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JAK
2010-09-12, 17:29
Not neccessarily a hammock question, but I thought you guys would appreciate this as much as anyone, and be more likely to have an answer.

So I got to thinking about sleeping bags and loft, and heat loss from humans when they are sleeping, or awake shivering in the middle of the night. We lose heat through conduction, convection, radiation, and evaporation (through both respiration and transpiration). Our skin temperature can vary some when we are in cold mode, to 93F around neck and chest area, and as low as 73F in the extremeties while still feeling warm enough to sleep if we are acclimated. It's mostly about loft, so they say, and it probably is, as long as everything is reasonably air tight. Of course we have to breath, and if its really cold a wool scarft helps, but other than comfort and frostbite that is still supposed to be a smaller part of the total equation if everything is well designed and working right.

So here is the thing. Say we are cranking out 100 watts in our sleep, and our cocoon has a surface area of 2 square meters. So our heat loss is 50 watts per square meter. It might be less as some of the 100 watts is breath, but lets say its all going out through our skin and through our cocoon. OK, so how much loft should be needed for 50 watts per meter squared. Let's go to an extreme of a temperature difference of 50 degrees C, or 90F, or an outside temperature of -10F and an average skin temperature of 80F.

That works out to an R-value of only 1.
That can't be right. So what am I doing wrong?

JAK
2010-09-12, 17:50
OK. I can see a big part of what I did wrong was I was mixing my units up. The R-Value would be 1 in purely SI or metric units, but that would be 5.75 in english units.
http://en.wikipedia.org/wiki/R-value_(insulation)#Typical_R-values_for_a_specified_unit_.28not_per_inch.29

Still, in theory we should be able to get that with only 1" of loft.
We know 1" of loft won't cut it for -10F, so what's up?

I'm thinking that even with a fairly tight nylon shell there must be a considerable amount of convection not just inside our quilts and sleeping bags and cocoons, but also passing right through the inner and outer shells. Especially when it is colder, and especially when it is windy, so the effective r-value actually goes down when we need it the most. So maybe that is why we need like 2 or 3 or 4" of loft instead of just one.

But what does that say about the common assumption that 1" of loft is 1" of loft, no matter whether it is 1" of 900fill down or 1" of 600 fill down or synthetic batt or fleece or batt or whatever. Also, maybe it is not so critical that these shells breathe a little. Maybe there is a better way. Maybe something totally airgtight we can slip over, and then remove to dry things out, and a VBL on the inside so the insulation isn't totally soaked.

Or perhaps it is just that we have alot of leaking around the neck area and we need to cut that down some.

Ray
2010-09-12, 18:35
Time out. Something seems screwey here. And Green Bay's about to score. Let me check.

Ray
2010-09-12, 18:44
OK, half time. So I assume youíre starting with the classic
H = g * (T1 - T2)

H : Heat flux (W/sq.m)
g : Thermal conductivity (mol/sq.m/s)
T1: Inside sleeping bag, so body temperature, let's call it a constant 37 C.
T2: Outside sleeping bag, air temperature (C)

Are we on the same page so far?

First, I don't like the application of that equation since it says that if H is constant then g is linear with decreasing outside temperature. And g is related to and linear wrt loft. So by the equation, needed loft is strictly linear with temperature difference. And that may work ok within some range but it seems to me when there's a big temp difference, when air temps get way cold, then some other form of heat loss must come into play.

Two guesses: My first guess is radiation, that really low temperatures are going to suck the heat out of the surface of the bag and that as temps get extremely low that value will go from negligible to significant. Or the fill itself, the down feathers or whatever they're called, become heat conductors since the loft in sleeping bags is assumed to be pure still air and at some point that conductance must be accounted for.

SGT Rock
2010-09-13, 04:50
As I thought about it, I think Ray is on it. The air outside the bag will constantly move and change, so it never equalizes heat with the body inside, or comes close. The parts of the bag also have some effect of conducting heat to the outside air as well as insulating.

That said, I've never looked into this and I could be completely wrong. But using Army beer math, we would say double whatever you come up with as the needed loft from this equation and you would probably be OK. That is what we would do with explosives if it didn't look right.

