Insulation in general seems to be a very polarizing topic. For one, it’s complicated — and that can lead to incomplete conclusions. And secondly, data points gathered through “experience” are not easily reconciled and analyzed in a rigorous way. While keeping in mind my general philosophy towards the outdoors (i.e. Just get outside), I wanted to provide some quick thoughts regarding sleeping bag (and quilt) temperature ratings.
50 years ago, there was less information and choice within the backpacking sphere (though I wasn’t alive to witness this). I’m not exactly sure how people back then went about accomplishing their objectives, but it was probably somewhere between “Just tough it out,” and “Let’s be prepared for everything.” Did they manage to stay warm though? I don’t know.
Today, we all want the very best sleep insulation. We’re trying to maximize on all three dimensions (price, weight, performance). Yet there’s a fourth dimension that is often overlooked — ourselves. For the winter season, I try to prepare my body and mind through a simple exercise: taking a cold shower. However, even that will only get me so far and at some point I’ve got to snuggle into my sleeping bag and trust that it’s warm enough.
Such a simple idea but complex execution
Now, let’s consider some factors that impact our feeling of comfort while sleeping. Internally, our metabolism sustains us and we maintain a steady body temperature through homeostasis. If we keep our body well-hydrated and well-nourished, it will do its best job. As a byproduct of our “internal flame,” we emit heat (thermal radiation). The clothing systems and sleeping systems that we use are passive and only serve to trap and contain this heat, but don’t produce any heat themselves. This is why it’s often suggested that you do some sit-ups in your sleeping bag if you are feeling cold.
Our body temperature could also be affected by the following external factors: air temperature, air composition (humidity, air density), wind, ground temperature, ground composition, and radiative heat sources/sinks. How can an outdoor gear manufacturer account for all of these variables when they market a sleeping bag? Short answer: they can’t (and maybe they shouldn’t).
These external factors affect change through three mechanisms: radiation, conduction, and convection. Radiation can be thought of as heat transfer, plain and simple. For example, the Sun transfers heat to the Earth via radiation, even through the vacuum of space. Conduction can be simplified as heat transfer through a stationary body. Backpackers like to think of thermal contact as conduction, like when you put your hand down on the cold ground. Convection is heat transfer through a fluid (gas or liquid). When you get hit by a gust of cold air, you’re feeling the temperature variations in the air caused by convection.
It can be a challenge to trace the path of heat flow because it doesn’t always neatly break down into just one of those three mechanisms. Fortunately, we might not have to think about all of it. To protect ourselves from losing heat to the ground, we typically use a sleeping pad. The R-value of the sleeping pad already accounts for heat transfer by radiation, conduction, and convection (woohoo). To protect ourselves from losing heat to the air and sky around us, we use sleeping bags.
Because of the “simplicity” of making purchase decisions based on a single temperature rating, you’ll be hard pressed to find a sleeping bag on the market without this number. Rarely do we ask about the underlying conditions and variables of that temperature rating — and therein lies the problem.
Manufacturers who use their own rating system
Some manufacturers choose to implement their own temperature rating system. There is a lot of strategy when it comes to choosing a temperature rating. If they choose a conservative rating, they might develop a good reputation for keeping their users warm (think Western Mountaineering and Katabatic). If they choose a more aggressive rating, they might better compete on paper against similarly rated but heavier products. As a result, there are generally two camps: those who buy based on reputation and those who buy based on specs.
I’ve noticed an interesting development among these two camps. Those who buy based on reputation tend to favor performance over weight and I’ve heard few complaints from them about their sleeping bags. Those who trust temperature ratings but also care about their baseweight usually end up with the aggressively rated sleeping bags. They’re the group that’s more likely to feel dissatisfied after some trail use (e.g. a large percentage of PCT hikers are dissatisfied with the Zpacks 20F quilt). And then there are a whole bunch of people trying to navigate in between these two extremes.
