 4x lighter, or even more. Imagine taking the baffle wall material, and punching a bunch of circles in it, about as tall as the baffle wall itself is, like so:
perfbafflewall.PNG
This already reduces the material weight by about a factor of 4, and that is significant. Remember when I said that these truncated circle shapes use about 3x as much baffle wall material as traditional baffles? This implies that using a truncated circle baffle with this kind of baffle wall would result in a lighter quilt than the regular baffle equivalents, while being more stable and allowing less down shifting! That is really cool! It will take some experimenting to see exactly how much we can cut from the baffle wall while still holding the shell firmly in place, but this is a really promising area for research.

Another neat fact about this baffle: unlike the elliptical baffle, which has a few "rule of thumb" aspects, this baffle is easy to optimize! The only factor you need to choose is the center-of-baffle height, and the rest follows from your material choices! For more information, see Part II

Of course, theory is one thing. Reality is another. Like I said, I'll be constructing a quilt using this theory later this month. I'll let you all know how it goes.

4. Conclusion

Thanks for reading my thoughts on baffle shape. I hope you all view it as as significant improvement on my first attempt. Again, see Part II for more on baffle optimization. And before you ask about differential baffles, that will be the subject of Part III, which will be posted in a week or so. Here is a quick teaser on shape:
diffbaffle.PNG

[Cruiser51]

This is an interesting read, but I still have a few concerns over theory and reality

3A - "And again, as a quick reminder, do not use overfill when using these shapes. "

If everything behaves like we predict and materials do exactly what they are supposed to do, this is a reasonable statement, however, if the myriad of discussions on down performance versus ratings have shown anything, it is that rated values rarely hold up in application. Also, because we design a baffle it doesn't mean we will be able to actually build it to spec .... material moves, sewing can suck, people have different skill levels, baffles can end up larger, smaller or both in the same baffle.

Overfill is pretty much something that is part of the DIY landscape, and I don't see anything here that changes that.


3B - "Let us go back to our truncated circle shape. The problem with that shape is that the baffle walls weigh too much"

I took a look at the design parameters of the quilt I am going to assemble this month, total weight of the quilt is ~15 oz (using 1.1q/yd2, shell material), for a -1.5C quilt .... using .66 oz/yd2 NoSeeUm mesh will account for much less than an ounce(~.75 oz), ~4% of the quilt weight .... what is this statement based on?

I do admit the idea that the down won't easily transfer between baffles that are appropriately filled, seems plausible ... but filling said baffles and getting it all balanced and even, will be a chore, plus keeping it that way in actual use could be a nightmare. If too much of the baffle wall is cut out it will impact the structures ability to maintain any semblance of shape, simply because of the flexibility and stretch properties of the fabric, and cutting away fabric will only make the material more flexible and stretchy.

Although, I like the thought process, i don't think baffle weight is an area that presents opportunities for any meaningful weight savings. I didn't run the areas to check the idea of 3X less baffle weight, but I suspect it doesn't allow for a reasonable sewing allowance at the edges and mesh is not a wonder material, it will tear easily of you make the cutouts as depicted, the center portions are just too narrow. Given it's weight contribution to a typical project, I think the full baffle is likely a better choice.

Brian

[Just Bill]

In the context of real world examples- how familiar are you with a Karo step baffle design?

That is on the extreme end of reduced baffle weight design... though I appreciate the thought you have here.
One of the drawbacks is the manual labor required at setup to redistribute the down. I do see the improvement your method could offer though.

My concern would be more of a practical one similar to Brian. Though doable....removing large clean holes from your baffle material would be a pain.
I would imagine you'd need to layout your baffle material and use a punch of some sort that would heat seal. I'm not sure if mesh would hold up, so likely that leaves you with fabric (and thus the heat seal).

A sharpened cookie cutter perhaps on a hot plate... but still not much fun. Realistically if you're really going for it... you'd want to use cuben.

Which can bring us back to .33 ounce baffle material being a possibility... which is already half the weight of .67 material.

Another rarely used but once discussed technique is some form of 'skip baffling'.
In that method you design the quilt as normal, but you basically do what you're suggesting by sewing in half the baffle material. 3" baffle, 3" skip and repeat.

This tends to require less work than a karo step for the user in the field and would back up your reduced shift ideas.
Your punched baffle method would be an improvement on skip baffling though as the circles would retain a more even baffle profile.

Skip baffling basically looks like a big agnes q-core sleeping pad- https://www.bigagnes.com/Q-Core-SLX

As Brian mentioned... seam allowance adds up.
In SUL materials you need to stick with at least 1/4" to reduce tearout.
So an 80" long baffle with 1/4" per side is (80*.25*2)40 sqin. If you go 50" wide quilt with 11 walls (50/5" per baffle plus one end) that's 440 SQ inches.
440/1296= .34 yards of material in seam allowances. That said, unless I miss the mark it seem you'd go with a tighter baffle spacing than a typical 2"x5" pattern... so the seam allowance factor would become more relevant as baffle count goes up.

I won't call them out... but not long ago there was a thread from a customer who washed their quilt and had huge voids in the baffles afterwards.
That is because of an open end baffle system that allowed the wet down to redistribute poorly. The fix is lots of manual manipulation to clean that mess up.
Again- not insurmountable or super relevant I suppose. But one issue with many of these reduced baffle systems is a high rate of user error. Even as the designer if you find yourself playing with the down at camp each day it can get frustrating fast. Careful packing can reduce some of the frustration... but I've never been a fan of manually manipulating down in the field.