Ray
2010-09-13, 10:25
OK, now I've had some sleep and much less beer...
H = g * (T1 - T2)

H : Heat flux (W/sq.m)
g : Thermal conductivity (mol/sq.m/s)
T1: Inside sleeping bag, so body temperature, let's call it a constant 37 C.
T2: Outside sleeping bag, air temperature (C)

which isn't particularly useful until we add that g = k / L, where
k = thermal conductivity, 0.024 W /m-C (http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html) for air
L = length or in our case loft in cm

Which makes sense, as the temp drops the loft has to increase to maintain the same heat flow.

So...rearranging, taking care of conversions...

L = 2.4 (37 - Ta) / H and plugging in your 50 W/sq. m and 0 degrees C, we get a loft of .... uh, 1.8 cm for 32F which doesn't sound right. So how confident are you in that whole 50 W/sq. m thing?

Googling "sleeping bag loft" and taking the first hit I get this chart (http://www.survivaltopics.com/survival/sleeping-bag-temperature-ratings/). And of course we have to believe it since it's on the internet but it doesn't look unreasonable.

Temperature Inches of loft
30 degrees F 1.8 inches
10 .. 2.3
0 .. 2.8
-10 .. 3.2
-20 .. 3.7
-30 .. 4.2
-40 .. 5.2

Plugging those numbers into the equation and solving for H gives a value of around 20 W / sq. m of heat flow. The number also decreases as the temperatures go lower so I'd guess over-fill is worked in to maintain comfort for the reasons Rock & I speculated about.

Plugging 20 W/sq.m in for H gives a loft of about 1.7 inches for a 32 degree bag, and that sounds right and it also matches up with the chart.

So there you go. L = .12 * (37 - Ta); L in cm & Ta in degrees C.

warraghiyagey
2010-09-13, 10:40
That's exactly what I was gonna say. . .

Ray
2010-09-13, 10:58
That's exactly what I was gonna say. . .You were going to say "boobs" and we all know it.

taildragger
2010-09-13, 10:59
I feel like the zippers and back of the bag as well as dead space, the fact that the bag can breathe and that we are a non-uniform temperature as well as differences in metabolism during sleep are all being ignored here...

Also, the loft is non-uniform, especially around the baffles, so you'll have another area of greatly increased heat flow.

IOW, there are a lot of complications that are left out of that simplified equation, if this was an insulated pipe with a flowing fluid and a 30C difference, I'd be more apt to trust it.

warraghiyagey
2010-09-13, 11:05
You were going to say "boobs" and we all know it.

Which is the most effective way I know to stay warm on the trail. . . :beer:

Ray
2010-09-13, 11:14
Consider a spherical hiker...

taildragger
2010-09-13, 12:01
Consider a spherical hiker...

If he's in vacuum I have an equation for this.

Oddly enough, it was solved by this guy http://www.funwithln2.com/img/Thu011sup.jpg

JAK
2010-09-13, 12:45
I've done the spherical hiker thing. Not so good up hills.

