I just recently finished making my own sleeping bag. So, let’s say you are like me and you want one as well. One of the initial things that you will want to do is gather your materials. Although it’s a lot more interesting picking out the fabric to use for the shell, the major material in terms of amount and cost is the insulation. Since you are building your own sleeping bag I am just going to assume that you will be using down feathers as your choice of insulation. I can not actually imagine why anyone would go through the trouble of making their own sleeping bag if they plan on using polyester filling as insulation. If you are that type of person, I suggest you just go purchase one of the ubiquitous synthetic material bags on the market. If you still want to do this, you might as well read this post as well, since it will at least give you some idea of what you need to think about for how much insulation.
The function of the sleeping bag is to provide long path of low thermal conductivity material between you, the thing being insulated, and the outside, the thing that acts as a heat sink. The way it does this is by trapping a large amount of air and changing the mode of heat transfer from convection to conduction. Air is a good thermal conductor when it is free to transfer heat via convection and a very good insulator when it is limited to only conduction. This is why sleeping bags can be so light; you are utilizing the air surrounding you as a thermal insulator by limiting it’s mobility. Down, it turns out, is very good at limiting convection in air without adding much additional weight. It also has the very desirable quality of being a resilient material that resists permanent deformations. These two qualities together make it ideal for outdoor activities. And of importance to the McChoppin’ mindset, it’s an old-timey material. You know, something that Shackleton himself could have used. 1
In the sleeping bag business world the lowest temperature that can comfortably be tolerated in the sleeping bag is normal equated to “loft.” Loft is the path distance between you and the outside, or, in other words, the thickness of the sleeping bag from the inside to the outside. The total thickness of the bag should be twice its loft. However, in determining things for making your sleeping bag, in particular, how much down feathers to purchase, we care more about equating the temperature to the weight of the down that we will need.
It turns out not all down is equal. It’s actually all equal from thermal conductivity stand point; down as a fill insulation is mostly air and therefore has essentially has the thermal insulation properties of air. The major difference in down from batch to batch is the density. The inverse of down’s density is expressed by down feather merchants as “fill power.” Because it is the inverse, a higher fill power is a lower density. Numerically, fill power is equal to the number of cubic inches 1 ounce of down occupies. I imagine they choose inverse density as the metric because, for the purpose of purchasing things, larger number means better to most people and therefore they can sell 900 fill power down for more than 700 fill power because most people would rather have less dense down.
The fill power rating for most down sleeping bags seem to range from about 650 to 900 fill power.2 As I said the only major difference is density. And since I’m not particularly concerned about the weight of my bag, I’m not terribly concerned about the fill power, other than cost. However, there does not seem to be much of a market out there for down feathers that is readily accessible directly to the individual sleeping bag maker. During my search I found my self pretty much limited to the 900 fill power end. And keep in mind that you pay for down by the ounce, but how much you need is dependent on the type of down you buy. Therefore the ratio of the fill power directly relates to the ratio of the price in down. In other words, let’s say that you find 900 fill power down that sells for 30 dukes for 3 ounces. You also find 700 fill power down that sells for 25 dukes. If you don’t care about the weight, which down should you purchase to reduce your cost? It turns out that you should go with the more expensive 900 fill power down.3 The 700 fill power down should be less than $23.33 if it is going to be more economical, assuming everything else is equal.
Alright, I’m going to assume that your pricing of the market is going to lead you to the same conclusion that it lead me to: 900 fill power down.4 So, how much do we need?
Well, I’ve gone through the little bit of trouble to get this equation that relates expected outside temperature to amount of down needed. It is , where T is the temperature in degree Fahrenheit you plan on taking you bag to in the outdoors and W is the weight of 900 fill down you will need in ounces to achieve this. A few thing recognize immediately is that at zero degrees you need 19 ounces of down. For every degree above that you need one third of an ounce less. For every degree below that you need one third of an ounce more. This equation also says that you need no down in your sleeping bag if you plan on going no colder than 60 degree Fahrenheit. That seem reasonable, as at 60 degree I can sleep clothed with little more than a light blanket.
So, that is pretty much all you need to know. Pick a temperature, plug it in, and that is now much 900 fill power down you need to buy to start making a sleeping bag. Go nuts. But there are a few more things you might like to know about the equation and what went into it.