All that said- nothing wrong with taking a very deep dive to see what the limits are.
As I am fond of saying... you have to find the edge and define it first. Then you can take a step or two away from it.
Beyond me punching holes in the physical practicalities of your punched hole baffle theory... I do like the idea and see how it is a potential improvement over Karo, skip baffle, or other methods.

[Bindle]

Thanks again for taking the time to reply Brian, it really helps me iron out my ideas and make sure I'm conveying things clearly.

This is an interesting read, but I still have a few concerns over theory and reality

3A - "And again, as a quick reminder, do not use overfill when using these shapes. "

If everything behaves like we predict and materials do exactly what they are supposed to do, this is a reasonable statement, however, if the myriad of discussions on down performance versus ratings have shown anything, it is that rated values rarely hold up in application. Also, because we design a baffle it doesn't mean we will be able to actually build it to spec .... material moves, sewing can suck, people have different skill levels, baffles can end up larger, smaller or both in the same baffle.

Overfill is pretty much something that is part of the DIY landscape, and I don't see anything here that changes that.

It's true that, for small amounts of overfill, the effect is well within the margin of error for quilt making. Overfilling by 5% won't shift things around enough to care. At around 10% is where things start to get noticeable, and beyond that the dimensions will be quite different for most baffle shapes. I think adjusting the baffle design from the beginning to achieve the desired affects is the superior approach, and would result in quilts closer to their specs (as I outline at the end of 1c). I do believe you are right that "never" is too strong of a term though: what I meant to convey is that there is not much point, compared to the alternative. I'll modify the post to reflect that.

3B - "Let us go back to our truncated circle shape. The problem with that shape is that the baffle walls weigh too much"

I took a look at the design parameters of the quilt I am going to assemble this month, total weight of the quilt is ~15 oz (using 1.1q/yd2, shell material), for a -1.5C quilt .... using .66 oz/yd2 NoSeeUm mesh will account for much less than an ounce(~.75 oz), ~4% of the quilt weight .... what is this statement based on?

"Weighs too much" is meant to be relative to a normal baffle. For the example of your quilt, using a truncated circle shape with solid baffle walls would result in about 1.5 oz of extra material use (assuming your baffle width to height ratio is between 2:1 and 2.5:1). It's not a huge amount of the total quilt weight, but it is notable, and hard to justify IMO. In contrast, switching to a perforated baffle would put it in the same neighborhood as your baffle construction. Any weight savings would be basically nothing, but the increased performance is the real benefit.

I do admit the idea that the down won't easily transfer between baffles that are appropriately filled, seems plausible ... but filling said baffles and getting it all balanced and even, will be a chore, plus keeping it that way in actual use could be a nightmare.

Theoretically, my baffles actually make the filling process easier, and keeping it even shouldn't be too hard either. To get a little technical, the baffles want to have equal amounts of down, it is the "ground state" of the system: uneven distribution results in more energy being stored via pressure in the quilt. The quilt wants to be in the ground state, so it shouldn't be hard to get it there.

This means, to fill the quilt, you really only have to measure the total amount of down, and get the distribution approximately even, and sew the baffle shut. Then, to even it out further, take a tennis racket or paddle or similar item, and beat the quilt like a rug for a couple minutes. That should cause the down to distribute itself evenly between the baffles. It won't be perfect of course, but it should be as close as anyone normally gets when filling baffles. And if at any time you sense the baffles are uneven, just beat the quilt into submission and it should be fine again. At least, that's the theory... we'll see how it works in practice.

If too much of the baffle wall is cut out it will impact the structures ability to maintain any semblance of shape, simply because of the flexibility and stretch properties of the fabric, and cutting away fabric will only make the material more flexible and stretchy.

This is actually my biggest concern. It will take some experimenting to figure out what exactly the tolerances of the baffle material are before distortion becomes a major concern. I have extra a lot baffle wall material for just this purpose, and I will be cutting it a few different ways and using clamps to test how the holes affect these properties.

Although, I like the thought process, i don't think baffle weight is an area that presents opportunities for any meaningful weight savings. I didn't run the areas to check the idea of 3X less baffle weight, but I suspect it doesn't allow for a reasonable sewing allowance at the edges and mesh is not a wonder material, it will tear easily of you make the cutouts as depicted, the center portions are just too narrow. Given it's weight contribution to a typical project, I think the full baffle is likely a better choice.

The 4x is a *very* rough approximation as is the 3x before that. Even the circle is probably not the right shape choice — caternary curves are a better direction to look at. I just wanted a quick, easy example. For the 4x just took the area of a circle relative to a square (~78%) and rounded it down. It will definitely change in practice.

I think you are missing one of my points though. The benefit of this baffle design isn't weight savings relative to a normal baffle. It is better down distribution and performance. My target is just to reduce the baffle weight by ~3x in order to keep the weight equal to a regular baffle, anything beyond that is just a bonus.

---

Bill, you posted while I wast drafting this reply. Some of your points are addressed here, but I will have a reply for you shortly.