I think you are onto somethings there Ray. Firstly, maybe less heat can be allowed to flow through the bag because more heat is escaping in other ways. So we need to reduce those other ways as much as possible also, but we can't blow the entire heat loss budget through the bag. So the bag needs perhaps twice the loft than it would if all the heat were going through it. Seems counter-intuitive in a way, but it makes sense. If only half your budget can go to heating your house, you need thicker walls, not thinner walls. Definitely should identify and mitigate those other losses though. Some other thing that might make things non-linear, or perhaps only seem that way. One is that our acts as insulation, especially with vasoconstriction. When cold, but still able to sleep, the average skin temperature can drop to 80F. Neck stays warmest at 93F, followed by chest, face, head, groin, back. Feet and ankles and hands can be coldest at about 73F, with calves and thighs and arms and butt somewhere in between. Anyhow so that means the insulation required is perhaps linear, but from 80F down, not from 100F down. So we need twice as much at 0F vs 40F, not just 1.67 times as much. The other thing is that some areas do need more protection than others. Although the bag does balance it out some, and the blood system circulates heat inside out bodies really well, if we have a neck baffle, as we should, the upper part of the bag around the neck and face and scalp really should have better insulation than the rest of the bag. Not alot more at -40F, but alot more at 40F. We tend to do the opposite though. We tend to open up the head and neck area at 40F, and close it up more when it gets colder, but that is usually because we are using the same bag over a wider range. So in general, same insulation everywhere, and more for colder design temperature, but linear from 80F, not from 100F. The thing that might make things actually non-linear is that convective forces and radiative forces increase with temperature difference. I am somewhat sceptical about radiative, especially under a tarp, but if the fill is loose enough, maybe. I think the real non-linear culprit is convective. I am not talking about outside the bag. All that does is strip away the boundary layer, so the outer surface can't act as extra insulation. I am thinking more about some convective currents, either generated by heat or forced by the wind, actually penetrating through the outer shell of your sleeping bag. If this is happening, you definitely loose R-Value, and you lose it most when you need it more. The solution could be more loft, or a more windproof shell, or a little bit of both. I think even with a bulletproof shell, there is still some air getting into and out of the opening into your sleeping bag, especially when you move around a bit. The effect of this might be linear with temperature, but I think to some degree it is non-linear. Not sure. I tend to move around less when it is really cold. it is easier to stay on my back with my arms in, unless for some reason I don't have enough insulation underneath. Then there is more moving around. The other thing is when we get up in the middle of the night, and get cold, we need to get warmed up again. I think it takes longer to get warmed up again when it is colder, so we might need a little extra insulation to allow for this. Not sure. I think there is definitely less margin of error at -30F vs 0F though. Most people would agree with that. So I think in addition to other things you need a little more loft for that then you otherwise would.

For ground sleepers there is some non-linearity working the other way. On snow, or even frozen earth, the temperature beneath the ground pad might add some extra insulation once it starts warming up and developing a temperature gradient. In the extreme, on snow, it might mean that the temperature get no lower than 32F. So that would mean that at least the insulation below you, the ground pad and compress fill, doesn't need to keep increasing so much as the temperature drops compared to the loft above you. Also, there would be no convection below you either. For ground sleepers this effect balances against the convective effect, so insulation probably does end up increasing more or less linearly with temperature difference. For hammockers, there is more convection above than below because heat rises, but you would have non-linear effects on both side, so you would need to increase loft a little more non-linearly with temperature difference, or take increasing measures such as tarps and such.

Not sure if there is anything to be gained from all this. I think I will pay more attention to having good insulation underneath to make up for compression. if its cold enough 2 blue foam pads vs one unless I know I will be on snow. Also, perhaps alot more attention around the opening, neck baffle, neck and face and scalp. I wear my alpaca neck tube and peruvian type hat now, but I think I might get some sort of down filled hood as well, in addition to my sleeping bag hood. I used a pillow once, inside of my sleeping bag hood, and found it warm as well as comfy, but I think a hoody pillow or pillowy hood might be even better. Something that doesn't compress too much at the back of the head. Where I was hoping to go with this was a better case for wool blankets. I think there is still a good case for wool as an inner liner, but I think long wool underwear does the job just as well and it more versatile, and you can wear it with you when you take a dash for a pee break. I think a wool sweater is excellent also, not just for added warmth and moisture heat recovery and management, but also because it acts as something of a baffle and heat recovery unit for air finding its way into and out of the opening, past the neck baffle. Not so sure what to do about extra space around the legs. Is it good to have some there or not? I do like the look of those sleeping bags with some sort of elastics that expand and contract to maintain contact.

I think the best conclusion is that more loft is good, but it is not all about loft.

