Orion wrote:So you are saying that!
A warmer shell will lose heat to the environment at a higher rate than a colder shell. You can't have both higher heat loss at the shell and "a lesser rate of thermal loss" overall -- that's a contradiction.
Come on, man. Don't ask a question, then misquote the answer for the sake of making a contrary rebuttal. I made
exactly that point in literally the next sentence.
jdeks wrote:Except usually it isn't[/i], for a number of reasons. ...
Secondly,[b] the outer shell radiates heat to the cold environment. The warmer the shell, the faster it does this. Conversely, the better the insulation, the slower it resupplies heat to the outer shell. Net result is, at a given ambient temp, an equilibrium loss rate with a colder shell. Probably.
So yes, more insulation = colder shell and less heat loss. Unless you OVER insulate but thats another scenario...
Takehome message here is that it's an interlinked multvariable system. It's not just "more x = more y".
Warin wrote:More insulation = less heat loss, less energy flow ..
The insulation from the outer surface of the bag to the heat sink does not change .. and the energy flow is less ..
So the temperature difference increases from the outer surface of the bag to the heat sink .. so the bags skin temperature will rise, not fall.
That different view help jdeks? Cannot reliably figure it for the heat source (inside the bag) as both insulation and heat flow change. Simpler to look from the heat sink, that way only one thing is changing.
...
Okay I'm goinna lay this out in a bit more detail. Whether folk read it is up to then, but dont come back at me unless you get to the bottom.
Heat energy creeps from the body outwards through the insulation, warming it as it goes. The speed of this is pushed by temperature difference, at first between the warm body and the inner skin of the bag, then between the warming inner skin and the first 'layer' of down, then between this layer and the next, and so on, right up to the outer skin.
As the outer skin finally receives heat energy, it too will warm. But - it loses heat faster, by convection and radiation to the cooler outside. It will heat a little, but tends to find balance closer to ambient, as the skin needs very little thermal difference to drive that radiation, but the outer insultion (being, yknow, insulative) needs a greater thermal differential to match in supply what the skin loses. That outer insultion will then also itself cool somewhat, choking its own transfer, until an equilibrium is established. This cooling will creep a little back up the insulation layers, and evetually establish a thermal gradient through the bag, with a heat loss rate to the outside defined largely by the insulation.
The specific temperatures of this gradient, depend on MANY things
Now, lets
drop the outer temperature. The skin sgain radiates faster than it can draw from the outer insulation at first, and its temp drops, until it reaches a new point where the thermal diffrence draws as much heat fromthe insulation as it emitts by radiation. The outer insultion also cools accordingly, and a slightly steeper temperature gradient establishes in the insualtion. More importantly, the rate of net thermal loss only drops marginally - the reduced ouside temps have in turn lowered the insulation and shell temps, in itself moderating heat transfer rates.
On the inside, you probably barely notice - interior insulation and skin temps likely stay the same. This is, roughly speaking, the comfort rating of your bag.
Now
drop the temperature more. Eventually, you reach a point where then skin simply cannot suck heat fast enough from the outer insulation to replace the rampagin radiation. What gets there may warm the skin fractionally, but that just accelerates the rate it radiates out, and it cools again until it ultimately approaches ambient. The skin just keeps leechnig heat out, until the outer insultion cools enough (ie also near ambient)to lower thermal drive to the shell and reach and new steady state, as it in turn draws heat from deeper in the bag. Eventually, it self limits as the increasingly cold insulation drives less heat outwards - but just when/where this happens depends on how cold it is outside and how hot the body inside is. You could end up with the skin, and several cms of outmost insulation all at ambient temps.
You can still feel warm like - in fact this is probably where most winter hikers are at. The overall heat loss isn't much less, and if the temp gradient doesn't lower too much, the inner insulation can still be at around body temp.
However- if ambient temp is at, or close, to dew point, this is one sceario where you can get condensate on, or in, your bag, depending on humidity. And if it goes too far, you then lower the temperature of your inner insualtion and inner bag skin. You feel 'cold', and any humidity, be it perspiration or otherwise, has no heat to vaporize it and drive it though and out of the bag wall (hence feeling clammy). This is the bottom end of your comfort rating, and this is where the OP is at.
Going much colder than this, and the temp differential will drive heat loss through the full insulation thickness faster than the human body can keep up with. You're just delaying the inevitable - how long for, is how you get the 'extreme' or 'survival' rating.
So now, lets increase the insulation. Skin's still ambient-cold and thus wont get colder, but the heat isn't leaking to it as fast . Ergo, the insulation warms up - first the inner layers, but then progressively to the outer layers. Some of this leaks to the skin, whih in turn radiates away, but it's slower because the insulation is better. The insulation builds heat until a new equilibrium is reached, this time with a much warmer insulation temp, as a much larger differential is needed to drive heat into the skin at the rate it then loses it to then enviroment.
Increase the insulation even more, and you can potentially hold so much heat that the bag skin will then warm, even with the consequent increase in radiant heat loss. In this case, you will indeed be losing
more heat energy, but youd want to be - or you'd probably overheat. THIS is the other 'clammy' scenario - bag is borderline too warm and you wind up sweating inside then chilling. I doubt this is the OPs problem though.