What is a good emergency-overnight-sleeping-setup when every ounce counts, like on long technical routes (grade V or VI) where the goal is to do it in one day (without hauling), but one wants to prepare for the unexpected? Building a fire is not an option.
I am interested in answers for all seasons, elevations, and most likely weather conditions.
I want to prepare for this situation, so I won't have to use the advice given in this question.
The most obvious thing is an emergency blanket. It will add a lot of extra insulation per gram. It'a good to have one in you bag on any trip.
However, a mere blanket is definitely not enough for all seasons, elevations and weather conditions. When planning at home, you should ask yourself a question: "What will happen to me if I have to be on the route overnight?" Than take this much insulation that will make the answer not worse than "no permanent injury".
While at home, think of where (at which points of your route) would you sleep if something happens. Good shelter is very important, because wind and elements take a huge amount of heat from you. On top of the mountain is the worst, near a wall is better, between 3 walls (rocks) is even better, snow cave is the best (so learn how to dig one, if you are going very high up or in winter).
Another lifesaver is water boiling gear (I'd recommend JetBoil), it also has a very high rate of heating per gram (compared e.g. to clothes). Of course you'll need water to boil, so know where you can find it or take some extra.
I always take a fist-sized SOL emergency bivy bag and a couple of strong black garbage bags. That way you can stuff food and even your body in the bags when conditions are cold and wet. I have also converted a garbage bag into a spare insulating clothing layer by tearing holes for arms and legs.
Essential for climbers are a whistle for signalling and a square of aluminum foil for signalling people on the ground or in the air.
Take spare alkaline batteries as a strategic energy reserve as alkaline performs better than Lithium in cold weather. If an overnight bivy is planned, you need a spare flashlight as well.
As far as food/water, I don't think you can get around bringing spare food and water unless there's snow/ice, which can be melted into water. In that case, a Jet Boil is a good option but it's not a very elegant option (not compact and need a lighter/matches, which you disqualified in your question). Another option is to collect dripping water off of the wall into a water bottle and filter, but that's not a guaranteed option. Similarly, collecting dew is an option, but that doesn't sound practical either. In mountaineering, there may be no running water at high elevations.
These are all besides for the 10 Essentials. However, these solutions are specifically directed towards the emergency-overnight possibility.
The most pressing points are good ground insulation(mat to lie down on) wind protection and then dry clothes. We lose 80% of body heat through the ground. Evaporation of sweat or humid clothes cool at an extremely fast rate too. The amount of heat transfer depends on the evaporation rate, however for each kilogram of water vaporized 2,257 kJ of energy are transferred.
Remember to bring a good hat, that covers your ears and cheeks. Always makes a big difference. You might not be comfortable like with a sleeping bag and tent, but you'll get through the night.
Expecting rain
When you expect rain things get more complicated. You have to stay dry at all cost, else you lost the battle against the cold. I never used a tent, just a rectangle piece of sturdy fabric, about 2 meters by 1 meter. I wax it regularly. Construction possibilities are many, depending on conditions, especially wind. Standard position is this:
You don't need metal poles, just use some sticks. As a habit from military I keep my tent as low as possible when I am only confronted with wind, so I can close the sides with some equipment, rocks, or vegetation. Imagine the previous diagram with the metal poles/wood sticks shortened and moved to the right 1/4 of the fabric length. When it rains hard, steeper tent sides will make it more efficient in throwing off water. You might consider bringing an aluminum field shovel to dig a small 'canal' around your Bivouac. If it rains hard without this canal bringing the water away, you will find yourself lying in a swamp quickly.
I would use a simple emergency bivy bag, a butt-pad (short sleeping pad) (thermarest, exped), some extra socks, buff and then you can sleep in all your clothes on the pad in this bag. this should work for emergency, it's not the best comfort but it works in 3 season conditions.
For more comfort you can use a light silk liner in your bivy bag.
All this together is quite light and not to big for a small backpack.
I think anaheim brought up a lot of good points, but I would choose a super light biwi bag that is able to keep you dry whether the humidity comes from the inside (sweat) or the outside (rain). If I take an insulation mat with me depends on existing possibilities to use it. If there are no pedestals you can lay or sit on, I would leave it at home.
I used to sleep some times in the Belgium Ardennes which have a very mild climate. For size, weight and especially cost reasons I used agricultural black plastic (don't know the official English term). I used it like a tent replacement; I had a simple sleeping bag but in case of emergency it can be used also as replacement for that (depending on the temperature).
Advantages:
Disadvantages:
Your needs will vary depending on expected temperature. Your provisions for a 35 C night and a -35 C will be drastically different. Ultralight gear will help, but can only go so far. Whether or not you sleep warm or cold will determine how comfortable you are, as well as your age. The older you are, the more warmth you'll need as your metabolism slows.
An emergency bivy is fine if you expect warm weather. For colder weather, you'll need a mat (usually 1 lb for winter) and an R value that matches the climate. Winter mats need to an R value of 4 or 5, depending on your body.
What do you expect to happen? In winds above 40 mph, you'll need a winter tent (ultralight winter tents start at 3 lbs). Summer tents are usually less than 1 lb. If you expect persistent precipitation, you'll need synthetic insulation. A down sleeping bag rated for 10 C will be under 1 lb. A synthetic sleeping bag rated for -20 will be 2 lbs or more.
