UKH

Hi, I'm looking at using a down jacket as part of my sleep system.  

I'm doing the Everest Three Passes and need a warmer system. I'm loathed to buy a sleeping bag that goes to -15/-20 as i'll never use it again, it'd also be quite heavy and as I'll need to carry a jacket anyway it makes sense to use it to increase the temperature rating of the bag. 

I wear an old Rab summit jacket during my winter camps at the moment but looking to change that to the Mountain Equipment Kyros jacket for its pack-ability and weight. This is a goretex Infinium jacket, it's sold as 'highly breathable' and 'almost waterproof' but would it be suitable to sleep in. I wouldn't for example sleep in my goretex waterpoof because it wouldn't let enough body heat out to warm the bag. 

Any advice/experience appreciated. 

You can try sleeping in anything - I suspect your main problem will be fit and the possibility of the insulation in both bag and jacket getting squashed, and then your legs getting cold. I've woken because I have cold legs before, where the leg insulation in a bag doesn't work as well as over the chest. 

I really don't get this. If it didn't 'let enough body heat out to warm the bag' that's great, you don't then need a sleeping bag. You don't worry about your sleeping bag not letting enough body heat out to warm the tent do you? If it did, it would be a rubbish sleeping bag. Basically that's not how the physics of sleeping bags work.

When I was selling sleeping bags two decades ago the training advised against wearing too much clothing because it would 'stop the heat getting into the bag'. 
Doesn't make sense if thought about logically - but I still prefer to have little/nowt on in a sleeping bag.

Funny one this. The comments about just sleeping in light layers and letting the bag do the work are quite right, this uses the bags design as it's intended. However I get your point about bag weight and not being stuck with an overspec bag. I use a down jacket and down trousers in my bag in winter as I want a suit I can wear outside my tent at night. Whilst this system doesn't use the design of the bag from a thermal efficiency point of view, it keeps my pack weight down and gives me options.

I do run very cold and I use an x-therm pad. I bought a pair of down trousers from Cumulus down, really fluffy.

Interesting - but has to be wrong surely? It's not like a bit of body heat kicks off some magical exothermic reaction in goose down, that then pumps warmth into the bag. It's insulation, it keeps heat energy that is present, well, present. Goose down is not turning some other form of energy into heat energy that then warms you up.

I sold a fair few sleeping bags almost three decades ago now, but fortunately no rep ever tried to blag me with that interesting bit of 'physics'. The only thing I can think of that makes a tiny bit of sense along those line is that top quality down bags always use a silky very fine downproof nylon inside the bag. When you first get into a bag like that if you are not wearing much beyond your skivvies, it feels noticeably cooler against the skin than cheaper cotton or poly cotton liners in cheap bags. But that literally last a few seconds until you zip up, move a bit inside the bag and your skin warmth warms the fabric. I guess it is something to do with the specific heat capacities of the different materials? Or the denier size of the actual weave - so that you are touching more material on the fine silky fabrics than on rougher looser weaves?

But anyway, it's not really anything to do with how bags work over a night and obviously is only an issue if you are not wearing any clothes - clearly not going to be an issue in the OP's case!

But what do you mean by 'letting the bag do the work'? I'm now worried that I'm basing all my thinking on this on my GCSE Physics that I did in 1989 - although I did get an A when that was the highest mark - and I haven't understood something fundamental! But sleeping bags insulate - they don't create heat, they just keep as much heat produced by you, a live human being, inside the bag, not escaping into the wider atmosphere. If you wear lots of clothes inside a sleeping bag, the clothes are your first line of insulation, your body heat to some degree stays close to you because of the clothes. All the sleeping bag does it then keep in any heat that does 'leak' through the clothes inside the bag.

I've thought about this lots over the decades because you often here people say wear less in your sleeping bag to be warmer. My conclusion from both my understanding of physics and lots of very cold camping is that it's bollocks. The only way lots of clothes could make you colder if you are such a tight fit in the sleeping bag it compresses the insulation - i.e. imagine you are in a bag lying on your back. That sleeping bag over your chest uncompressed has the outer shell separated from the inner lining by say 4 cms - that 4cms of space is filled with down. If you wear loads of clothing your chest in effect get bigger - and it pushes the inner liner of your bag upwards, towards the outer shell. You then have only 1 cm of compressed down between the inner and outer shell. Compressed down doesn't insulate as well, and there is less depth of insulation. Your bag doesn't work as well.  But lots of winter bags are slightly oversized because they expect you to need to wear more inside. Can some physicist or sleeping bag engineer (is Mr Fuller still around?) tell me whether I'm missing something fundamental in my understanding of this? 

