Sleeping bag liners

12 Mar 2013 16:04 #56267 by Josh of the Bushveld
I got the Mercury jacket this afternoon (at full price), will try test if over the next few days

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12 Mar 2013 16:39 #56269 by Josh of the Bushveld
Just as I finished my last post, the heavens opened.
I went for a walk down the hill by my flat, then all the way up it at a brisk pace (its quite steep) to generate some heat and vaopur. I'm very happy with the water-proofness of the jacket, and with the hood system. I'm reasonably happy with the breathability - I was a little bit sweaty when I stopped, but I asked my wife to feel my shirt on front and back when I got back and she said it was slightly moist. (I wore a short-sleeve button-up cotton shirt and jeans). My head was quite sweaty though.

There wasn't any wind, only rain.

All in all I'm happy so far and am pleased that I can drop my shell jacket weight from 850g to under 300g.

(interpid, maybe this should be put into a new thread too?)

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22 Apr 2013 12:07 #56726 by Josh of the Bushveld
I decided with the cold weather in Jo'burg to do a sort of field test of some new gear, specifically the K-Way Extreme Lite sleeping bag, and a home-made fleece liner.

Some notes on the liner: I bought some fleece from a local fabric shop, I think about 1.7m x 1.4m or so. We decided not to put a zip in the side, to try save some weight (and because they're not the easiest things to sew). We just folded the fabric in half and made seems along the bottom and side. We also made a hem on the top for a drawstring (elasticated, with a stopper). The liner is big enough to fit me from feet to shoulders (drawstring cinches around neck).

Back to the 'field test'. I slept outside on my balcony on Saturday night. My car thermometer read 10 degrees as we got home, and the forecast minimum was around 9/8 for Randburg. I'm reasonably confident that it was high single-digits.

The clothers I wore: long-sleeved thermals, Cape Storm tracksuit pants (I think '100 weight'), Cape Storm Puff Adder fleece, polar Buff, K-Way beanie and Bridgedale socks (I couldn't find my Falke TK4s that night).

The balcony is concrete/brick. I slept on top of a thermal/space blanket (Coghlans), with the Klymit Inertia X-Fram Recon in the sleeping bag.

While falling asleep I overheated slightly and had to uncinch the liner once or twice. There were some cold spots, but almost exclusively from the ground (ie not the top). I'm quite sure that this is due to the sleeping pad, and other pads (like my self-inflating), or the addition of a foam pad, would have been a lot warmer.

I woke up around 04h30 due to the call of nature, and I wasn't warm but I wasn't cold.

Overall I was quite pleased with the combination of the fleece and sleeping bag, and I'd be confident with the system in temperatures of upper single-digits. The fleece weighs around 600g so the combination of the sleeping bag and liner weighs about 1.1kg, around 300g less than my FA IB. I'm not sure if this weight saving is big enough to cause me to leave the Ice Breaker at home. I am planning on making another liner from lighter-weight fleece which should weigh significantly less, and should be sufficient for 3-season use.

The pad adds 250g and my pillow 125g.

The biggest issue was the lack of insulation from the sleeping pad, and the addition of a foam pad would make a big difference I think. (I'm planning on buying a Therm-a-Rest RidgeRest SOLite Sleeping Pad which would add 260g with an R-value of 2.8).

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22 Apr 2013 19:13 #56738 by ghaznavid
Replied by ghaznavid on topic Sleeping bag liners

joshilewis wrote: The clothers I wore: long-sleeved thermals, Cape Storm tracksuit pants (I think '100 weight'), Cape Storm Puff Adder fleece, polar Buff, K-Way beanie and Bridgedale socks (I couldn't find my Falke TK4s that night).


Well there's the problem - as Tony Marshall told me when I was complaining that my -7 bag was not great in positive temperatures - the colder you feel in your bag, the more clothes you should take off. He sleeps in shorts, personally I sleep in thermal inners, sometimes with thin socks.

So many VE members had told me this in the past and I ignored them because it didn't make sense to me, but it is true and eventually I learned...

Getting to the top is nothing, the way you do it is everything – Royal Robins

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22 Apr 2013 21:21 #56743 by Josh of the Bushveld
I know that piece of advice, should have remembered it.

Would you say the same holds true when using a (fleece) liner?

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23 Apr 2013 11:19 #56746 by ghaznavid
Replied by ghaznavid on topic Sleeping bag liners
I'm going to take a shot at explaining the physics behind all of this - by no means my strongest subject (didn't even take it past grade 9), so anyone feel free to correct me if I'm wrong :thumbsup:

Ps. lots of this terminology is me trying to remember names I haven't used in almost 10 years, so I may have names muddled.

Heat and its transfer

As we know, heat is a form of energy, and when something is cold it is in such a state due to having less energy than the object to which its temperature is being related.

Heat can be transferred by conduction or convection. I think there's a 3rd one, something like radiation - but I'm not sure.

Convection

Convection is when air moves and basically mixes with air of a different temperature.

As we know, air of different temperatures doesn't naturally mix - unless it is stirred or something along those lines occurs. E.g. a cold front is cold air that has moved up from the poles - it doesn't merely mix with the air around it, it pushes the warm air in front of it and carries cold air behind it. The further it moves from the poles the more it begins to get stirred with the surrounding air and thus it dissipates at some stage.

Conduction

This is where heat energy is transferred from a molecule to the molecule next to it and the process continues to repeat itself, each time at a lower volume of transferred energy. I'll throw a bit of a topic that falls into my general field in here - this is basically the same as the economic concept of the law of diminishing returns.

Picture a hand in the middle of a fish bowl of marbles. The marbles are all cold and the hand is warm. The had begins to warm up the marbles that its touching and in the process loses some of its heat, those marbles then warm up the marbles they are touching and thus lose some of their heat.

