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...
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