DIY - My Designs - Sleeping Bags

Please note: this page is a work-in-progress!
Send any questions or comments to the designer Roger Caffin


Sleeping Bags: warmth

Remember the key point about being warm at night: the warmth comes from you, not the bag. All the bag does is slow down your heat loss. This heat loss will depend on two major factors: how good is your metabolic rate tonight, and how cold is the surrounding air relative to your skin. The only other really critical element is your head: if you have that outside your sleeping bag losing heat at a great rate, your nervous system will let the rest of your body freeze before your head even starts to feel cold.



Quilts and Bags

Two things do fascinate me about sleeping bags and typical use. The first is how people are quite happy to sleep under a quilt at home, but seem to think a bag is essential when in a tent. OK, I used to act that way too before I started to really think about it. Why put all that lovely down under your head and under your body to be squashed flat? It simply does not make sense. You don't do that at home: why do it in the bush? These days I use my sleeping bag as a quilt. I put the hood on top of my head, loosely, with the rest of the bag spread out over the top of me, and don't bother to do up either the drawcord or much of the zip. Lot's of room, and warm as toast - just like at home under a quilt. OK, I do have my feet in a bag section at the bottom. I use a bag around my feet because they are not on the nice warm 3/4 length air mattress, and anyhow this anchors the rest of the bag in position over me.

The other thing which fascinates me is how everyone buys a heavy 4-season winter bag with 800+ grams of down in it when they usually only go walking in the warmer months of the year. Then they either sweat most uncomfortably all night, or sleep on top of the bag anyhow. I guess there is more profit in selling an expensive winter mummy bag compared with a light summer bag.


Designs and materials

There are several sleeping bag designs to choose from, some of which are illustrated to the right:


Marketing Myths

Along with these traditional classifications there are several myths about sleeping bags. Let's have these out here now because they influence my designs. Be aware that I may be a little biased about marketing 'spin'.

Why do our Australian manufacturers include many of these useless 'features'? One could be very rude, but I suggest it is a combination of several factors. The marketing guys tell the manufacturers it will boost their sales; the higher prices are popular with the shops which thereby get increased levels of profit; and the customers are conned into paying for them by the marketing spin. Sadly, it seems that anyone who starts making good gear ends up being hypnotised by the lure of increased sales, and never mind the original goal of 'a premier product'. Here endeth the sermon.


Requirements for Design

So, what good things should go into a quality bag design? I list here some criteria I believe to be 'the right stuff', with my explanations. I have also added comments about the general state of the market. In doing so I am focusing on the top-of-the-range gear, and ignoring the cheap end of the market.


Commercial summer bags

I mentioned the Mont Nitro bags in the main Sleeping Bag section. With 150 g of down they have gone down to 0 C (yes, freezing point)and kept us just warm enough. There was some frost around in the morning. We were wearing thermal tops at the time, and my wife was also wearing her thermal longs. In this context it is worth noting that many women sleep 'colder' than men. Maybe men have some use after all? Yes, we were snuggled up together too. We were sleeping on Therm-a-Rest Deluxe mattresses at the time, with spare day clothing under our feet, along with a small bit of 2 -3 mm closed cell foam I carry for the foot end of the tent. However, this was in a single-skin tent which was not sealed against all drafts. We were at the bottom of a valley, and you know how the cold night air flows down valleys. Anyhow, this suggests that we just don't need 800 g of down in the summer time.


Ultralightweight Prototype

The trouble with the Nitro bag is that it weighs 910 g with only 150 grams of down. This means the fabric shell weighs 910 - 150 = 760 g. Unfortunately, all the commercial summer-weight bags I have seen appear to have 600 - 750 g shells even when they have only 150 g of down in them. This seems way out of balance, doesn't it? Surely we can do better?

Well, more modern materials such as Pertex Microlight fabric are available and are much lighter, while still being down-proof. Yes, they are slightly dearer than the cheap Asian fabrics. Using Microlight I made up a shell for a fairly conventional design of bag roughly similar to the Nitro: more or less 'tapered' but with a 3/4 length zip and 20 mm walls, shown as A to the right. The shell weighed about 250 grams. Yes, 250 g, instead of 650 - 750 g. The zip is a #3 coil rather than the great big clunky #5 or #8 toothed jobs found in commercial bags. Into this I put 300 grams of 800 loft down for a total weight of 550 grams. We found this worked to about -5 C - with thermals and snuggled up of course. I made this design for my wife.

I made a slightly different design for myself, as shown in B. In conventional terms the zip is in the middle, compared with current Australian designs which have the zip at the side. This is not that radical: older sleeping bags used to do this all the time, and we still have a (heavy) NZ Fairylight bag like this. But I use the bag as a quilt and sleep under it. The zip in general is not be done up; the bag is spread out like a quilt over me. The edges of the hood are slightly flared out to give better coverage of my head: this could be simplified. This bag weighs the same 250 g as above, and has had the same 300 g of down put in it. The little rectangle at the left end denotes a small foot end wall: this was included as an experiment. yes, with a thermal top and snuggled up, I slept down to about -5 C happily.

Note that both bags are shown opened right out, before the lowest 1/4 length is sewn up. There are some heavy red lines at the top of each pattern: these represent the 3/4 length zips which are included. There is a drawcord around the hood in each case, but mainly as a conservative token gesture, 'just in case'. The string was very light - about 1 g total.

We have used these bags down to -5 C around Kosci, and we used them for 8 weeks in the Pyrenees in Europe. The 800 loft down is still fine, and so is the Pertex Microlight fabric. In fact, this bag would be adequate for many walkers right through the winter. Whether it would be enough for ski-touring - well, that remains to be seen. I could of course add Polartech longs and wear my duvet jacket - I would be taking the duvet anyhow. Hum ... maybe. The #3 coil zips have worked just fine, and have not given any trouble. I didn't add the conventional heavy 'anti-snag' tape along the inside of the bag next to the zip, and had no trouble. Yes, I did catch a little bit of fabric occasionally, but the coil construction is very smooth and the fabric is very light and quite slippery. I was able to gently ease the zip back off the fabric with no damage at all every time. The secret is to treat such gear with a little care and love.

Actually, I didn't put the down in these bags myself: that is a rather frightening task! I had OnePlanet do it for me. If you look at their web site you will see 'Custom Bags' under 'Sleeping Bags'. They mention here that they can stuff bags for you, with 600, 700, or 800 loft down. They gave good service.


© Roger Caffin 6/6/2002