Manufacturers who use the EN 13537 standard
Manufacturers who adopt the EN 13537 standard for their sleeping bag temperature ratings make their products more comparable to one another. While sometimes people get upset at an EN temperature rating, I think it’s rarely the manufacturer’s fault. It could be poor testing that leads to an improper temperature rating, but more often than not I’ve observed that it’s a lack of understanding and transparency about the EN standard. I’m a proponent of more standardized testing of outdoor gear so that consumers have a strong data point in their decision-making process, but I recognize the need for the EN standard to improve in both testing and marketing.
There are some papers that highlight the shortcomings of the EN 13537 standard in its current iteration here and here (Aside: a scientific paper is not the truth — it’s just a way for scientists to share their findings). They do go into more detail than I could about improving the testing procedures and hopefully it gets incorporated in the next iteration.
From a marketing standpoint, I think there’s just too much confusion all around and therefore this is where the most impact can be had. First, there are four temperature ratings: Extreme, Lower Limit, Comfort, and Upper Limit. I would say most people who need to care about this Extreme rating already have the experience to overlook the rating. And if it gets too hot, you should probably get out of the sleeping bag. Why don’t we just get rid of the Extreme and Upper Limit for starters?
Next is the idea of a Lower Limit, which is defined as the lowest temperature at which a standard adult male will have a comfortable night’s sleep. The Comfort temperature is defined in the same way but for a standard adult female. Seems simple right? In actuality, I think the EN standard intended for the emphasis to be on the range of temperatures for comfort, rather than a temperature endpoint. So maybe manufacturers and retailers are doing a bad job by focusing on the endpoint temperatures (kudos to Therm-a-Rest for trying to explain this on their website).
There are a few important details that most consumers don’t know about the EN Lower Limit and Comfort temperature ratings though. First, EN testing is performed using a non-standardized sleeping pad with a range of R-values — but they use a pad with R-value of 4.8 for calibration. Very few consumer pads have this level of R-value today, and it’s unclear whether consumer temperature ratings are over-stated as a result.
Next, the physiological responses in this “Transition Range” (between Lower Limit and Comfort) might not be what the consumer signed up for. In this Transition Range, a standard male (25 years old, 5’6″, 160lbs) is in thermal equilibrium and not shivering, but he can be curled up inside the bag. Well, I can say from personal experience that while I fit this “standard male” requirement quite well, I definitely start feeling cold and uncomfortable before I start shivering. I’m led to believe that any temperature above the Lower Limit means I should feel comfortable as a standard male, but that is rarely the case even in good conditions.
Without knowing the type of ground insulation that their consumers are using, or how far the consumer deviates from the “standard male” and “standard female,” it’s hard for manufacturers to market the EN temperature rating with consistently positive results. I think the major hurdle is the amount of educating necessary to help consumers pick the right sleeping bag in sub-optimal conditions.
How I would try to solve this temperature rating issue
At the end of the day, I think a lot of consumer satisfaction and dissatisfaction comes from the expectation that these manufacturer-issued or EN temperature ratings are applicable to them. If you’re not trying to get into the sleeping bag business, it’s easy. Just find out what works for you. For a sleeping quilt with minimal features, I’m able to compare different models based on fill amount, fill power, size, and fabric type, and then factor in anecdotal evidence to determine whether it might suit my needs. This is because I’ve tried out enough EN-rated sleeping bags to know where my personal “Lower Limit” and “Comfort” temperatures are relative to the EN standard. What a tedious process to go through though!
Here’s my suggestion: instead of using an absolute temperature rating for a sleeping bag, market the sleeping bag as “comfortable up to an X degree drop in overnight temperatures.” It’s far from perfect because of the other external factors mentioned above, but at least it means that if you’re appropriately dressed before bed, this sleeping bag will help you stay warm and comfortable as your metabolism slows and as temperatures drop overnight. Rhetorical question: If we ask our sleeping bag manufacturers for a temperature rating, why don’t we ask our clothing manufacturers for one also?
Fortunately, you can now justify some of your trips (to those who may not be quite so enthusiastic) as necessary field-testing of gear. Have fun!
Disclosure: I may have a B.S. in Materials Science & Engineering but by no means should I be considered a subject matter expert. I’m just an enthusiast.