JAK
2010-09-13, 12:49
Thinking about using my CF gortex bivy as the outer shell of some sort of bedroll. Synthetic batt insulation, with a soft nylon shell on just one side, that wraps around me so that it is one layer underneath and two layers over top and the sides. It would be removeable from the bivy. I would cut the bivy down to be more tapered, but still fairly boxy. Some sort of foot box but a flap and adjustable tie up the front, rather like a sneaker or those old anorak neck openings. Blue foam pad underneath, or inside, not sure. Maybe a wool liner also. That could be added or removed. Some sort of mummy hood and neck shoulder baffelt is needed though. I might make something from an old parka and sew it to the gortex. Not sure. I would like to make something for -25F, but adjustable so that it might weigh something like 4 pounds in total for 20F, 5 pounds for 0F, 6 pounds for -20F. Bivy is 2 pounds now though, empty. My old synthetic mummy is 3 pounds. They are not a great fit.

taildragger
2010-09-13, 13:04
Jak, I assume that both of those posts could be taken down at least 32 notches, bullet points would help.

warraghiyagey
2010-09-13, 13:12
Jak, I assume that both of those posts could be taken down at least 32 notches, bullet points would help.

Interesting that you mention bullet points cuz after the first paragraph I wanted to shoot myself. . .

saimyoji
2010-09-13, 13:14
Interesting that you mention bullet points cuz after the first paragraph I wanted to shoot myself. . .

you should always trust your instincts. go read it again and this time do what you feel.....

warraghiyagey
2010-09-13, 13:22
Now I just feel like shooting you. . . :beer:

saimyoji
2010-09-13, 14:09
boobs......:itsme:

JAK
2010-09-16, 02:14
I'm A Cuckoo. :bath:
http://www.youtube.com/watch?v=NwDLpFqyxz8

cool video showing last 100m olympic gold medal won by a Scot

JAK
2010-09-16, 02:30
There it is again...
http://www.youtube.com/watch?v=j3KLzUYF0Uo

JAK
2010-09-16, 03:21
Not to be seen anywhere in this video...
http://www.youtube.com/watch?v=BzrI15uw92k

Ray
2010-09-16, 11:34
Heat transfer is extremely relevant to Scottish sprinters.

Superman
2010-09-16, 12:08
Now I just feel like shooting you. . . :beer:

How are those anger management classes working for you?:angel:

Skidsteer
2010-09-16, 13:05
Heat transfer is extremely relevant to Scottish sprinters.

Scottish sprinters?

There's the oxymoron of the day.

JAK
2010-09-16, 16:04
Wells
1982 http://www.youtube.com/watch?v=aL417SkHtME
1981 http://www.youtube.com/watch?v=1_Zr0Zy0iy4
1980 http://www.youtube.com/watch?v=RQeNlSoq88E
1979 http://www.youtube.com/watch?v=30pmaXj1Omg
1978 http://www.youtube.com/watch?v=CgPmr5PU0pM

Liddel
1924 http://www.youtube.com/watch?v=fRdrtp5YAxU

Also:
Elliot Bunney
Menzies Campbell
Lee McConnell
Henry Macintosh
Bob Murdoch
Douglas Walker
Brian Whittle
Ronald Wylde
Ian Young

And that is not counting alot of Scots sprinters from abroad, places such as USA, New Zealand, Canada. Percy William of Canada was a Scot, and won double gold in the 1928 Olympics. Peter Snell of New Zealand was a Scot, though he was a middle distace runner, doubling in 800m and 1500m in Tokyo.

Percy Williams:
http://en.wikipedia.org/wiki/Percy_Williams
http://www.youtube.com/watch?v=DN-XuGxFTyA

Carl Lewis and Jessie Owens were Scots as well.

JAK
2010-09-16, 16:09
Most of the 100m records were broken by Scots.
Just look at the list of names...
http://en.wikipedia.org/wiki/Men's_100_metres_world_record_progression

Where do you think Bolt got his speed from? There's some Scots blood in there as well.

taildragger
2010-09-16, 16:31
Scotch helps with the heat transfer

JAK
2010-09-26, 21:13
On a different take, how much control do we have over our metabolism for when we are sleeping, or awake and resting? I am thinking about such things as the type of foods we eat, and when we eat them. Protiens are supposed to generate the greatest thermogenic effect, but I am not sure how much of this heat is generated in the stomach, small intestines, or even later in the kidneys. I've heard figures like 80kcal/100kg after fasting, and 100kcal/100kg on a full stomach, but I would like to know more specifically what the heating response of food is over time, and how much it can vary with quantity and with type of foods. How much water should be taken with meals, to get the maximum heating effect.