[Just Bill]

Some of the reason I mentioned in the other thread why a Primaloft Gold quilt may be more efficient than a down quilt at a low loft/lower temp application has to do with the hard realities of baffles.

Primaloft Gold has a CLO value of .92. Apex is .82 CLO. A not insignificant difference in low weight designs.
In my experience PLG also retains that value longer and is more uniform in density. Being continuous filament insulation it is much easier to 'crack' Apex in use and I have always found a 5* or so ding in performance after limited use. The PLG packs, feels, and behaves much more like down... the short staple fill design is the primary reason.

As you go into lower loft down you really should have a narrower baffle width to reduce shift.
I believe you are finding something similar in your efforts.

Somewhere around 45* or so is where you 'tip the scales' between a highly efficient synthetic and down construction simply because of shell weight. Most simply say... you need a shell on each design so what is the difference. Of course the answer is there is no difference in the outer shell you see, but in the baffle material itself that you don't see. In the 11 baffle wall design above at 80" by 1.25" loft (80*1.25*11)=1100 sq inches. Or .85 yards.

Add to that our seam allowance of .34 yards and we are at 1.19 yards of baffle material not needed in a synthetic.

And as I believe we'd agree... that's going to result in a listless quilt at that size and a more properly designed shell would have double the chambers at roughly 1.25"x 2.5 wide" maximum. So you are up to over 2 yards of baffle material alone.


There is also the issue of cluster size that few examine.
My layman's example is a landscape bed with black weed blocker fabric laid down.
If you take pea gravel... you can spread a fairly thin layer and cover it with little or no of the fabric showing through.
If you take 1" stone and lay it one inch thick... you will see a large portion of the landscape fabric poking through.

The point being... down clusters of high fill power can be as large as a golf ball. While there is some overlap... if we are talking ideal distribution with no waste... we have the landscape bed problem at low loft. With the rock representing the down and the weed blocker fabric poking through representing un-insulated portions of the quilt.

You could likely more accurately do the math than I could...but the gist of it is simple. You need a second layer of material to fill in the gaps.
The second layer of spheres would not double the depth, but give you roughly 150% more depth and material to 'fill the holes'.

Point being... my rule of thumb is never to design a piece of down gear lower than 1.5" of loft. (roughly 40*)
Ideally I don't bother with anything less than 2" of loft if chasing efficiency (roughly 30*).

From there (increasing loft as you go) the down piece becomes more and more efficient from a shell to fill ratio standpoint, as well as from a simple material properties standpoint. The lower loft down is the most prone to cold spots and shifting both due to design flaws (baffle spacing too far apart) as well as the landscape bed problem. (cluster size). One little bump of the hand, bulge of the shoulder, or flick of the quilt and you skitter the clusters of down just as you would displace the rocks in the landscape bed.

That said... this is less of an issue in warmer temps. A loss of 5-20% of your coverage isn't the end of the world as the temp difference isn't large enough. If your sleeping body is roughly 70* and it's only 50*... these 'gaps' in coverage are not as big a deal. It's the simple reason a sewn through down piece of gear still works fine. Some parts of you have zero insulation, other parts have 200% of what you need and it washes out.

As it gets colder... say around freezing... this sort of gap coverage is no longer an option.
If your as little as 5% of your body is cold you will wake up. Cold Butt Syndrome is the practical proof of this.
A chilled hip and shoulder will awaken a side sleeping ground dweller who is otherwise warm as well.

Which brings us back to your efforts....
If one were to try to design a highly efficient quilt I would limit your effort to a 2-3" loft target.
2.5" is generally agreed to be roughly 20* and the gold standard of three season gear.

It's also about that spot where the quilt weight is more of a concern, and minor savings present at very low lofts start to pay off in something that results in double digit gram results that actually reach into the gold standard of ounces. You shave 10% off a 13 ounce quilt and you'll get a shrug... you shave the same 10% off a 26 ounce quilt and folks get more interested.

It's also about the spot that pack size is an issue... less shell means less volume as shell material does not pack as cleanly or efficiently as down.

Further it's about the spot that baffle depth and baffle width fall into a better range to support your truncated circle ideas as the baffle spacing falls more closely in line at that point with others.... so your design is not 'suffering' from requiring more baffle walls than the competition.

The final reason to limit yourself to that range is thermal calculations. I'm no expert in them, but I understand enough to know that at roughly zero degrees the CLO formulas break down. As do others. By limiting yourself to a 20-30* F range you avoid encountering this issue. You wanna dive in and crack the code on why these formulas break... be my guest. From my practical minded point of view there isn't much need to design HVAC systems for zero degrees... so there isn't any real need for those formulas to work well at those temps.

A highly efficient 2.5" quilt is a worthy goal.
As would be a highly efficient 3" Under-quilt to match it. ($)
Both of those items would be about the most favorable to your ideas in terms of real world payoff.

($)Not many officially agree that a top quilt and underquilt should have a separate loft chart.
But unofficially most everyone vendor comes to that conclusion... something to consider for you as well.

[Bindle]

In the context of real world examples- how familiar are you with a Karo step baffle design?

That is on the extreme end of reduced baffle weight design... though I appreciate the thought you have here.

