Extending the range of your sleeping bag

My experience tells me that much is to do with how much room is inside the bag you are working with. if the bag doesn’t have enough room to allow the clothing to loft and expand to its maximum then you are chasing your tail trying to extend its range downwards by wearing clothing inside it. Now I have to clarify here and say that I always buy a sleeping bag in Expedition cut to allow for the use of high loft clothing inside and that for summer use I have LW synthetic bags.

If you have a skinny cut bag [ and so far all the UL bags I have experience with are very narrow to get the weight minimised] the only real way to extend the range is with an overbag and because an overbag is; by its very nature large the weight is going to be correspondingly high compared to a sleeping bag with the correct temperature rating

Secondly for me personally it is my legs that get cold/lose heat rapidly and an UL half bag is warmer than insulated pants and sleeping socks but in very cold conditions I will use a half bag plus insulated clothing. If my legs are not losing heat I need less insulation on my upper torso

Also I now sleep very cold but that is age related.

Common wisdom here is that for every decade after 35 YO you need an extra 5 to 8C added to the lower temperature rating of your sleeping kit, so my -4C climbing bag [ rated -30C with clothing] is now only good to 5C/8C as a conservative rating and why I always use my Goretex Bivvy with it.

@Martin, the plan is to get the jacket first, but I’d like to try it this autumn so I have to be quick :)
Also, the option of combining a jacket with a sleeping bag makes sense, but I don’t want to depend too much on a particular jacket size/model + sleeping bag combination.

@Edward, thanks for your reply! Bag size is an issue of course as well – although this will be hard to estimate – depending on which clothes I chose for which activity.

For example, if I know I will be standing around taking pictures all day, I’ll have to take a heavier jacket than if I know I’m very active – e.g. ski touring. So it makes sense to have a bit of room in the bag rating.

At the moment I’m looking at the WM Lynx MF as a sleeping bag, Rab Electron as a medium winter down jacket – the Lynx might be overkill for my purpose, but I need to start testing somewhere.

What I’m unsure about is a VBL – if I use one to extend the rating (and for example go with a WM Antelope) I cannot use the down jacket in the VBL. I would have to take individual pieces, that I do not like to carry.

You can use a VB bag and still wear a parka, you just stop the VB bag at the waistline, better tho may be to also wear a VB shirt and a VB halfbag or do what we did in the 60s Wear our non breathable waterproofs under our down jackets and then hop into our very marginal sleeping bags.

VB liners work but they are perhaps less flexible, I suggest trying it first by using a big rubbish bag costing a few cents before rushing out and buying

“What I’m unsure about is a VBL – if I use one to extend the rating”

Remeber that using a VBL doesn’t extend the rating of anything, since it doesn’t have a measurable R value. It simply restricts your body’s vapor from pushing through your sleeping system, and freezing somewhere in between. Of course one can still get condensation in their system, simply due to relative humidity in the surrounding air, and a reasonable temperature difference.

While I don’t have personal experience with them, I understand that using a VBL is only a consideration for sleeping in <i>very</i> cold temperatures.

Matt I’ve used them, they do extend a bags rating; I have no idea why; perhaps it stops evaporative cooling to some degree but wearing insulated clothing works better because it is thicker.

VB liners are more comfortable at lower temperatures although the lowest I have trialed them is -25C

If you’re in the winter Alps anyway, like Christoph, you might want to take an emergency bivy.

Mountain Equipment does one that weighs 110g – and people have used it as a makeshift VBL liner, reportedly with success.

Rain gear, which I presume the OP would be bringing, can make a good multiuse quasi VBL.  Worked for me last month in the sierras anyway.

I often wear my Patagonia down sweater inside my down sleeping bag. But, I have discovered from hard experience that if you go to bed cold, the sweater holds in your heat. That makes it hard to warm up the sleeping bag. So, what I do now is start with the sweater off, but keep it inside my sleeping bag. I can put it on later as the night temperatures drop.

I carry a separate set of sleep socks. These can be light weight hiking socks or special loose-fitting socks for colder weather. The possum down socks from Z Packs are great. If my feet are warm I am toasty.

VBL adds about 5F by eliminating the evaporative cooling of passive perspiration.  This, in addition to the benefit of keeping the evaporated moisture out of your loft where it may condense.

If you’re not familiar w/ passive perspiration – our skin isn’t actually 100% water proof.  Even when at rest, and not actively secreting water through our sweat glands, water still evaporates out of our skin.  A VBL creates a micro climate close to the skin where relative humidity reaches 100%, thus halting this evaporation.  And that 100% relative humidity is why you don’t want any insulation inside the VBL.  If you do, the air in that insulation will also be saturated with water vapor and the moment you leave the bag, the temp inside it will drop, causing all that humidity to condense.

VBL’s work at any temp, but you only need to keep the vapor out of your loft in colder conditions, hence the association w/ lower temps.