First, if you don’t get 900 fill power down you can still use the equation. Just calculate the amount of down you would need if you were getting 900 fill down and multiple it by the ratio of the fill powers. So, if you needed 22 ounces of 900 fill down you will need 28 ounces of 700 fill power down.
Second, I made this calculation based upon my size: six feet four inches. If you are a lot different in size, than you will need more or less down. By “a lot” I mean that for this rough calculation, anyone less than 4 inches less than me in height will need at least 5% less down. A similar thing for your girth: I’m fairly skinny.
Also, keep in mind this is a rough calculation. The exact amount of down you will need is related to how much space you need to fill in your sleeping bag. Due to difference is construction and actual down quality you may end up needing a little more or a little less.
All right, all those caveats aside, you’re probably wondering where I got that equation from: using freely available data of the intertubes I did a linear fit to all the data points and got the equation. How good is the linear fit? What was the data?
The data I gathered from Peter Hutchinson Designs, Feather Friends,Western Mountaineering, and Thru-Hiker. The PHD results are easy: they already give 900 fill power down in terms of weight needed for a specific temperature. For TH, they give you a loft for a desired temperature. So I just calculated the total volume given the loft and my dimensions and divided by the fill power. Because WD and FF don’t give much information about the down that they use, I had to assume a fill power and correct for it to get everything into 900 fill power. Despite the fact that both WD and FF seem to hint at using a higher fill power, an assumed fill power of 750 seem to get the data inline with the PHD and TH results. FF and WM still seem to be pretty conservative with the temperature. But they all agree that no matter which down you choose you don’t need any at about 60 degrees Fahrenheit. Personally, I choose to more heavily weight the PHD and TH results for a couple of reasons. First, they were the first two data sets I looked at: blatant human bias. Second, they both went for a more conservative approach with the amount of down needed: I wanted to buy less down. And finally, I suspect I will need less down then the market that FF and WM are going after: total hubris. So, keep in mind that my equation is likely to be on the absolute low end of the amount of down that you’ll need. I don’t recommend getting less than it tell you. If you think you might be want a little extra thermal protection, get more down.
All right, so I was going to talk about how well this simple model for projecting the amount of down you need compares to an actual (but still relatively simple) thermal model of you, the sleeping bag, and the outdoors, but this post is too long already. For example, my equation predicts a number for sleeping in outer space. Which, for the sake of this discussion, is 0 Kelvin.5 Or -460 degrees Fahrenheit. It tells you you need some 170 ounces of 900 fill power down. At 30 dollar for 3 ounces, that equates to a cost of 1645 dukes. Down is by far the highest cost involved in a home-made sleeping bag, so we can use that as an estimate for the total cost of the bag. Western Mountaineering’s best bag only goes to -40 degrees F, but costs half as much! By the way, it turns out that that much down (170 ounces) would offer 3 feet of loft. I’d be in a sleeping bag that had a diameter as much as I am tall. But would that really work? I suspect not. But we will check it out in a future post.
1 Shackleton, if my memory serves me correctly, actually used sleeping bags stuffed with Reindeer fur. An intriguing idea, one that I’d be tempted to try, if it weren’t for the fact that during their 800 mile open-ocean voyage to South Georgia the bags got wet and started to rot. Shackleton and this men later complained that everything was covered in hair and that it often found its way into their mouths. It does not sound like a pleasant experience.
2 I will ignore the “+” part of the description of down because I feel that it is even more of a marketing ploy; if 650+ down was really significantly different than 650 fill power, why not just label it 700?
3 Unless you want a sleeping bag that needs less than 3 ounces of down insulation at 900 fill power. But I’m imagining that you aren’t planning on making sleeping bags for for pet canary, in the quantities that you will likely need we can ignore the fact that the economics depend slightly on much down you are getting.
4 Unfortunately there just doesn’t seem to be a whole lot of other reputable options. If you can find any I would be interested to know of them.
5 Also, nevermind that the major component of heat loss in outer-space is is radiative heat loss and not convection and conduction, which is what your sleeping bag can protect you against. If you want, you can imagine that your in an air filled bubble at standard pressure floating around in outer-space and you need a sleeping bag to keep warm. And please don’t bring up any other objections, after all I am a physicist.