Why are sleeping bags the way they are? Humans must have been making sleeping bags for thousands of years, right? You’d think so, but it ain’t so! My name is Bjørn (which means bear in Norwegian) and I’m a designer and gear geek here at Norrøna and today we’re taking a closer look at some of the details to consider when designing a sleeping bag.
The sleeping bag, an ancient invention. Or is it?
The modern sleeping bag was invented in 1876, patented by mail-order pioneer Pryce Pryce-Jones. And the world’s first commercially produced sleeping bag with an insulating filling was actually made in 1890 by a Norwegian company called Fuglesangs Sønner A/S. Who? Indeed. The bags were marketed under a different name which you might be more familiar with; Ajungilak.
And here’s yet another fun fact you might not know: we at Norrøna have actually made sleeping bags before! Way back in the 50’s and 60’s we made sleeping bags for the casual camper. Then we had a total makeover in the late 70’s, shifting our focus towards higher end gear for professional outdoor people, and in 1979 we debuted our new down bag range. We designed the bags ourselves, but can you guess who produced the bags? Here’s a hint: the world’s oldest sleeping bag manufacturing company, which also happened to be Norwegian. This range was only produced for a handful of years though, as our manufacturer (who shall remain nameless) deemed the bags too good and as such too much of a competitor to their own brand of bags. We can only take this as a compliment, showing that our focus on user driven and functional design really does make an impact. And now, 40 years later, we truly believe we can make an impact yet again.
Overall, the general construction of sleeping bags these days is quite similar to the bags of yore. So what’s new? What’s left to improve upon? Well, it’s mostly in the details. Simply put, a sleeping bag can be broken down into three parts; fabrics, fill, and construction. After you’ve chosen fabrics and fill, all you have left to play with is the fit and the construction. Our goal when designing a sleeping bag is to combine these elements in the most efficient and sustainable way as possible. So let’s break it down and look at each step of this process.
There’s roughly one point three bazillion different fabrics one could possibly choose from when making a fabric based product. But, luckily for me, there are certain criteria a sleeping bag fabric must meet in order to be usable, which limits the selection at least somewhat.
Here’s a brief list of the most important criteria that are relevant when searching for a sleeping bag fabric:
- Down proof
- If using down and feather as insulation fill, both the lining and the shell fabric has to be down proof. This means that the fabric is so densely or tightly woven that the tiny yet sharp down feathers can’t poke through the fabric and escape, leaving you with an empty two layer body sock, bereft of any insulation value and totally useless when it comes to keeping you warm throughout the cold, dark night.
- But, and of course there’s a but, the fabric can’t be so tightly woven or coated that it won’t let any air or moisture through. This would be like sleeping in a plastic garbage bag, and let me tell you, that ain’t comfortable. For instance, the average adult human loses about 0.5 liters of water during the night (mostly by breathing and sweating). If you were to sleep in a totally waterproof plastic bag, you’d be soaking wet by the time you woke up. So, breathability is key.
- Yes, as I just mentioned, breathability is key. We need to find a fabric with just the right amount of breathability. Both the shell and the lining fabric needs to be breathable in order to let moist air out so that the bag can dry after use.
- The fabrics also need to be breathable so that air can come into the bag so that it lofts up and becomes soft and fluffy (and hence warm), and then in turn so that the same air can escape when you want to compress the bag into its transportation vessel.
- Most of the weight of a sleeping bag comes from the insulating fill material, but that’s mainly because over time fabric mills have been able to weave lighter and lighter fabrics. Most lightweight sleeping bags these days use fabric in the 10-30 denier range, which is incredibly light.
- So why don’t we go even lighter? What about choosing a 7 denier fabric? Well, we could if you the customer wants to pay for it. But most of you aren’t filthy rich, so we do have to restrain ourselves The lighter the fabric, the more expensive it gets. Or weaker. Or not yet available in high quantities. Or not made using sustainable yarns. You can’t check all three boxes at the same time, unfortunately. Not yet at least. Material technology is the future of most outdoor gear. You heard it here first!
- Durability can be split into two: tear strength and abrasion resistance. If you want a fabric that’s abrasion resistant, has high tear strength, is lightweight, cheap, and sustainable, all at the same time, well look no further, it’s called Imagination-Tex and doesn’t exist. So it’s all about finding the right balance.
- And then there’s the question of colors. Different fabric suppliers will have different MOQ and MOC for different fabrics, meaning you have to order a certain minimum amount of fabric per Color for the fabric supplier to bother making it. This can put a limit on the amount of colors you can use on a product. A production run of a sleeping bag using two colors might reach MOQ (total quantity of fabric), but not MOC (quantity per color). When choosing colors we therefore might have to consider choosing materials and colors we are also using for other products. But there are also other considerations we need to take when choosing the colors. For example, a rich and deep color might bleed onto other fabrics or even your skin if the fabric has poor color fastness. But a light color will get dirty and be too transparent, and for some reason a lot of you don’t like seeing the down inside your down garment. Now, please note that a dark colored fabric will get just as dirty, you just won’t see it. Out of sight, out of mind, eh?
- What material is the fabric made of? Where does this material come from? How is it made? Can it be used for something else later? Does it last a long time before getting worn out? Is it treated with chemicals? So many questions! Both the lining and the shell fabrics on our new Falketind bag are recycled and traceable all the way back to the raw material. Our goal is to make the mostest sustainablest bag of them all.
Down is more than just down. The most common type of down is water fowl, which we are using. Our down is 800 FP with a 90/10 blend. So what does this mean?