Pretty minimally. I know what it is in theory, and seeing how it was set up is what gave me confidence that I was on the right track. I've been too intimidated to do a deep dive on it though: the reason the design works is because of how a square of fabric resists being distorted into a sphere, and that seems really hard to model. Because of that, my plan of attack was to optimize box baffles as much as I could and see how close I can get to an equivalent Karo step design. I think I am going to fall short of beating it on a warmth-to-weight basis, but we'll see how the final product turns out.

One of the drawbacks is the manual labor required at setup to redistribute the down. I do see the improvement your method could offer though.

As I mentioned in my reply to Brian, this part should actually be easier, at least if I am right. You'd just have to beat the quilt.

My concern would be more of a practical one similar to Brian. Though doable....removing large clean holes from your baffle material would be a pain.
I would imagine you'd need to layout your baffle material and use a punch of some sort that would heat seal. I'm not sure if mesh would hold up, so likely that leaves you with fabric (and thus the heat seal).

A sharpened cookie cutter perhaps on a hot plate... but still not much fun. Realistically if you're really going for it... you'd want to use cuben.

Which can bring us back to .33 ounce baffle material being a possibility... which is already half the weight of .67 material.

Yeah, this is kinda where the theory starts butting into reality. I came to the same conclusion on punches being the best way — lucky for me, I married a metalsmith! It will really be interesting to see how different materials hold up to different hole shapes and sizes, and I bought extra of the 0.67 fabric I'm using for the inner shell just in case the mesh doesn't hold up. Cuben is always so tantalizing... but like I said, I'm married

Another rarely used but once discussed technique is some form of 'skip baffling'.
In that method you design the quilt as normal, but you basically do what you're suggesting by sewing in half the baffle material. 3" baffle, 3" skip and repeat.

This tends to require less work than a karo step for the user in the field and would back up your reduced shift ideas.
Your punched baffle method would be an improvement on skip baffling though as the circles would retain a more even baffle profile.

Skip baffling basically looks like a big agnes q-core sleeping pad- https://www.bigagnes.com/Q-Core-SLX

Skip baffling is a method I considered, though as you mention the perforated baffle should retain more profile. Skip baffling works really well for air-filled mats and the like though, since there is no harm in needing to add more air to maintain a shape. On the real extreme end of skip baffling, you have "tufted" baffles as a possibility, but it would take more down and runs into the modeling issues I mentioned earlier. I bet it would really work well for synthetic insulation though.

As Brian mentioned... seam allowance adds up.
In SUL materials you need to stick with at least 1/4" to reduce tearout.
So an 80" long baffle with 1/4" per side is (80*.25*2)40 sqin. If you go 50" wide quilt with 11 walls (50/5" per baffle plus one end) that's 440 SQ inches.
440/1296= .34 yards of material in seam allowances. That said, unless I miss the mark it seem you'd go with a tighter baffle spacing than a typical 2"x5" pattern... so the seam allowance factor would become more relevant as baffle count goes up.

You are absolutely right, those seam allowances do add up. This is especially for thinner quilts, where seam allowances are a greater portion of the baffle wall material. I'm hoping that I can cut enough material so that my quilt and a more standard design have about the same baffle wall weight, and I think that is doable. We'll have to see how it works out though.

I won't call them out... but not long ago there was a thread from a customer who washed their quilt and had huge voids in the baffles afterwards.
That is because of an open end baffle system that allowed the wet down to redistribute poorly. The fix is lots of manual manipulation to clean that mess up.
Again- not insurmountable or super relevant I suppose. But one issue with many of these reduced baffle systems is a high rate of user error. Even as the designer if you find yourself playing with the down at camp each day it can get frustrating fast. Careful packing can reduce some of the frustration... but I've never been a fan of manually manipulating down in the field.

As I mentioned before, I think redistributing the down is easier than normal. There is an important caveat that your story reminded me of though, that I forgot to mention: this is only true so long as baffles are at 100% fill or more. This means that, if the down loses too much fill power over time, my quilt can start becoming uneven. This makes overfill a little more important than usual.

All that said- nothing wrong with taking a very deep dive to see what the limits are.
As I am fond of saying... you have to find the edge and define it first. Then you can take a step or two away from it.
Beyond me punching holes in the physical practicalities of your punched hole baffle theory... I do like the idea and see how it is a potential improvement over Karo, skip baffle, or other methods.

Thanks for taking the time to read and critique my post. I'm excited to see if my idea ends up working out, and I hope to have some preliminary results on the baffle cutting stuff that I can post this weekend.

---

I see you managed to get another post in before I finished my reply! Thanks for all the thought you are putting into these. Hopefully I'll get a chance to read it and respond before the end of the day.

[Just Bill]

Well I doubt I'd marry Kyle for his laser... his butt is just too skinny. But perhaps I can get enough IPA together to pay off Carter....
https://ripstopbytheroll.com/pages/b...ntract-cutting

Ironically the waste from the circular patches they cut for samples and reinforcement might make DCF baffles with clean holes much more affordable.

I think you miss my point on the manual down distribution issue.
Agree- no big deal to reach for the tennis racket at home... that's a handy trick to bust clumps during assembly.
What is much less fun is packing said racket... or sitting down after a long day of hiking and deftly massaging and redistributing feathers. If a quick shake, 20 minutes to loft up and one more shake before bed doesn't do it then the design is a fail in my book. That's been an ongoing problem for all minimal baffle designs once they go from workbench to field test. As you note... the problem gets worse if humidity creep reduces you from 105% fill fresh from the dryer to 95% filled after a few days in the woods.