Yes this is why bacpackers are using Quilts, no point in having a layer of down compressed between you and your mat.

The only time I've felt any truth to it was a night where I was camping wearing jeans. My feet were freezing all night and only got warm when I took the jeans off inside the bag to get changed in the morning.

My "letting the bag do the work" comment was a ref to keeping the heat in, rather than it somehow magically generating heat so I think we agree on that.

This idea of wearing less in the bag does puzzle me and after some thought I can only offer theory. I wonder if wearing too many or certain layers in the bag make it hold sweat in the fibres causing the body to feel cold, perhaps less clothing allows the system to wick away the body sweat. I think your understanding of GCSE physics will be much better than mine!

The compression of down reducing the performance was a lesson learnt for me some time back in a hammock. Those things are freezing, regardless of how good your bag is.

Why is wearing a down jacket and trousers in the sleeping bag any different from using a liner bag to boost the rating, as promoted by PHD?

It would be interesting to see in test how a single bag with say 300g of down in it performs against 2 bags inside each other with 150g each. Which would be thermally better?

Assuming that neither bags insulation got crushed by the other, I would imagine two bags would be warmer due to it being able to trap more air, which ultimately is the thing that keeps us warmer.

Can't be certain but I'm sure that multiple layers is warmer than one thick one.

When in the scouts (40 years ago) i remember often being told that it was best to sleep with minimal clothing. Even to my teenage mind that didn't make sense, so I took it to be ensuring that we removed any sweaty/damp clothing before getting into our sleeping bags.

That does make sense.

Wow. Quite a hot topic (pun intended) 

Some really interesting considerations, thank you. 
 

I do intend to wear down trousers too, as someone mentioned, these will be for wearing in the evening in unheated tea houses and then back here during winter camps. 
 

lots to mull on  

Or... People putting this inside the down bag https://wildbounds.com/products/inertia-x-frame-camping-mat-yellow in addition to a normal sleeping pad/mat underneath sleeping bag

I've been to Nepal many times, most recently in Oct and I have done the Three passes route. With regards to a sleeping bag, I understand the logic that you won't use it after the trip so I would hire one for the trip and it will be a much more comfortable and simpler experience. You can hire in the UK or Kathmandu.

You didn't mention when you are going but I note you plan to stay in tea houses, its never that cold that you would need down pants.

Someone in the office has just flagged that my name was mentioned in this thread, so here I am…

In answer to the original question, I’d have no concerns about wearing a Kryos jacket inside a sleeping bag. It’s something that all of our sponsored team do on their trips and it’s something we’d recommend to boost the warmth of a sleeping bag. Toby’s right that the main thing to consider is if the sleeping bag fits tightly. If so, then you’d be better off laying the jacket over the top of the sleeping bag than wearing it inside the bag. You’re also right to consider some insulated pants: wearing five jackets and two pairs of legwear – like plenty of people do if winter climbing – is like putting a foot of insulation in your loft and then leaving your windows open downstairs. That's been one of the biggest things we've learnt from nearly a decade of environment chamber testing.

On the ‘two sleeping bags or one’ question, it depends… Generally, you’d be better off with two bags, because as Schmiken says you’re going to trap more air. However, if it were two stitched-through sleeping bags – which they might well be with 150g of down in – and the 300g bag was boxwall or similar then there might not be much difference in warmth at all. Also, few sleeping bags are designed to be used as ‘pairs’ and aren’t sized with that intention, which would again cause issues.

The clothing-inside-the-bag myths are weird – they seem to come back around every now and again. Wearing more clothes inside a bag is always a good way to keep you warm assuming that the clothes are dry, and that they don’t make the bag too tight. The source of your warmth is the human, and it’s the job of your sleeping bag/clothing to prevent that warmth leaving too quickly. Dry clothing is >25 times more effective at insulating than wet clothing; if it's wet it is not only not as warm but also requires energy - from you, the heat source - for it to dry.

Dr Matt @ Mountain Equipment

It's always a lovely warm feeling of smugness (probably enough to warm up a sleeping bag) when the experts tell you that you are right. Cheers Matt!

General question: wouldn't it help to put the sleeping bag inside a breathable bivvy bag, assuming you're in a cold room or a cold tent?  It must make some difference.  Pretty sure I did once somewhere.