Afraid your bag won’t keep you warm? Sleeping bags are rated for certain temperatures by their manufacturers. Look around and you will see some are rated for warm weather while others are rated for extreme cold.
The truth is that sleeping bag temperature ratings are a source of some controversy.
The disagreement stems from the fact that some people report feeling cold in a sleeping bag even though the bag is rated to, or below, the temperature they’re experiencing. Someone else may find the same bag warm in the same conditions. Even more confusing, you may find one company’s ratings accurate while a different company’s temperature ratings feel way off.
There are two important reasons for this problem.
Lack of Standardized Testing
The method by which sleeping bags are tested varies. Some companies use proprietary methods, information about which may or may not be available to the public. These tests could involve wind tunnels, walk-in freezers, sleeping with and without a pad, or hardy volunteers spending a night on a mountain.
There is no way to compare sleeping bag temperature ratings without understanding the conditions under which the bags were tested.
The European Norm
What does an EN rating mean for a sleeping bag?
In 2002, the EN (or to be more specific, the European Norm 13537) rating criterion was published. The EN rating system was an attempt to standardize sleeping bag temperature testing so that labels could be more accurate. It was superseded in 2017 by a new but similar standard, ISO 23537.
There is no legal obligation for any manufacturer to adhere to the published EN/ISO standards. Some have chosen to use this system and others have not. As a result, you will see some sleeping bags with EN/ISO rating labels and other bags without.
Breaking Down The Norm
What does an EN rating actually measure?
To measure sleeping bags, or anything else, in a standard way, it is necessary to control as many of the variables as possible. The EN system standardizes certain variables and also explains the procedure so people can understand under what conditions bags are rated.
For instance, the EN ratings assume:
- the subject is using a sleeping pad
- the subject is inside a tent
- the subject is wearing one base layer of thermal underwear
You will either be happy or annoyed to realize that, for the purpose of these measurements, a “standard male” is 25 years old, 5’ 8” and weighs 160 lbs. Just as likely to be inapplicable to many female hikers, a “standard female” is 25 years old, 5’ 3” and weighs 132 pounds. The EN standard even goes on to explain that it does not apply to children’s bags or bags built for extreme conditions (below zero F).
A specially-designed mannequin, outfit with wires and sensors, is used to measure four temperatures:
- Upper Limit
- Lower Limit
Upper Limit — the temperature at which a standard male can sleep without excessive perspiration. It is measured with the hood and zippers open, with the arms outside of the bag.
Comfort — the temperature at which a standard female can expect to sleep comfortably in a relaxed position.
Lower Limit — the temperature at which a standard male can sleep for eight hours in a curled position without waking.
Extreme — the minimum temperature at which a standard female can remain for six hours without risk of death from hypothermia, through frostbite is possible.
Consumers can now look at the labels of many sleeping bags and see these ratings, giving them a better idea of how that sleeping bag will function in various temperatures.
Summing Up The Test Talk
We talked about the standardized testing protocols of the EN/ISO. The important things to realize are:
- the EN/ISO system allows us to compare temperature ratings between bags tested in the same manner
- the EN/ISO system is not perfect and doesn’t apply equally well to everyone
- not all manufacturers adhere to the EN/ISO testing standards
Consumers do not currently have a consistently-applied system to judge the accuracy of sleeping pad temperature ratings.
The proverbial elephant in the tent
No rating system – be it scientifically standardized or individually idiosyncratic – can ever produce temperature ratings that work for everyone. That’s because we aren’t identical.
There are a host of well-documented factors that influence human thermoregulation (how well we maintain our body heat). This directly effects how warm (or cold) we feel in a sleeping bag. A short list of those factors include:
- Height and weight
- Hydration status
- Metabolic rate
- Environmental factors (temperature, humidity, wind speed, etc.)
The list of physiological differences between men and women is a long and complex one but we can say a few general things about body heat.
- On average, men have more muscle tissue and higher metabolic rates. This helps keep them warm.
- Research on circulatory patterns suggest that women’s bodies may focus on keeping the core warm at the expense of the extremities, resulting in cold feet and hands.
- Fat is distributed differently in men and women, meaning that there are differences in where they lose heat. This is why sleeping bags designed for women often have increased insulation in certain areas.
Our muscle mass and metabolism change as we age. Overall, our muscle mass and our metabolic rate decreases, resulting in feeling cold more often.
Height and Weight
A person’s height and weight interact to yield a measurement of body surface area. Shorter, heavier people have a greater surface area (in comparison to their total body volume) than taller, thinner people. More surface area means more heat loss.
Weight is also a factor because fat is the human body’s best insulation. More body fat results in less heat loss.
Even a minor level of dehydration of drastically effects your body’s functioning. A well-hydrated person’s body will keep them warmer.
People who may look the same (gender, height and weight) could still have slightly different metabolic rates. People with higher rates physiologically produce more heat.
This one is obvious but people often discount it when talking about sleep. For both men and women, wearing insulating clothing decreases heat loss. This is true whether you are hiking on the trail or lying in a bag.
This is a complex topic unto itself. The sheer number of variables is mind-boggling. Of particular importance are cold air temperatures, high humidity and high winds. Exposure to these factors draw heat away from us and cool our bodies.
Our clothes, our metabolisms, our stature, our age and other factors make each of us more or less unique in how well we produce and lose body heat.
Sleeping bag temperature ratings will never be perfect because the people out there hiking and camping are simply too diverse. We are men and women, young and old. We are tall and short, fast and slow. Some of us will sleep on a thin foam mat in a glacial valley at -20 degrees while others slumber atop an inflatable mattress at 50 degrees. Some wear a hat to sleep while others wear nothing at all.
This wonderful variety of bodies, minds and actions gives us strength as a community but it also prevents manufacturers from being able to produce a magic number that will make us all happy.
Understanding this situation empowers each of us to know ourselves.
It isn’t important that a manufacturer says a bag is comfortable at 30 degrees – it matters if you agree.
The unvarnished truth is that there is currently no one-size-fits-all system for sleeping bag temperature ratings and there may never be.
The benefit of the standardized EN/ISO system is that once you know what temperature rating works for you, you can buy another EN-rated bag from any manufacturer and be confident that you’ll get similar performance.
However, not all companies adhere to the EN/ISO system. Manufactures who don’t use the EN/ISO system continue to do their best to estimate the comfort ratings of their bags.
The ultimate goal is for each of us to find a bag that keeps us safe and comfortable. Instead of focusing solely on temperature ratings, use a combination of:
- the manufacturer’s rating
- the knowledge about how you sleep
- your experience sleeping outdoors
- a judicious dash of educated guesswork
If a 30 degree bag keeps you comfortable at 30 degrees, use it. After reading this, you won’t be surprised when your partner needs a 20 degree sleeping bag for the same conditions.