On the Karo step- https://enlightenedequipment.com/revolt-stock/

Here's the cliff notes...
Tim (owner of EE) was one of the bigger and earlly developers of this idea.
It's mostly a good idea as it's a compromise between some form of dot baffling (literally using small points of baffles) and full baffles.

It is a decent idea bordering a good idea for Top quilts, especially in a hammock.
If you are a violent sleeper it is very prone to shifting though.

It is a bad idea bordering on a horrible one for Under quilts.

As much as I like Tim and Enlightened Equipment and recommend their gear; his karo step UQ is the only UQ made by a respected vendor I would not recommend.
In real life it simply doesn't work as planned. Gravity wins the fight over the fill power and the down falls.
It might be fixed with a pair of lengthwise baffles along each long edge... with a karo pattern only in the center 3/4 of the quilt... but that's debatable.
You may be encouraged to debate and or suggest that Tim simply didn't put enough fill in... and you might be right.

However if the name of the game remains efficiency; then we must try to do the most we can with the down as well... not simply reduce shell weight.
Or said another way... if you trim out one ounce of baffle, but need to stuff in 2 ounces of down to compensate you are at a net loss.

There were many folks at BPL who didn't mind waking up in the middle of the night to pat the down back up from the edge of the karo step and into the center of the quilt... but I'm not one.
Simple breathing, let alone any tossing and turning can be enough that your down can settle off you during the night.
This point also goes to the 'field manipulation' problem. If your quilt needs constant maintenance to function... how big a deal is that to you?
If you're willing to physically move the down each night and day... you can get away with less baffling.
I don't mean to tell you how to do it though... as I understand for some folks they deeply enjoyed 'tuning' their quilt each night. Seriously.

But as I once pointed out to an extreme gram weenie on the subject...
If you shave 1 ounce from your sleep system... yet burn more calories each night to stay warm...
How many ounces of food are you down to cover that loss in the bargain?

Insulation (and down specifically) remains one of the most weight efficient systems one can carry.
It weighs less than hot drinks, food, or spare clothing by far.
Good sleep will net you more miles and efficiency as well.

I like where you're going overall... just a reminder that down tends not to 'stand up for itself' in real life as well as we hope.
So I would suggest sticking with a 5-10% overfill above max fill regardless... it's still relatively light.

One interesting question that may then present itself (and has in real life)....
Let's say you designed a hyper efficient quilt at say 2" loft... but had to add a little more down than ideal to do it... which erodes your 'gains'.

If you compared that to a poor efficiency 2.5" quilt... they may be the same warmth in real life.
So if your 30* (by loft) is as warm as a poorly built 20* (by loft) then you can make a more favorable comparison on your weight efficiencies.
Brandon from Warbonnet argued for rating by fill weight... and implied that he could stuff a 30* shell with 20*'s worth of fill and achieve the 20* rating (paraphrasing).
I don't think that's exactly right... but it's not exactly wrong either. You may find something useful in that idea as well.

[Cruiser51]

When I do sewing allowance, I usually make sure there is a solid amount, say 1/2" to 3/4", I don't want the baffles tearing loose because I scrimped on a seam ... if you intend on beating the quilt, I think that will be even more important. Also, as an aside, for the previous post about baffle material weight, in that example the total weight of baffle material was .7 oz, not sure where you got the larger numbers, but baffle material just doesn't weigh much, in the context of the other build materials weight. I like the full baffles simply because (as Bill pointed out) ...when I set stuff up, I really don't want to play around in the field or get up in the night to shift stuff .... the full baffles get a look over and maybe a pat or 2 and they good to go.

I don't use .67 fabric in UQs, I just feel that it is a bit too light for "external to the hammock" use, I will typically bump up to the 1.1 oz for external use and use the .67 oz for internal use. Could you do it? .... sure, but is the weight really worth the risk (obviously, I have decided in favour of a bit heavier).

Lastly, it seems that the issue of the down actually behaving as it is "rated" hasn't really been given the importance that is required for a solid design IMO. You buy 850 rated down(I suspect we all buy the best we can get/afford), that rating is for the lab and very specific test conditions, conditions that aren't likely to occur outside the lab, they have been tuned to give the highest values, not a practical number for everyday use. I also suspect with the complexity of the final "in use" shapes (the way the quilt will hang or shape to a body), some error in the determination of the volume (face it, cloth will attempt to approach a circle/spherical shape and the model will likely fall short ... any model), will all combine to significantly under estimate the amount of down required. After some trial and error, I settled on adding ~30% to the design down weight and that has been working out quite well for my quilts. I do recall a thread that involved one of the cottage vendors (whose experience I greatly respect.. all of them) saying that was also the level of overfill they were using.

Bills comments on the stability of the temp equations and suggestion (paraphrasing) that staying in a practical range of loft, say 1.5 - 3.0 inches would go a long way to bound the edges of the subject .... where above or below would require additional or different treatment .... is likely quite sage and helps put things in perspective.


Brian

[Bindle]

Hi all,

Sorry for the late response, I had a busy week. I'll cover responses in a moment, but the good news is that I did get some (preliminary) tests done on how the holes affect the strength of the baffle wall material.

