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Sleeping bags are rated according to temperature, with the lowest temperature a bag can withstand indicated by a number. This is known as the degree rating and is based on lab-tested temperature ratings. The ISO Test, for example, measures the amount of electrical energy needed to maintain a manikin's temperature when placed inside a sleeping bag, which is then cooled. However, real-world comfort may differ from lab-tested ratings due to variables such as clothing, body type, weather conditions, and food intake. The degree rating is an important consideration when choosing a sleeping bag, but it's not the only factor. Other considerations include the number of seasons the bag is designed for, the level of insulation, and personal preferences for sleeping temperature.
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What You'll Learn
- Sleeping bag temperature ratings are estimates, not exact ratings
- The ISO Test: the comfort rating is based on the air temperature at which the average female can sleep comfortably
- The LIMIT rating is based on the temperature at which the average man can maintain thermal equilibrium in a curled-up position
- Women's sleeping bags use the comfort rating as the average woman will feel colder in the same bag as the average man
- Season-based ratings: summer bags are for temperatures of 32°F and above, while winter bags are for temperatures below 20°F
Sleeping bag temperature ratings are estimates, not exact ratings
Sleeping bag temperature ratings are based on lab tests that simulate how a sleeping bag will be used. A sleeping pad is placed under the bag, and a test dummy—dressed in base layers—is placed inside the bag. This protocol ensures that all bag tests are done the same way. However, it does not account for variations in people's clothing, gear, body types, weather conditions, and more. Therefore, temperature ratings are estimates and should be used as a rough guideline.
The International Organization for Standardization standard, ISO 23537 ("the ISO Test"), is used to rate sleeping bags. The test involves placing a heavily instrumented electronic mannequin, clad in long underwear, inside the sleeping bag in question, and positioning both on top of a foam sleeping pad. The temperature of the air is then lowered, and the amount of electrical energy needed to maintain the mannequin's initial temperature is measured. While this test helps predict what kind of comfort a person can expect from the bag, it does not account for all variables.
The ISO Test has two main ratings: "Comfort" and "Lower Limit". The "Comfort" rating is based on the temperature at which the "average female" can sleep comfortably through the night without curling up in a ball for warmth. The "Lower Limit" rating is based on the temperature at which the "average man" can maintain his thermal equilibrium in a curled-up position. It is important to note that the "average man" at the lower limit rating may not be comfortable, but he can sleep without shivering or risking hypothermia.
In addition to the ISO Test, there is also the EN rating system, which is still voluntary in the EU. Some high-end sleeping bag brands, such as Feathered Friends and Western Mountaineering, set their own temperature ratings instead of using the ISO or EN systems. These brands are widely used by serious backpackers, alpinists, and guides, and their ratings are generally trusted.
When choosing a sleeping bag, it is important to consider the lowest temperature you are likely to encounter and your personal internal thermostat. If you tend to feel cold, use the comfort ratings as a baseline and choose a bag with a lower comfort figure than the coldest expected temperature. If you sleep warm, you may rely on the lower limit ratings, but it is still recommended to give yourself a buffer of a few degrees Celsius. Additionally, it is worth noting that sleeping bag ratings vary from brand to brand, so it is always good to do your research and read reviews before purchasing.
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The ISO Test: the comfort rating is based on the air temperature at which the average female can sleep comfortably
Sleeping bags have temperature ratings that indicate the range of temperatures they are suitable for. These ratings are based on standardised tests, which help consumers make 'like-for-like' comparisons between sleeping bags from different brands.
The ISO Test, also known as ISO 23537, is one such standardised test. It involves placing a heavily instrumented electronic mannequin, dressed in long underwear and a hat, inside the sleeping bag, which is itself positioned on a foam sleeping pad. The temperature of the surrounding air is then gradually lowered, and the amount of electrical energy required to maintain the mannequin's initial temperature is measured. This measurement is then correlated with real-world outcomes through rounds of experimentation, allowing the testers to predict the level of comfort a person can expect from the sleeping bag.
The ISO Test includes two key ratings: Comfort and Limit. The Comfort rating is based on the temperature at which the "average female" can sleep comfortably through the night without curling up into a ball for warmth. This rating is targeted at colder sleepers, as women's bags typically have more insulation, making them 10–15 degrees Fahrenheit warmer than similarly-rated men's bags. The Limit rating, on the other hand, is based on the temperature at which the "average man" can maintain his thermal equilibrium in a curled-up position. It is important to note that the "average man" at this temperature is not necessarily comfortable, but he is able to sleep without shivering and is not in danger of hypothermia.
It is worth mentioning that temperature ratings are only estimates, and real-world comfort may differ from the lab-tested temperature ratings due to various factors such as differences in clothing, gear, body types, weather conditions, and individual variations. Therefore, it is recommended to consider your own internal thermostat when choosing a sleeping bag and to select one with a comfort rating that matches your needs. Additionally, pay attention to the lowest expected temperature at your camping location and choose a bag with an appropriate rating to ensure a cosy night's sleep.
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The LIMIT rating is based on the temperature at which the average man can maintain thermal equilibrium in a curled-up position
Sleeping bag temperature ratings are a complex and sometimes confusing topic. The LIMIT rating is a specific aspect of this, and it is based on the temperature at which the "average man" can maintain his thermal equilibrium in a curled-up position. This does not mean that the average man will be comfortable at this temperature, but rather that he can sleep without shivering and is not at risk of hypothermia. The LIMIT rating is therefore aimed at warm sleepers, who may find that their sleeping bag is neither too hot nor too cold at this temperature.