What got me curious was reading about deer in deer yards in winter. Apparently when it is really cold out, if you feed them a high quality food like corn they will freeze to death in their sleep, but if they browse on their usual winter fare of bark and moss and other stuff the heat generated from this stuff in their digestive tract, in addition to their fat reserves and the food value obtained from earlier digestion, the combined heating effect is what keeps them alive.

So what is the best possible meal for us? What about activity level that day? What is the right ammount of activity during the day to maximize our metabolism during the night? I have found I am hottest in the evening when I have been fairly active all day, but not to the point of total exaustion. So what's all going on there?

Ray
2010-09-26, 21:31
It would seem to me that 100 kg is a lot of food to eat. Something like 220 pounds? If you were cold you could just bury yourself in it.

Hooch
2010-09-27, 07:11
I've done the spherical hiker thing. Not so good up hills.

I think you are onto somethings there Ray. Firstly, maybe less heat can be allowed to flow through the bag because more heat is escaping in other ways. So we need to reduce those other ways as much as possible also, but we can't blow the entire heat loss budget through the bag. So the bag needs perhaps twice the loft than it would if all the heat were going through it. Seems counter-intuitive in a way, but it makes sense. If only half your budget can go to heating your house, you need thicker walls, not thinner walls. Definitely should identify and mitigate those other losses though. Some other thing that might make things non-linear, or perhaps only seem that way. One is that our acts as insulation, especially with vasoconstriction. When cold, but still able to sleep, the average skin temperature can drop to 80F. Neck stays warmest at 93F, followed by chest, face, head, groin, back. Feet and ankles and hands can be coldest at about 73F, with calves and thighs and arms and butt somewhere in between. Anyhow so that means the insulation required is perhaps linear, but from 80F down, not from 100F down. So we need twice as much at 0F vs 40F, not just 1.67 times as much. The other thing is that some areas do need more protection than others. Although the bag does balance it out some, and the blood system circulates heat inside out bodies really well, if we have a neck baffle, as we should, the upper part of the bag around the neck and face and scalp really should have better insulation than the rest of the bag. Not alot more at -40F, but alot more at 40F. We tend to do the opposite though. We tend to open up the head and neck area at 40F, and close it up more when it gets colder, but that is usually because we are using the same bag over a wider range. So in general, same insulation everywhere, and more for colder design temperature, but linear from 80F, not from 100F. The thing that might make things actually non-linear is that convective forces and radiative forces increase with temperature difference. I am somewhat sceptical about radiative, especially under a tarp, but if the fill is loose enough, maybe. I think the real non-linear culprit is convective. I am not talking about outside the bag. All that does is strip away the boundary layer, so the outer surface can't act as extra insulation. I am thinking more about some convective currents, either generated by heat or forced by the wind, actually penetrating through the outer shell of your sleeping bag. If this is happening, you definitely loose R-Value, and you lose it most when you need it more. The solution could be more loft, or a more windproof shell, or a little bit of both. I think even with a bulletproof shell, there is still some air getting into and out of the opening into your sleeping bag, especially when you move around a bit. The effect of this might be linear with temperature, but I think to some degree it is non-linear. Not sure. I tend to move around less when it is really cold. it is easier to stay on my back with my arms in, unless for some reason I don't have enough insulation underneath. Then there is more moving around. The other thing is when we get up in the middle of the night, and get cold, we need to get warmed up again. I think it takes longer to get warmed up again when it is colder, so we might need a little extra insulation to allow for this. Not sure. I think there is definitely less margin of error at -30F vs 0F though. Most people would agree with that. So I think in addition to other things you need a little more loft for that then you otherwise would.

For ground sleepers there is some non-linearity working the other way. On snow, or even frozen earth, the temperature beneath the ground pad might add some extra insulation once it starts warming up and developing a temperature gradient. In the extreme, on snow, it might mean that the temperature get no lower than 32F. So that would mean that at least the insulation below you, the ground pad and compress fill, doesn't need to keep increasing so much as the temperature drops compared to the loft above you. Also, there would be no convection below you either. For ground sleepers this effect balances against the convective effect, so insulation probably does end up increasing more or less linearly with temperature difference. For hammockers, there is more convection above than below because heat rises, but you would have non-linear effects on both side, so you would need to increase loft a little more non-linearly with temperature difference, or take increasing measures such as tarps and such.

