Complimenting a sleeping bag with extra insulation

I'm planning to hike the Great Himalaya Trail this summer (April - August). I'll be camping between 4-5,000m (13-16,000ft) for 23 nights, and between 5-6,000m (16-20,000ft) for 9 nights. I'm expecting -15C (5F) or maybe even -20C (-4F) on the coldest nights.

My plan was to take my Z-Packs 20F (-7C) sleeping bag and also a down jacket, pants and socks... On the coldest nights I will layer up inside the sleeping bag. This means I have the flexibility to wear the extra insulation layers around camp and my sleeping bag is nice and light.

However, my hiking buddy thinks I'm mad, and perhaps they are right?

Has anyone deliberately under-specified a sleeping bag and complimented with extra insulation layers? How did it work out?

Don't wear too many layers that compress the insulation of your bag but your technique is fairly common. I'll also drape extra layers over the sleeping bag occasionally, particularly over my feet.

I'll add that you may want to test this out beforehand and see what works for you. 32 nights in bag with untested hypotheses seems to carry a lot of risk. An uncomfortable night or two might be worth the weight savings but several or 20 I'd probably look at a warmer bag. Also don't neglect your sleeping pad, lots of heat loss through the earth.

Sleeping bag ratings are pretty hit or miss, with the advertised rating often not being very representative of what the bag can actually handle. I've had some 20 degree bags from a few different manufacturers that left me pretty uncomfortable even in the mid 30's. Depending on the quality of your bag/ground pad combo and your other insulating layers, it MIGHT work, but it would be a bit of a gamble. I'm sure under most conditions it would keep you alive, but if you are planning on 30+ nights in a bag you want to make sure you can get some sleep (especially if you are factoring altitude as well).

If I were you, I would take you setup and head out for a night of car camping in the coldest conditions you can. Pitch a tent somewhere where you can sleep for a night or two at 0-5 degrees. You will know around 3 am if your system works.  

Sleeping bag rating come from the U.S. Army. The rating means a soldier will live, it has never meant you will be warm or comfortable, it just means you should live to fight another day.

test your gear. i wouldn’t want to bring a -20 if i could get away with a 10 degree bag. you could pack and probably should pack an emergency bivy just in case, it will keep you way warmer than extra layers but i wouldn’t want to supplement a month in the wrong bag. 

I’ve bivied around 5F in a 20F Nunatak quilt. I’ll sleep in my all my layers other than my shell.  Having a warm pad is a big help. The Thermarest XTherm neo air is awesome. Wind can also be a big concern. If you have a shelter you should be pretty good. All this being said, test out your system for a bit. There’s a fine line between regular discomfort and hypothermia. My system has worked for me on shorter trips, not huge trails. I’d wear at least a down jacket, fleece, and possibly synthetic jacket too for bed at that temp. 

petzl logic wrote: test your gear. i wouldn’t want to bring a -20 if i could get away with a 10 degree bag. you could pack and probably should pack an emergency bivy just in case, it will keep you way warmer than extra layers but i wouldn’t want to supplement a month in the wrong bag. 

^ this. Also, don't underspec your sleeping pads, the ground will conduct heat away faster than air. I take a closed cell foam as well as an insulated inflatable for snow and cold conditions. The foam pad can also be used to line your backpack for rigidity and as an emergency bivy pad on summit day.

"You're a nice sleeping bag!" There, that's a compliment.

A good sleeping bag liner can make a huge difference, like the difference between a 15 degree bag and a 5 degree bag. Using a sleeping bag liner and sleeping my layers I've been pretty toasty inside my big agnes 15 degree bag even in sub zero temps. 

Warmth of your tent is a big deal. I’ve voiced in the open in my 17F bag around 20F and wasn’t very warm even with all my puffy layers. In a tent the outside temp is irrelevant if the indie of your tent is warm. In a direkt 2 or similar mountaineering tent it’s easy to get away with a lighter bag as poorly ventilated tents will warm up inside (and get wet). 

Concept is pretty normal, look at the feathered friends vireo. The key though is that the bag is cut in a way that lets your puffy and other layers loft fully. A lot of bags won’t be large enough for a parka to loft.

Compare the shoulder and hip girth numbers from z pack to the vireo and you’ll have an idea whether your parka can loft up 

Mark Frumkin wrote: Sleeping bag rating come from the U.S. Army. The rating means a soldier will live, it has never meant you will be warm or comfortable, it just means you should live to fight another day.

No. 

To answer the question, it really depends. If your a larger individual you might already take up all the room in the bag if your on the slimmer side (like I am) you can easily get a full puffy on it also depends on the cut of the bag some are super restrictive while others have tons of room, some bags like the feathered friends viero and the patagonia hybrid bag and do lots of options for spacious over bags and half bags that are made specifically to allow space for a huge puffy jacket. If you are lacking in space on the inside you could drape something onto the outside of your bag but this probably won't be particularly effective and will shift if you move at all in the night.

The sleeping pad is just as important (high R-value) as the bag as well. A liner makes a big difference too.

I like silk for a liner. On cold nights, a bottle of boiling water placed at your feet. 

I totally understand with the comments about sleeping pads. I'm planning something a bit different for the sleeping pad... I'll take a the NeoAir XLite (R=3.2) and also the Z-Lite SOL (R=2.6). This will weigh a good deal more than an XTherm on it's own, but has a lot of benefits:

* Can use the Z-Lite Sol as a camp chair outside the tent
* Combined R value above XTherm (5.8 vs 5.7)
* Much more resilient to punctures

I'm going to try to mitigate the weight issue by trimming the Z-Lite SOL down to the size of the NeoAir. Hopefully that'll save quite a bit.

You're absolutely right - I should test this system in cold weather before starting a long trek like this. I'll be doing that soon and I'll let you know how I get on!

Aah just realised that's exactly what @hikingdrew was recommending.

BTW the tent I'll be using is a HMG UltaMid 4 with the full insert (with floor).

If anyone's interested - here's my full gear list: docs.google.com/spreadsheet…

Have you slept with the Zpacks bag on cold nights?  I think I run colder than most people but at 20 F in the winter, I need two down jackets and a vest, plus a 15 F liner to keep me warm ( I tested it last weekend in the Sierras).  I feel that the zpacks doesn't trap heat like a mummy bag.  I also threw in a Nalgene bottle full of boiling water in the bag too. I love my bag for summer thru hiking trips, but it does not hold up at 20 F, unless that is a survival rating.

Boil water before you go to bed and throw it in a nalgene and then into your bag to pre-heat the bag.  I'll also zip my rain shell over my feet on extremely cold nights. Prevents any condensation or frost from forming on the shell down by the foot box. I'll also drape my extra layers over the top of my chest. Wear your down jacket with a hat and the hood, works fine for me. Z-lite and then an insulated pad on top works wonders for preventing the conductive heat loss to the ground. 

There are tons of way to squeeze more warmth out of a bag, all of which are great if you want to save ounces on a climb. (Infact, I personally only have 10F and 30F bags.)  None of which are anywhere nearly as comfortable as just having a warmer sleeping bag. 

For you, are you certain you want to spend a whole month with an underrated bag with a bunch of layers and hot water bottles? 

As mentioned above, boiled water in a Nalgene bottle is my go to trick on cold nights.  Wrap this in a jacket or wool hat to trap the heat longer and also protect your body from the heat. It can be too hot to handle!

It's usually still warm in the morning.