I did my test on on worst case scenario material: I got a 50 yd spool of 6 inch wide tool for $1.50 from Walmart. Real garbage stuff, and the grid spacing was pretty wide. It was still quite light though (How light? Who knows!). I cut it into 3 inch strips and haphazardly cut circles out of the material with scissors. I then taped the tool to paint mixing sticks to I could evenly distribute the tension through the system.

The good news is that strength is not much of an issue. Even with strips less than 1/4 inch at the narrowest point, the tool was never at risk of breaking. The bad news is distortion did occur, and it was permanent. Distortion varied in proportion to how narrow the spacing was. It maxed out up to 100% of the height of the material, depending on how close the circles were. It took some force to get there, more than the baffle walls would normally face, but the permanent damage makes it pretty risky. However, these tests were done with the grid of the tool at a 45° angle, i.e. a diamond shape. Distortion came from this diamond collapsing. I suspect that re-orienting the grid to be squares will prevent the material from distorting, though it may reduce the strength and force distribution. I'll do more experiments and let you all know how it goes.

---

Bill, I can't say there is anything I disagree on in your first post. This sort of stuff definitely becomes more worthwhile with thicker baffles. I making as thick a quilt as I can, but I am a little constrained by budget. The best deal I could get on down was by the pound, and so I have exactly 1 pound of 750 fill down, and I intend to use it all. I'm making a top quilt and a slightly-less-than 3/4 length under quilt, so that ends up being ~2 inch baffle. Not ideal, but sufficient. However, I am not familiar with the concept of CLO values breaking down around 0°F, and I would be interested to see more info on that.

As for your second post, you raise some interesting points. The Karo step stuff is definitely interesting. It seems like it has some of the problems I feared, meaning that it is hard to determine the right amount of down (or even that the right amount of down is more than typical). You can always add more down to a quilt to eliminate shifting issues. Hell, throw enough down into that Karo step underquilt, and you can turn it into a mattress. The question is if it is worthwhile to do so, compared to other options. I can do out the math for a hypothetical answer for regular box baffles, but I wouldn't know where to start for karo step, and the unique design means it could require "overfill" just to work as expected. I am worried Karo step requires the fabric to be under a higher tension than box baffles in order to work right, which requires more fill and could easily cancel out its weight advantages.

As for the debate between loft vs. overfill, I have done out the work on that. The major resource I used to determine how it works is a post by Richard Nisley on the backpacking light forum. Just reading post, you may think that Brandon is right, adding more fill just increases the insulation value at a linear rate. That is not the full story though.

Take a look at that last graph. CLO units work just like R-values, so increasing the loft 50% would increase the CLO value by 50% (or more, depending on baffle shape). However, the graph shows adding 50% more down only increased the CLO value by about 16%, about a third of what we expected. Likewise, the previous graph shows a doubling in down density, which we would expect to cut the k value in half (k is inversely proportional to R-values). Instead, it is cut by a little less than a quarter. These two graphs suggest that each ounce of added overfill down is about 1/3 as effective compared to an extra ounce of down at full loft. In other words, a 30°F quilt overfilled with with 20°F quilt's worth of down is a 26.6°f quilt — provided that the 30°F quilt was full to begin with. As I have been stressing with these posts, overfilling an oval baffle design does not increase the down density, it only increases loft (for normal amounts of overfill). That means that adding 10% overfill to a typical quilt will increase the loft, and the R value, by ~10%. So it is possible a 30°F quilt with 20°F quilt's worth of down is a 20°f quilt, depending on how much extra room was in those baffles.

---

Brian,

The truncated circle baffles are ~3x narrower than typical baffles, so building a quilt with them will ~3x the baffle wall material. I took your 0.75 oz figure and multiplied by 3 to get 2.25 oz, which is 1.5 oz more than your quilt. That increase in baffle wall material use for the truncated circles is why it is important to cut the material weight if I can. And if it works as I think it does, down massaging won't be necessary.

For my quilts, I decided to go with a 0.67 taffeta inner shell and a 1.1 ripstop outer shell. I used the thinner material not out of serious weight concerns, but because it tends to be softer. This strategy is a bit riskier than 1.1 all around, but seems pretty common, so I'm not too worried.

30% is a hearty amount, though I think that it makes sense for a typical baffle design depending on the parameters. I am curious what the width and height of your baffles ends up being in practice. I am aiming for 5-10% overfill on my quilt, but again, the truncated circle baffle is the same as an oval baffle at "maximum overfill", so the 5-10% is quite a lot. I agree that the 1.5-3.0 inch range is good advice

[Just Bill]

Hi all,

Sorry for the late response, I had a busy week. I'll cover responses in a moment, but the good news is that I did get some (preliminary) tests done on how the holes affect the strength of the baffle wall material.

I did my test on on worst case scenario material: I got a 50 yd spool of 6 inch wide tool for $1.50 from Walmart. Real garbage stuff, and the grid spacing was pretty wide. It was still quite light though (How light? Who knows!). I cut it into 3 inch strips and haphazardly cut circles out of the material with scissors. I then taped the tool to paint mixing sticks to I could evenly distribute the tension through the system.