Assume the hand is 10 "unit" of temperature, the marble won't warm up to the full temperature of the hand (well, not quickly anyway). So say the marbles touching the hands reach 9 units, the hand naturally cools down a bit, but fortunately its heated from the inside.

Now the 9 units warm marbles begin to heat up the marbles they are touching, but again, not to the temperature at which they are, so say 8 units.

The pattern continues until eventually, maybe at about 30 marbles out, the heat transfer will be so minimal that it is irrelevant.

The best insulators

As the above makes clear, when air moves around in small spaces, it mixes and thus cools down the air closest to the hand faster faster. This is basically why a ceiling fan cools a room down.

By contrast, when heat transfer is mainly by conduction, much less heat is lost by the object loosing the heat.

The best insulator known to man is a substance called aerogel - developed by a scientist in the 1930's. It is a gel that has been completely dried, but in a manner that prevents the gel from collapsing as it dries. It is 96% air and weighs practically nothing. Sadly it's very brittle and absorbs water - not at all suitable for hiking!

The reason that it works so well is simple - heat transfer by conduction occurs in air/gasses very slowly, liquids are faster and solids are the fastest (e.g. boiling a kettle vs putting a fork in hot water - the one takes 5 minutes to heat up, the other is almost instant).

Aerogel is basically vast amounts of air that barely move within the structure of the dried out gel. Thus you have little or no convection and thus heat transfer is by conduction only.

By contrast, an air mattress which only contains 4% less solids than aerogel has no matter restricting movement of air, thus making it a fairly poor insulator.

Down and hollow fibre

Filings in sleeping bags are designed to create internal spaces thus reducing the amount of convection in the bag itself. This is the reason that sleeping on a down mattress that isn't inflated doesn't do much (its just a squashed lump of feathers), while an inflated down mattress is incredibly warm. In the same way, a wet down bag will be of little use as it is all squashed up.

Application of the above to sleeping bags

Lets assume a hiker is sleeping in underwear only. There is a minor gap between him/her and the down sleeping bag. This is quickly heated by the body which radiates a vast amount of heat. Once the surrounding air is warmed (by convection), the air in the bag itself (between the down) begins to warm up. The further from the person you get (inside the down part of the bag), the colder it gets. But as its nice and thick, and energy transfer is almost exclusively by conduction, the cold air on the outside doesn't mix with the warm air in the middle - thus keeping you nice and warm.

A fleece liner is designed to sit right against bag itself, thus the heat is radiating from the hiker directly to the liner and from there to the bag. I imagine it would be better if the liner was on the outside, but I'm not sure on that one.

Now lets take a hiker all wrapped up in clothes. The clothing will be retaining the heat in the same way as the sleeping bag would, but now the air that was previously between the hiker and the bag is only being warmed by the heat that escaped from the clothing. This air will still be warmed by convection, but now it will be warmed by the nominal amount of heat that escapes your clothing. This basically stops the bag from attaining any heat and thus effectively makes makes your 1.5kg sleeping bag into a paperweight. In essence, its basically as warm as the clothes you are wearing - maybe a bit warmer due to a barrier between you and the inside of a tent/cave.

So then why would a Ghaznavid want to sleep in thermal inners?

Because he hadn't thought this one through properly :P Just kidding. Basically I own a 1.8kg -2 K-Way bag and a 1.4kg -7 Mountain Hardware bag. But because I don't like sleeping with an open zip on my bag and I use this bag in the middle of summer - thermal inners help to reduce the heat of the bag itself. If its really hot I can always add my thermal fleece :silly:

Getting to the top is nothing, the way you do it is everything – Royal Robins

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23 Apr 2013 15:45 #56748 by Josh of the Bushveld
By that logic, one shouldn't use an (insulating) liner.

If one wears long sleeve thermals (or any long sleeved clothes), is a liner necessary to prevent a down sleeping bag from getting dirty?

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23 Apr 2013 15:59 #56749 by ghaznavid
Replied by ghaznavid on topic Sleeping bag liners

joshilewis wrote: By that logic, one shouldn't use an (insulating) liner.


While I was writing that I was thinking the same thing. If I'm correct the difference is that there is very little air between your liner and your bag (I believe that that's why liners are usually bigger than sleeping bags). Thus the heat escaping the liner is transferred to the bag by conduction and basically no convection should occur. It comes down to where the open space is - between 2 layers or between the human and the combined layers.

joshilewis wrote: If one wears long sleeve thermals (or any long sleeved clothes), is a liner necessary to prevent a down sleeping bag from getting dirty?


Interesting question. I hope so - I don't like the idea of washing a sleeping bag :laugh:

My guess would be that it might help a bit, but not much. The average human loses something ridiculous like 750ml of liquid during the night (hence that most people wake up feeling thirsty). Most of this is through breathing, but lots is through the skin. Since inners are designed to move sweat from your body to the outside of the clothing, I imagine they don't help much with keeping a sleeping bag clean.

Getting to the top is nothing, the way you do it is everything – Royal Robins

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24 Apr 2013 12:02 #56751 by intrepid
Replied by intrepid on topic Sleeping bag liners

joshilewis wrote: If one wears long sleeve thermals (or any long sleeved clothes), is a liner necessary to prevent a down sleeping bag from getting dirty?

Think also about stinky feet! I once rented a high-altitude bag in Nepal and it was seriously smelly inside at the bottom!!

The other part where a bag gets dirty without prevention is from your neck and from your hair.

Take nothing but litter, leave nothing but a cleaner Drakensberg.

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24 Apr 2013 12:48 #56752 by Captain
Replied by Captain on topic Sleeping bag liners
:sick: lol!

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