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So I am going on a 6 days wildness hike. I have a down sleep bag that can comfortably cover up to 3 degrees if I have thermos on. I am considering getting a sleeping bag liner. Main concerns are:

  1. Weight
  2. Volume
  3. How many degrees this liner can increase in a sleeping bag

Can someone give some recommendations on how to select a small, ultralight sleep bag liner that can improve the the warmth in the sleeping bag as much as possible?

  • what temperature do you expect you'll face? – njzk2 Dec 22 '16 at 18:33
  • What does "have thermos on" mean? Please edit. – Martin F Dec 23 '16 at 0:49
  • @njzk2 Apparently according to some tourist site, the average minimum for the month is about 9 degrees celsius. But people who have been there said that it could snow at any time of the year. So I am preparing for the worst case scenario - maybe to -3? – TelKitty Dec 23 '16 at 2:34
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    @MartinF: By "have thermos on" I guess, OP has put on thermal clothing, which generally is an inner layer of clothing. – WedaPashi Dec 23 '16 at 5:25
  • Related: outdoors.stackexchange.com/questions/645/… – ShemSeger Dec 23 '16 at 6:38
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For your three criteria the one that jumps out for me is a silk liner. It's warmth to bulk ratio is going to be the best. You'll prob get a couple of degrees (5C at most) for this.

That said, A sleeping bag liner is mainly to keep your sleeping bag clean, not particularly add warmth, though they do do this also. A better approach to add warmth when in a bag is to wear clothes. I also find this more comfortable as you can get in a right tangle in a bag liner in a bag.

I've even slept in down jackets, etc. when needed. This also keeps your sleeping bag clean as well!

  • How does a silk liner compare to cotton one? – TelKitty Dec 22 '16 at 15:03
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    Thinner, lighter, better insulation and more breathable. Cotton is not good as a breathable material. It tends to make you cold when wet. Polyester is actually a much better choice than cotton, though silk is generallly used as it's less "scratchy". So basically silk is comfortable like cotton but offers better breathablility than cotton (like polyester) – user2766 Dec 22 '16 at 15:05
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    cotton is terrible, especially in the temperature that you describe (3C). It will weight 3-4x as much as a silk liner, take 2-3 time more volume, and feel very cold if you sweat in it. – njzk2 Dec 22 '16 at 18:08
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    I have a cotton liner, and I've used it at -15°C and colder. It works just fine as long as you keep it dry, and makes your bag that much more comfortable as well as warmer. It feels like sleeping in your bed at home. Silk is lighter, and less bulky, but also more expensive. – ShemSeger Dec 23 '16 at 6:36
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    I actually disagree with your approach Liam. Wearing SOME cloaths, by that I mean mostly first-layer stuff, can help. And then head cover as well and perhaps a pair of good socks. But for the rest, IMO your best bet is to NOT over dress in your sleeping bag. The point of a sleeping is to create a pocket of warm air around you. By wearing cloths you actually hinder that process. IMO a better approach to exercise a bit just before going to sleep (perhaps do a bacon dance in your sleeping bag). I would stuff emptiness in my bag with clothes before wearing them. – Francky_V Dec 28 '16 at 14:22
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Vapor barrier liner gives about 10 F. You wake up damp and they are not real comfortable but they work. Not only does it hold some heat in it prevents moisture from building up in the bag. Especially over multiple cold nights a bag will build up condensation. A buddy of mine used one year round for ultralight.

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    If sleeping multiple nights in under freezing temps, you definitely want to keep the moisture out of a down bag especially! Get it out in the sun during the day if possible. VB clothing is a must if you'll be out a lot under freezing. – topshot Dec 22 '16 at 18:06
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The basic criterion should be keeping the sleeping bag clean, and allowing moisture to wick away from the persons skin. If it adds to the warmth, its win-win situation.

First and far most obvious thing I'd consider is the shape of my sleeping bag: whether it is a Mummy-shaped or a regular rectangular one.

Considering that your sleeping bag is going to keep you warm enough, I'd recommend using a Synthetic liner, such as CoolMax. You'll find these ones slightly cheaper than Silk-made ones, though I do understand that you didn't consider cost as one of the criteria.

I've used one such liner just once, and it was great.

  • What do you mean by 'keeping the person dry off the moisture'? – TelKitty Dec 22 '16 at 15:02
  • Wiking properties @Telkitty. You want any moisture (sweat, etc) moved off your skin before it can make you cold – user2766 Dec 22 '16 at 15:07
  • a down-vote? Thank you! – WedaPashi Dec 28 '16 at 10:42
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If you're looking for a liner to add some serious warmth (10-15 degrees Fahrenheit), look into the Thermalite Reactor series. They weigh generally in the 8oz range.

  • for serious warmth, imagine a sleeping bag that has ~225g more down than your current bag. That would bring way more than those 9.4 C – njzk2 Dec 22 '16 at 18:10
  • Maybe, but then you only have an expensive sleeping bag suitable for very cold clinates. I prefer a modular system where I can add warmth as needed. – Ramrod Dec 22 '16 at 18:21
  • Would you care to elaborate somewhat, just stating a single product recommendation (which will probably get obsolete in time) without any specs (except weight) seems not very useful to me. – imsodin Dec 28 '16 at 10:13
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I would go with a silk liner.

Be sure your thermals and socks are polyesther or poly-cotton blend. They should be no more than 30% cotton.

Also, take caution with that down bag. On a 6-day hike, if it gets wet, you won't be able to dry it in time for the next night you'll need it. Down compresses well, and is unbeatable for warmth - when dry.

It is up to you to take that risk, I'm just throwing that out there for you. I've seen people pack two bags, one down, the other poly - and both meant for warmer weather, thus allowing both to pack down more easily, and yet together offer better insulation. That's too complicated for my blood, but some people like that arrangement.

You mentioned the temps can range from 9C to -3C, which means, you could be crossing the melting point. When temps stay really low, your risk of getting wet is lessened; when temps are really high, your chances increase - but then you don't need the down. When you are at the melting point thresholds, snow turns to wet, whereas if it was really cold, snow just blows off.

  • Why the minus one here? – user2766 Dec 23 '16 at 20:10

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