The good news is that strength is not much of an issue. Even with strips less than 1/4 inch at the narrowest point, the tool was never at risk of breaking. The bad news is distortion did occur, and it was permanent. Distortion varied in proportion to how narrow the spacing was. It maxed out up to 100% of the height of the material, depending on how close the circles were. It took some force to get there, more than the baffle walls would normally face, but the permanent damage makes it pretty risky. However, these tests were done with the grid of the tool at a 45° angle, i.e. a diamond shape. Distortion came from this diamond collapsing. I suspect that re-orienting the grid to be squares will prevent the material from distorting, though it may reduce the strength and force distribution. I'll do more experiments and let you all know how it goes.

---

Bill, I can't say there is anything I disagree on in your first post. This sort of stuff definitely becomes more worthwhile with thicker baffles. I making as thick a quilt as I can, but I am a little constrained by budget. The best deal I could get on down was by the pound, and so I have exactly 1 pound of 750 fill down, and I intend to use it all. I'm making a top quilt and a slightly-less-than 3/4 length under quilt, so that ends up being ~2 inch baffle. Not ideal, but sufficient. However, I am not familiar with the concept of CLO values breaking down around 0°F, and I would be interested to see more info on that.

As for your second post, you raise some interesting points. The Karo step stuff is definitely interesting. It seems like it has some of the problems I feared, meaning that it is hard to determine the right amount of down (or even that the right amount of down is more than typical). You can always add more down to a quilt to eliminate shifting issues. Hell, throw enough down into that Karo step underquilt, and you can turn it into a mattress. The question is if it is worthwhile to do so, compared to other options. I can do out the math for a hypothetical answer for regular box baffles, but I wouldn't know where to start for karo step, and the unique design means it could require "overfill" just to work as expected. I am worried Karo step requires the fabric to be under a higher tension than box baffles in order to work right, which requires more fill and could easily cancel out its weight advantages.

As for the debate between loft vs. overfill, I have done out the work on that. The major resource I used to determine how it works is a post by Richard Nisley on the backpacking light forum. Just reading post, you may think that Brandon is right, adding more fill just increases the insulation value at a linear rate. That is not the full story though.

Take a look at that last graph. CLO units work just like R-values, so increasing the loft 50% would increase the CLO value by 50% (or more, depending on baffle shape). However, the graph shows adding 50% more down only increased the CLO value by about 16%, about a third of what we expected. Likewise, the previous graph shows a doubling in down density, which we would expect to cut the k value in half (k is inversely proportional to R-values). Instead, it is cut by a little less than a quarter. These two graphs suggest that each ounce of added overfill down is about 1/3 as effective compared to an extra ounce of down at full loft. In other words, a 30°F quilt overfilled with with 20°F quilt's worth of down is a 26.6°f quilt — provided that the 30°F quilt was full to begin with. As I have been stressing with these posts, overfilling an oval baffle design does not increase the down density, it only increases loft (for normal amounts of overfill). That means that adding 10% overfill to a typical quilt will increase the loft, and the R value, by ~10%. So it is possible a 30°F quilt with 20°F quilt's worth of down is a 20°f quilt, depending on how much extra room was in those baffles.

---

Brian,

The truncated circle baffles are ~3x narrower than typical baffles, so building a quilt with them will ~3x the baffle wall material. I took your 0.75 oz figure and multiplied by 3 to get 2.25 oz, which is 1.5 oz more than your quilt. That increase in baffle wall material use for the truncated circles is why it is important to cut the material weight if I can. And if it works as I think it does, down massaging won't be necessary.

For my quilts, I decided to go with a 0.67 taffeta inner shell and a 1.1 ripstop outer shell. I used the thinner material not out of serious weight concerns, but because it tends to be softer. This strategy is a bit riskier than 1.1 all around, but seems pretty common, so I'm not too worried.

30% is a hearty amount, though I think that it makes sense for a typical baffle design depending on the parameters. I am curious what the width and height of your baffles ends up being in practice. I am aiming for 5-10% overfill on my quilt, but again, the truncated circle baffle is the same as an oval baffle at "maximum overfill", so the 5-10% is quite a lot. I agree that the 1.5-3.0 inch range is good advice

Tulle (spell check got you, lol) probably isn't that much crappier than netting... so your test maybe more relevant than you think perhaps. Guess it depends if it's woven or fused a bit.
Solid fabric is probably better regardless.

And if'n budget is an issue I strongly suggest you do a sample. This is all well and good to a point but wouldn't be the first Nisley, Jerry Adams, or other wonder theory that fell apart in real life. I still have a Jerry Adams inspired baffle quilt laying around someplace. All good folks with good ideas that inform the discussion but Mountain Hardware still more or less builds their sleeping gear the same way they have for decades for a reason.
Many of us do a 'pillow' say 20"by 30" we can test out an idea on. Gets you a little scale but not a big $$$ commitment.

When you chain the baffles together- this affects them too. Heavier fabric like 1 ounce may alter things... as does checking out something laying on a table vs hanging in space.
Karo step works pretty well in top quilts at mid range volumes around 2".... much less so at 3" and way less so as an UQ. Same theory, same math, same idea... but different applications.

Many of us do a 'pillow' say 20"by 30" we can test out an idea on. Gets you a little scale but not a big $$$ commitment or a huge time suck to make.

As far as CLO breaking down...
I'm still a carpenter. Not even a tinknocker so I can't supply you with a paper or run you a number string.