It is important to note that the LIMIT rating is just an estimate, and real-world comfort may differ from the tested temperature rating due to variations in people's clothing, gear, body types, weather conditions, and other factors. The LIMIT rating is also not stated as an exact temperature rating, and brands may round up or down to a number ending in 0 or 5.
To choose the right sleeping bag, it is recommended to consider the lowest temperature you are likely to encounter and your own internal thermostat—whether you tend to feel cold or run hot. If you often feel cold, you may want to use comfort ratings as a baseline and choose a bag with a significantly lower comfort figure than the coldest expected temperature. If you tend to run hot, you may be able to rely on lower limit ratings, but it is still advisable to allow for a buffer of a few degrees Celsius.
Additionally, it is worth noting that sleeping bag ratings can vary from brand to brand, and it is generally recommended to trust the temperature ratings of respected outdoor equipment companies. The EN and ISO testing methods have been adopted by some manufacturers for consistency and accuracy, but not all companies use these systems. Therefore, it is important to do your research and consider factors beyond just the temperature rating when choosing a sleeping bag.
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Women's sleeping bags use the comfort rating as the average woman will feel colder in the same bag as the average man
Sleeping bags are rated according to the International Organization for Standardization standard, using ISO 23537 (the ISO Test). The test involves placing an electronic mannequin, dressed in long underwear, inside the sleeping bag, and positioning it on top of a foam sleeping pad. The temperature of the air is then lowered, and the amount of electrical energy needed to maintain the mannequin's initial temperature is measured.
However, real-world comfort may differ from lab-tested temperature ratings. This is because the tests cannot account for variables such as differences in body types, clothing, gear, weather conditions, and more. For example, the ""comfort rating" is based on the air temperature at which the "average female" can sleep comfortably through the night, while the "limit rating" is based on the temperature at which the "average man" can maintain his thermal equilibrium in a curled-up position. The ""average woman" will feel colder in the same bag as the "average man" due to physiological differences, and so women's sleeping bags typically have more insulation, making them roughly 10–15 degrees Fahrenheit warmer than similarly-rated men's bags.
Women's sleeping bags are usually shorter than men's, reflecting the average height difference between men and women. They may also have a higher concentration of insulation around the torso and in the footbox, as studies have shown that women lose heat at a higher rate in these areas. Additionally, women tend to lower their internal body temperature more than men during the night, and more blood flow will be pulled from the extremities, causing these areas to become colder. Therefore, women's sleeping bags are rated to a ""warmer" temperature rating to reflect how much insulation is needed for a comfortable night's sleep.
When choosing a sleeping bag, it is important to consider the lowest temperature you are likely to encounter, as well as your own personal internal thermostat. If you often feel cold, use the comfort ratings as a baseline and choose a bag with a significantly lower comfort figure than the coldest expected temperature. If you tend to sleep warm, you may be able to rely on lower-limit ratings, but it is still recommended to give yourself a buffer of a few degrees Celsius.
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Season-based ratings: summer bags are for temperatures of 32°F and above, while winter bags are for temperatures below 20°F
Sleeping bags are rated according to the International Organization for Standardization standard, using ISO 23537, also known as "the ISO Test". This test involves placing a heavily instrumented electronic mannequin, dressed in long underwear, inside the sleeping bag in question, and positioning both on top of a foam sleeping pad. The temperature of the air is then lowered, and the amount of electrical energy needed to maintain the mannequin's initial temperature is measured.
The ISO test has two types of ratings: "comfort" and "lower limit". The "comfort" rating is based on the air temperature at which the "average female" can sleep comfortably through the night, while the "lower limit" rating is based on the temperature at which the "average man" can sleep without expending energy shivering and without being in danger of hypothermia.
Season-based ratings provide a helpful guideline for choosing the right sleeping bag for your needs. Summer bags are typically rated for temperatures of 32°F and above, making them suitable for warm, summer-like conditions or lower elevations. They are a great option for saving weight, space, and money. However, it is important to remember that temperatures can still drop at night, even in summer, so it is always a good idea to check the weather and prepare accordingly.
On the other hand, winter bags are designed for temperatures below 20°F. These bags are essential for winter camping and mountaineering, providing the necessary warmth in cold conditions. It is worth noting that the temperature rating for cold-weather adventures depends on the specific conditions, and expedition bags can be rated for temperatures as low as -40°F or lower.
It is important to consider your own personal preferences and internal thermostat when choosing a sleeping bag. If you tend to feel cold easily, opt for a bag with a lower comfort rating than the coldest expected temperature. Conversely, if you sleep warm, you may rely on the lower limit ratings, but it is still recommended to allow for a buffer of a few degrees Celsius. Additionally, it is crucial to prepare properly before spending a night outdoors, checking the weather forecast, and ensuring your sleeping bag is rated to handle the expected low temperatures.
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Frequently asked questions
The numbers on a sleeping bag refer to a temperature rating, indicating the lowest temperature that the bag is designed to provide a comfortable night's sleep in.
Temperature ratings are determined through lab testing. A test dummy, dressed in base layers, is placed inside the sleeping bag, which is then subjected to decreasing air temperatures. The amount of electrical energy required to maintain the dummy's initial temperature is measured, and the results are correlated with real-world outcomes.
"Comfort" ratings are designed for colder sleepers, indicating the temperature at which the average woman can sleep comfortably through the night. "Lower limit" ratings are for warmer sleepers, indicating the temperature at which the average man can maintain thermal equilibrium in a curled-up position for six hours.
First, consider the lowest temperature you're likely to encounter. If you tend to feel cold, choose a bag with a comfort rating significantly lower than the expected temperature. If you sleep warm, you may rely on the lower limit rating, but it's advised to have a buffer of a few degrees Celsius.