The blend ratio means that there’s 90% down clusters and 10% feathers. Down clusters look like tiny star-shaped balls of fluff, great at retaining heat. And feathers look like, well, like feathers, and they help the down clusters with some strength, loft and sticking-together-ness. 90/10 isn’t actually any warmer than 80/20, it’s just faster to fluff, yet more reasonably priced than the much more expensive 95/5.
800 FP means the down has a fill power of 800, which in turn means that one ounce of this down can loft and sustain a volume of 800 cubic inches. So in theory, 100 grams of 400 FP down is just as warm as 50 grams of 800 FP down. Or to put it differently, the higher the fill power, the less down you need to be just as warm. Higher fill power down is also inherently more compressible, so for the same weight of down you’ll have a warmer bag that packs down smaller when using higher fill powers.
What about 800 FP goose down vs 800 FP duck down? 800 is 800, so technically there’s no difference.
What about white vs grey down? The difference is the color. So why are products using white down more expensive? Because you have to pay someone to separate the pure white down from the grey.
What about synthetic fills? Down is lighter and more compressible. I could easily write a whole blog post on just this topic. Actually, I might, so I just leave it at that for now.
Want to know more about the specific down used in your actual bag? I mean literally the bag you are holding in your hands (after you have bought your new Falketind bag, which you should, but only if you actually need a new sleeping bag)? There’s a QR code on the hang tag. Scan it with your mobile phone and you can trace the down in your own bag all the way and find all sorts of data on the down used. It’s actually really neat, so check it out!a
So, we’ve chosen the lining, the fill, and the shell of our insulating tube. All that’s left to do now is to put it all together, and voila, we have our sleeping bag. Here are just some of the things we have to consider when putting the pieces together:
- Profile shape of baffles
- The baffles are the chambers which hold the down in place. Changing the profile will make the bag more or less efficient in terms of either weight or temperature, but you can’t have both. Any other shape than the good ol’ fashioned rectangle might be slightly more thermally efficient by creating overlapping segments of down, but will also use more fabric to create these overlaps, in effect making the bag warmer but also heavier.
- Baffle shape, size and layout
- Again, a large, straight and simple baffle will weigh less, but a smaller or curved baffle will be better at keeping the down in place. Too small, and there’s not enough room for the down to loft and provide maximum insulation. Too large, and the down will not keep an even spread throughout the baffle.
- Fit and size a sleeping bag
- We don’t want you to have to sacrifice on comfort so the Falketind sleeping bags are roomy. Especially the thicker and warmer bags, they’re even extra roomy! We wanted to make sure you could change underwear or get dressed inside your warm and cozy bag, and not have to freeze half to death before you got your shirt on in the morning. But we can’t just make the bag huge in every direction, or else the thermal efficiency drops. So the Falketind is roomy in just the right places, yet has a distinctive contoured fit around the shoulders and hood, just like on our puffy down jackets made for cold weather mountaineering.
- Even though the Women’s bags are shorter, they still have the same amount of down as their larger brethren, which makes them roughly half a degree warmer overall. But most of the excess down is placed around the hips and feet to boost performance in just the right places. And remember, if you’re a short guy or a tall girl, I recommend you size down or up. Get the bag that fits you best!
We test all of our sleeping bag prototypes. A lot. Me and my fellow designers test our gear. Our ambassadors test our gear. Even some lucky friends have been testing our gear. I myself had a rather unfortunate weekend in the mountains of Hemsedal, trying to get some sleep on a leaking mattress in -34°C. Luckily I had two prototypes with me and could double up. I was cold and miserable and didn’t get much sleep, but I did it for you, my fellow gear nerds!
In addition to extensive field testing, all of our bags are of course also tested at an independent lab according to the internationally recognized standard EN 23537:
A mannequin with heating elements and sensors is placed inside the sleeping bag on a mattress on a table in a room. The room is kept at constant temperature and humidity. You can then measure how much energy is required to keep the mannequin at body temperature. The less energy that’s required to keep the mannequin warm, the better the bag is performing. These numbers are then used to calculate the key temperature ranges that we use to rate sleeping bags.
Interpreting the results can be somewhat confusing, as there’s only one test (performed three times to get an average reading) that gives us the complete list of temperature ranges, and it’s the same test regardless of whether it’s a Women’s or a Men’s bag. Allow me to try and explain the three main elements:
- Comfort range and T-comf
- The comfort range is the temperature range where an average adult female can sleep comfortably in a relaxed posture, without sweating profusely or shivering to keep warm. The comfort temperature (T-comf) is the lower limit of the comfort range.
- Transition range and T-lim
- The transition range is the temperature range where an average adult male should be able to sleep, but in a curled up position and possibly starting to feel a bit cold. The limit temperature (T-lim) is the lower limit of the transition range. Most sleeping bag brands use the limit temperature in their marketing, which can be quite misleading. Remember, T-lim means you’ll be cold, and you’ll start to lose sleep.
- Risk range and T-ext
- The risk range is not a happy place. This is where an average adult female will struggle to keep warm and your body starts to shiver. There’s little to no sleep to be found at these temperatures, and you’re likely to end up suffering from hypothermia. Too much time spent on the highway to the danger zone and you’re not gonna have a good time. They don’t call it the extreme temperature (T-ext) for nothing! High five if you got both my 80’s music references.
Wow, there’s so much more to say. I could go on and on, but I have to do some actual design work now.
Stay dry, stay warm, and sleep well.
Sincerely yours, Bjørn, your friendly neighborhood Gear Bear.