I can point out one very simple real world application- Take my Neo-air Xtherm mat which is the gold standard of winter mats with thousands of people reporting warm nights at zero degrees easily.
Other than one fella here with one night reported... they don't work much past freezing in the air. Now you and I can convert with 1 CLO= .88R all day long and discuss the specs and insulating properties... but the simple fact is that the pad wasn't designed for convection losses and once it goes up in the air it's simply not as effective as it is on the ground.

While a bit simplistic- R value of a pad based upon air chambers and mylar simply isn't the same R value as its closed cell foam equivalent. Nor are any of the laboratory tests performed on anything higher than cot in a closed room or a slab with no airflow. It's just not an application that applies.

One could say the same for down as well. As far as I know... nobody has studied how down works when it's under you.
Above you... heat rises. Sides... more or less covered.
Below you... now what?
Gravity pulls the down to your body when on top. In theory you have increased density fill right next to you and low density fill farther way at maximum loft. This would put the best insulation right at the heat source.
A good bit of 'cold butt syndrome" is also the simple fact that cold air falls and can easily pool within your underquilt.
If the quilt fits poorly- your butt crushes out the insulation. If the quilt fits well- but not perfect- it's easy to get a small boundary layer of air to form or even the hot air rising out any leaks to cause cold air to be drawn in.

The specific reason the CLO breaks down though I don't have the science to express but likely you follow along.

There is a point of thermal equilibrium in all these insulation scenarios. As it gets colder- the difference between the 'inside' and 'outside' is greater.
CLO specifically wasn't really meant for applications outside building science as far as I understand... but was adopted and used at some point.

Point being...
We only throw off so much heat. (Watts)
Around 50*... we are tossing enough watts out so that the outside surface of our sleeping gear is still (more or less) equal to or above the outside air temp.


I toss 70*... it's 50* out... the surface of my summer quilt is probably around 50* (or close enough).
The point of thermal equilibrium is at the edge of or outside our sleep system.

I toss 70*... it's zero out... the surface of my winter quilt is probably around zero degrees... maybe 10*.
The point of thermal equilibrium is within our sleep system.

Or perhaps my science terms are a bit scrambled and I should be saying thermal gradient... regardless I'll push on.

So how much loft do you actually have? Do you have 4" of down in your zero degree quilt? Or do you have 3" of down you can heat... and 1" outside the reach of the watts of your personal output?

I always explain sleep gear like a lightbulb. (as that's basically how the calculation works anyway with W and surface area/mass dictating heat output of the sleeper.)

If you are a light bulb and you put out 40W or so... you can only light up a room 8'x8'.
If you are a hot sleeper you might put out 60W... you can light up a 10'x10' room.

But put either of you into a 12'x12' room... and there are some dark corners your bulb cannot reach. Your bulb is only so bright... and it's on a dimmer switch set to turn down by about 20w when you fall asleep and you metabolism drops off.

This is the reason the CLO formula breaks down.
It is a linear formula for a non-lineal equation... or perhaps a 'capped' equation is a better term.

Watts is a fixed number for us in the outdoors unlike the other numbers it is not infinite. It is why most old woodsman here can tell you that 2am fire tending, hot water bottles, midnight snacks, pre-bed jumping jacks, chemical packs or a dozen other little tricks are needed in deep cold. The only way to push that heat into the dark corners is to ramp up your metabolism just before bed. To increase your watts, supplement the old heater.

Otherwise increasing CLO/Loft/R-value only makes the dark room bigger and bigger. It's wasted space since your light simply cannot reach it. Sure it may cut down convection loss... but at some point if you are not pushing out enough heat... there is nothing left to conduct through that insulation as it is outside your 'range'. Insulation only traps the air we warm so if we never warm it... might as well not be there at all.

If you can 'go to bed hot' or supplement your warmth... then you can turn up the bulb and illuminate more of the darkness. Only then that insulation can be used to hold that heat longer.

The limiting factor in the CLO formula is us... watts. Generally speaking 40-60w is about as bright as we get when laying down. I'm sure you can look up your own met chart of choice but 80 at rest but fully awake rings a bell as a maximum realistic number.

I am unclear why this isn't talked about... other than the fact its a fairly limited subset of an already limited application in overall insulation science. Arseholes who camp in subzero temps are not that high on anyone's radar... even for military application it's a very small subset. But nearly any of us who do camp in deep cold understand this limitation pretty intimately.

This is also why limiting yourself to three season temps of 20-40*F is also more valuable... because you don't have to deal with this issue as badly.
While 40 is not 40.... zero is zero. There is much less ambiguity when we reach zero degrees than at higher temps.

There is actual truth to the term 'experienced sleeper'... what happens to those who acclimate is that their metabolism doesn't kick down that 20W or so most of us habituated folks do when we fall asleep.
It's also one reason when you may read an 'Artic gear' study for military use you can pretty well discount it for Joe Average coming from his house to a Frozen Butt hang for the weekend.
That is one of the flaws in the EN testing and some of the US military tests that run numbers based upon fit healthy mid twenties males who are 'experienced' sleepers... They are literally the brightest bulbs in the bunch! That same military member 10 years older may have lost 10W of output as his metabolism slows. It would drop another 10W if he stopped being active and put on a few pounds once he got assigned to desk duty.



However that is another subject for another day...