Not sure if there is anything to be gained from all this. I think I will pay more attention to having good insulation underneath to make up for compression. if its cold enough 2 blue foam pads vs one unless I know I will be on snow. Also, perhaps alot more attention around the opening, neck baffle, neck and face and scalp. I wear my alpaca neck tube and peruvian type hat now, but I think I might get some sort of down filled hood as well, in addition to my sleeping bag hood. I used a pillow once, inside of my sleeping bag hood, and found it warm as well as comfy, but I think a hoody pillow or pillowy hood might be even better. Something that doesn't compress too much at the back of the head. Where I was hoping to go with this was a better case for wool blankets. I think there is still a good case for wool as an inner liner, but I think long wool underwear does the job just as well and it more versatile, and you can wear it with you when you take a dash for a pee break. I think a wool sweater is excellent also, not just for added warmth and moisture heat recovery and management, but also because it acts as something of a baffle and heat recovery unit for air finding its way into and out of the opening, past the neck baffle. Not so sure what to do about extra space around the legs. Is it good to have some there or not? I do like the look of those sleeping bags with some sort of elastics that expand and contract to maintain contact.

I think the best conclusion is that more loft is good, but it is not all about loft.

You make me want to tell you to shut up. Holy shit, do people acutally pay attention to you at all when you speak in real life?

Superman
2010-09-27, 08:25
You make me want to tell you to shut up. Holy shit, do people acutally pay attention to you at all when you speak in real life?

I'll wait and read the reader's digest abridged version...too many words.:angel:

JAK
2010-09-27, 20:22
You guys aren`t actually reading all this shit are you?

Sorry, the 100kcal/100kg was meant to mean 100kcal/hr per 100kg of body weight. It would probably be better stated per square meter. Anyhow, my question is really about how much we can raise our heat production by sleeping on a full stomach, and how much it can vary depending on what you eat and how much you eat, and how long through the night the effect lasts.

Skidsteer
2010-09-27, 20:28
You guys aren`t actually reading all this shit are you?

You had me at heat transfer.

SGT Rock
2010-09-27, 21:00
You guys aren`t actually reading all this shit are you?
Usually only the first sentence and the last sentence. :hammock:

Ray
2010-09-27, 21:41
Usually on my back, sometimes on my side but I can't sleep on a full stomach.

Unless you mean sleeping on someone else's stomach. That doesn't work either. But I'd guess it'd keep me warm.

taildragger
2010-09-27, 22:21
I find that beans keep me the warmest, and consequently, the loneliest (unless there is a barking spider contest).

Also, I think Ray's sig should be noted during this discussion, as it is very true, and would provide the warmest nights sleep

Two Speed
2010-09-28, 09:20
Yeah, but ya gotta stoke the fire with a puppy once in a while or the fire goes out.

Superman
2010-09-28, 09:30
We sewed a mating zipper onto rip stop material which can then be zipped to one of my LL Bean rectangular down sleeping bags. We did that so we need to carry only one sleeping bag for two people and the trophy babe can't cacoon with the sleeping bag. Spring and fall hiking/camping is the best and the trophy babe puts out enough heat to melt the polar caps. There may be ice on the pumpkin but I'm warm, warm, warm.:angel:

taildragger
2010-09-28, 10:12
Why are you letting your pumpkins get so cold? I always make sure that they are the warmest part

warraghiyagey
2010-09-28, 10:20
I don't understand why you would put pumpkins in the hammock with you . . . :albertein

Superman
2010-09-28, 10:26
LMAO...Hammock, pumpkins...what the fuck did I write this time?:angel:

Two Speed
2010-09-28, 10:50
Superman, language check please.

Superman
2010-09-28, 15:19
Superman, language check please.

If I'd had one more year of college I would have had spelling and gramma.:angel:

JAK
2010-09-28, 21:57
This threads definitely getting better. :)
Trying to come up with some more half baked slit your wrist stuff for you all to feed off, but I'm currently at a loss. Too much sleep lately. Give me another day or two.

