Sleeping Bag Construction

A Sleeping bag can be a technical piece of kit. A down filled sleeping bag will be completely useless if it is lined with cotten that retains moisture.

When you are choosing a sleeping bag you should consider finding one with a construction that matches your actual needs.

Shape - Mummy / Square

A Square bag is typically only found on the very cheapest bags. It is also the shape that will hold the least heat and feel the coldest. For practical outdoor use a square bag should be avoided.

The almost standard shape for a modern sleeping bag is a Mummy bag. This has a shape which is contored to match the body with a narrow foot box and a wider area for the shoulders. Most mummy bags will also have an integral hood. This hood is designed to help keep in the warmth.

When shopping for a bag you should buy one that is big enough for you to sleep in when fully clothed but not so big that it feels spacious.

One thing that you should be aware of is that most mass-market sleeping bags are manufactured so that anyone can use the. In practice this means that they are built for a 6ft 6" gorilla. If you in contrast are a 5ft 2" waif then you are buying a bag that is too big for you. If you can buy a smaller bag then you will save weight. Some manufacturers such as Bozeman are now producing 'regular' and 'long' bags where 'regular' will be good for most people.

One modern variation on bag design is to have a zip baffle that can be zipped into the bag to add an extra few inches of girth. This is quite a nice design touch and can make a bag feel more comfortable if you are a restless sleeper.

I have personally found that the hoods on mummy bags are useless. I tend to move around when I sleep and so the hood never stays on my head. Instead what I tend to do is to tighten the hood up so that it forms a shoulder baffle (see below).

In hot weather the 'hood' will form part of my pillow. In cold weather I tend to find that a fleece balaclava is a much better option for sleeping.

Filling - Down / Synthetic / Fleece

These are the three main fillings. Your choice of filling will basically come down to the conditions that you expect to use your sleeping bag in and the amount that you are willing to spend. High performance down sleeping bags can be obscenely expensive for what is no more than 2-3 layers of pertex and some bird feathers.

Filling - Down

Goose down or duck down is the lightest and most luxurious of all the sleeping bag fillings. The main benefit of it is that it is very light for the volume of air that it will trap. This means that it is very warm for the weight.

I personally feel that down is needlessly expensive for what you are getting and that often the weight benefits do not justify the additional pricing. Of course it is your money and your call.

If you are buying a down bag then it is going to be worthwhile to buy a high-quality down bag and not bother with 'budget' down bags.

This year I bought a new 3-season bag. I looked seriously at down and at synthetic bags, at the weight and at the pack volume. A good quality down bag was going to cost me at least £200with £300 for lightweight performance bags wheras a good quality synthetic bag was around £80.

Down needs to be treated carefully. If it gets wet it will be basically useless. So you will have to keep it in a dry stuff-sack and have to use it as part of a sleep system where the bag is well protected.

When I looked at the actual weights of the various bags I found that down bags were weighing in at around 1200g and that synthetic bags were weighing in around 1400g. So that 200g saving in weight was going to cost me at least £120 and the pack volumes were not that different.

Filling - Synthetic

This is by far the most common choice for a sleeping bag filling. The filling tends to consist of some sort of random mesh of fibres that are usually hollow. These fibres trap air and so keep the bag warm. The fibres also tend to have some insulating properties of their own.

Synthetic is always going to be heavier than down and more bulky when packed simply because feathers compress down easily.

In practice, unless you are spending serious money the synthetic option will be better than down for most people. A lot of cheaper down bags use heavier outer fabrics and so the difference in weight and pack volume for a £100 synthetic bag compared with a £150 down bag will not be worth worrying about.

These days I would not worry about pack volume too much. When you can buy a 45/50L ultralight sack that weighs 600g - which is less than most ordinary daysacks then pack volume is not really an issue.

Damp synthetic bags are not much colder than dry synthetic bags and will work better than a damp down bag.

Filling - Fleece or Pile

Fleece bags are a little bit of a speciailist item. The main manufacturer for these is probably Buffalo. Fleece bags are very simple in concept - take a normal piece of micro-fleece or 'furry' fleece, add a windproof/pertex outer and you are done.

in favour of fleece is that it is bomb-proof and will stay warm even when wet.

Construction

This is one area where you pay a premium for quality. The simplest way to construct a sleeping bag is to take some insulating material and stitch it into a pertex case. This can mean that on every stitch line there is a potential cold-spot.

A more sophisticated construction method is to have two overlapping layers of filling and stitching so that there are no potential cold-spots. These features only tend to be seen on high-end and expedition bags.

	This is a 1-season bag that has a rip-stop pertex outer. The synthetic filling is very thin. This bag weighs 650g.
	This is a 4-season bag from the same manufacture. You can see that the synthetic filling is much thinker. The outer is rip-stop pertex. It is an entry level quality bag. This bag weighs 2Kg - over 3 times the weight of the 1-season bag. The only difference in weight is due to the filling. The fabric inner and outer elements are comparable in weight.

Outer and Inner - Pertex, Nylon, Cotton

One way that manufacturers can save money is by using cheap materials for the inner and outer linings. The penalty for this is that the bag will be heavier than is necessary and will retain excessive moisture.

In all but the budget bags the standard construction these days is to have a rip-stop pertex outer and often a pertex inner. Pertex is very light, is windproof and very breathable. A pertex bag will feel much warmer than a cotton bag and will shed sweat and condensation much better.

If you are not expecting rain and you have a pertex outer on your bag then you may want to consider whether or not you need a bivi bag at all...

I would not bother with any bag that uses cotton as a lining. Cotton is clammy when wet and will sap heat from you very quickly. It is also very heav when compared with pertex.

Features - Baffles, Zips etc.

The two key features are zips and baffles.

This picture shows a typical shoulder baffe. It is a separate piece of insulation that has a drawcord through the centre of it. In cold weather what you do is tighten the drawcord so tha the baffle fits neatly around your shoulders and neck. Shoulder baffles are I think essential in anything other than a summer bag. They make a huge difference to the warmth of the bag and will stop most of those draughts. A shoulder baffle and a nice close-fitting hood or a balaclava will keep you lovely and warm whatever the weather.

This shows a typical zip baffle on a 4-season bag. The baffle is the same thickness as the rest of the sleeping bag. The idea is that the baffle will stop draughts through the zip. This is only true most of the time. The warmest bags will have no zips. A bag with a zip but no baffle will be colder than a bag with a zip baffle

Pack Size

The two main things that will affect the pack size is the temperature rating of the bag and the filling.

Here you can see a 1-season and 4-season bag together before they have been compressed down to minimum size. The bag on the left weighs about 3 times the weight of the one on the right but has about 4-6 times the volume. The bottome line is that if I take the 1-season bag then I can use a 25L daysack and if I have to take the 4-season bag then my only option is to use a 45L sack.