OK. Here's something. Working on a new wood stove idea for throwing a little heat at me in my bivy/tarp, or if I have to sit up in the night. Basically like a Swedish Candle, which is one of these.
http://www.youtube.com/watch?v=wEHSJftQ9Vg

But then instead of lugging a chainsaw, you just bundle 7 two inch sticks together. The jam a 6" coffee can over the top of them, with a 2" hole in the bottom, which is the top. Then you set it on the ground but drive the center 2" stick into the ground, so the whole thing is supported, and the stove has a cavity in the center. You light it from the top after you stuff it with kindling. As it burns, you might be able to shove the coffee can down further. You can cook stuff on top, or maybe just put a reflector and wind shield behind it, like a space blanket or tarp or something.

Ok. Last sentence for the Rock.
Ray's signature would fit in really well once some of the stakes burn down enough.

JAK
2010-09-28, 21:58
These are pretty neat...

http://www.youtube.com/watch?v=gMLzY48YwEM&feature=related

Iceman
2010-12-13, 15:41
Not neccessarily a hammock question, but I thought you guys would appreciate this as much as anyone, and be more likely to have an answer.

So I got to thinking about sleeping bags and loft, and heat loss from humans when they are sleeping, or awake shivering in the middle of the night. We lose heat through conduction, convection, radiation, and evaporation (through both respiration and transpiration). Our skin temperature can vary some when we are in cold mode, to 93F around neck and chest area, and as low as 73F in the extremeties while still feeling warm enough to sleep if we are acclimated. It's mostly about loft, so they say, and it probably is, as long as everything is reasonably air tight. Of course we have to breath, and if its really cold a wool scarft helps, but other than comfort and frostbite that is still supposed to be a smaller part of the total equation if everything is well designed and working right.

So here is the thing. Say we are cranking out 100 watts in our sleep, and our cocoon has a surface area of 2 square meters. So our heat loss is 50 watts per square meter. It might be less as some of the 100 watts is breath, but lets say its all going out through our skin and through our cocoon. OK, so how much loft should be needed for 50 watts per meter squared. Let's go to an extreme of a temperature difference of 50 degrees C, or 90F, or an outside temperature of -10F and an average skin temperature of 80F.

That works out to an R-value of only 1.
That can't be right. So what am I doing wrong?



Old thread, but I have been elsewhere for years and thought I would post up a comment on this topic....

Each year we camp up on Mt St. Helens out here in Washington. Our snow camp at night ranges from 20 degrees to as low at 4 or 5 degrees. We sleep in a tent, on doubled foam Wallmart hiking pads, layed 90degrees to each other to avoid gaps... We have elcheapo -25 degree bags, with elcheapo zippers, and elcheapo insufficient baffles at the zippers...

I am rather barrel chested, and over 300lbs.

What I have learned over the years of sleeping in cold weather is this....

In my not so humble opinion, as we sleep in any sleeping bag, our lung motion, the motion of breathing in and out causes the bag to slightly rise, and then to lower. You suck in cold, then push out warm...every breath.

Ever had a cold spot that would not just go away?

I am a believer in tossing another thin layer over your entire bag as you sleep. I have been using a weird little product for years at our snow camps. The product is called a NeatSheet. It is a microfiber sheet designed as a beach or picnic ground cloth. It is incredibly lightweight, it breathes, yet sheds water.

We sleep with these NeatSheets draped over ourselves at our snowcamp each year and find that the extra layer of trapped air seems to heat up a bit. Now as you breath in and out, your zipper is pulling in substantially warmer air and you cool less in your bag. My wife is rather claustrophobic and does not zip her bag past her waist, and just uses the bag over her, laying her top half of her body (in thermals) on the foam pad, the draping Neatsheet keeps drafts out and she is happy.

I guess my point is about loft, I think that it doesnt even matter how much you put on, that drafts occur/robbing you of your stored warmth, and that for the weight of extra loft, you would be well advised to create this second layer of warmed air around the bag, sort of like layering....