Herbie Murray
Sleeping bag temperature ratings are a somewhat complicated subject. The key factor when choosing a sleeping bag is its temperature rating. If you have a reasonable idea of the coldest conditions you'll experience on your trip, you can select a bag that will keep you warm at or below that temperature. Sleeping bags are rated according to the International Organization for Standardization standard, using ISO 23537. The 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 cooled, and the amount of electrical energy needed to maintain the mannequin’s initial temperature is measured. The current ISO 23537 sleeping bag test standard is recognised globally, with the standard itself undergoing regular reviews.
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What You'll Learn
- Lab testing: Dummies in standardised conditions determine a bag's thermal resistance
- Limitations: Real-world comfort varies due to differences in clothing and gear, and physiology
- EN/ISO ratings: Lab-tested ratings that allow for comparisons between bags from different brands
- Gender-specific ratings: Women's bags have more insulation, catering to lower metabolic rates and muscle mass
- Season ratings: Bags are designed for specific seasons, ranging from three-season to five-season (high-altitude) bags
Lab testing: Dummies in standardised conditions determine a bag's thermal resistance
Sleeping bags are assigned temperature ratings based on standardised laboratory tests. The ISO 23537 standard, and its predecessor, the EN 13537 standard, use a heated thermal mannequin with multiple temperature sensors inside a climate-controlled room on a standardised mat. The mannequin, wearing standardised clothing, measures the power needed to maintain warmth in a cold room, determining the sleeping bag's thermal resistance. The thermal resistance is then converted into temperature ratings based on historical data.
The standardised clothing worn by the mannequin typically consists of a base layer, similar to what a camper might wear. The mannequin also wears a cold-weather mask. Before testing, sleeping bags are kept at a constant temperature and humidity and allowed to loft fully. The test runs until equilibrium is reached.
The results of the test are used to establish a range of temperatures, with two temperature ratings within that range specifically called out: a comfort rating and a lower limit rating. The comfort rating indicates the temperature at which a cold sleeper might feel comfortable. This is the temperature rating typically used for women's bags. The lower limit rating, which is always lower than the comfort rating, indicates the temperature at which a warm sleeper might still feel comfortable. This is the temperature rating typically used for men's bags.
It is important to note that lab-tested temperature ratings are only estimates and may not perfectly match real-world comfort levels. The tests cannot account for all variables, such as differences in people's clothing, gear, body types, changing weather conditions, and individual sleep patterns. Therefore, it is recommended to consider a sleeping bag as part of a sleep system, including appropriate insulating pads and liners, to ensure adequate warmth in various conditions.
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Limitations: Real-world comfort varies due to differences in clothing and gear, and physiology
Sleeping bag temperature ratings are based on standardised testing, such as the ISO 23537 test, which provides three temperature thresholds: a comfort rating, a limit rating, and an extreme rating. However, it's important to remember that these ratings have certain limitations and may not perfectly reflect your real-world experience.
Firstly, the comfort of a sleeping bag can vary depending on the clothing and gear used. During lab testing, the test dummy is typically dressed in base layers, long underwear, or standard pyjamas, with socks and a hat. However, in real-world situations, the type and amount of clothing worn can differ significantly, impacting the overall comfort and warmth experienced.
Secondly, individual physiology plays a crucial role in determining comfort. The standardised tests use mannequins or dummies that represent an "average" male or female, with assumptions about muscle mass and body surface area. However, in reality, each person's body composition, metabolism, and sensitivity to temperature can vary, affecting their perception of warmth or cold. For example, older individuals tend to have lower muscle mass and may feel colder than younger people.
Additionally, factors such as diet, mindset, and sleeping posture can also influence an individual's comfort level. While the tests attempt to standardise these variables, they may not perfectly reflect the diverse range of human experiences.
Furthermore, the shape and size of the mannequin used in the tests can vary slightly between labs, affecting how the sleeping bag compresses insulation and retains warmth. This means that the test results may not perfectly align with your personal experience, depending on your body size and the specific fit of the sleeping bag.
Lastly, external factors such as the sleeping pad or liner used in conjunction with the sleeping bag can also impact its overall warmth. Sleeping bags are often rated with the assumption that an appropriate sleeping mat with a suitable R-value will be used, which can significantly affect the bag's performance. Therefore, it's important to consider your entire sleep system, including the sleeping bag, pad, and liner, when planning for a comfortable night's rest outdoors.
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EN/ISO ratings: Lab-tested ratings that allow for comparisons between bags from different brands
Sleeping bags are assigned temperature ratings through standardized testing, which allows for comparisons between bags from different brands. The ISO 23537 standard, also known as "the ISO Test", is a widely recognized testing method for sleeping bags. It involves placing an electronic mannequin, dressed in long underwear, inside the sleeping bag, which is placed on a foam sleeping pad. The surrounding air temperature is then lowered, and the electrical energy required to maintain the mannequin's initial temperature is measured. This process is repeated in a series of experiments, and the results are correlated with real-world outcomes.
The ISO Test provides three temperature thresholds: a comfort rating, a limit rating, and an extreme rating. The comfort rating indicates the temperature at which the "average woman" can sleep comfortably throughout the night without curling up for warmth. The limit rating represents the temperature at which the "average man" can maintain thermal equilibrium in a curled-up position without excessive shivering or risking hypothermia. The extreme rating, which is not always specified, indicates a potentially dangerous temperature for the "average woman".
It is important to note that these ratings are estimates and may not perfectly match your experience in the outdoors. Individual factors, such as body type, clothing, gear, and sleeping habits, can influence the actual comfort level of a sleeping bag. Additionally, brands may round the temperature ratings in their product names for simplicity, which can make direct comparisons between bags more challenging.
The EN rating system is another standardized method for assigning temperature ratings to sleeping bags. Similar to the ISO Test, the EN test utilizes a heated mannequin wearing light clothing and a well-insulated pad. The EN ratings include the upper limit, comfort, and lower limit thresholds. The upper limit is the temperature at which a standard male can sleep without excessive perspiration, while the comfort rating is based on a "standard woman" sleeping comfortably in a relaxed position. The lower limit rating indicates the temperature at which a standard male can sleep for eight hours in a curled-up position without waking.
When choosing a sleeping bag, it is recommended to consider your personal preferences, sleeping habits, and expected weather conditions. It is generally advisable to select a bag with a temperature rating that is slightly lower than the expected temperature to ensure comfort and safety. Additionally, paying attention to the EN/ISO ratings from reputable manufacturers can help you make an informed decision when comparing sleeping bags from different brands.
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Gender-specific ratings: Women's bags have more insulation, catering to lower metabolic rates and muscle mass
Sleeping bags are typically categorised by their temperature ratings, with most designed for use in conditions ranging from 10 to 50 degrees Celsius. Temperature ratings are estimates that help consumers make comparisons between different brands. These ratings are determined by lab-testing procedures that simulate how a sleeping bag is used. The current ISO 23537 sleeping bag test standard is recognised globally, with the standard itself undergoing regular reviews. The test involves placing a heated mannequin with multiple temperature sensors inside a sleeping bag in a climate-controlled room on a standardised mat. The mannequin measures the power needed to maintain warmth in a cold room, determining the sleeping bag's thermal resistance.
The temperature ratings are based on the assumption that the "average man" can sleep without expending energy shivering and without the risk of hypothermia. However, gender-specific ratings also come into play, as women's bags often have more insulation. This is because women tend to feel colder faster than men due to lower metabolic rates and muscle mass. Women's sleeping bags are designed to be roughly 10–15 degrees Fahrenheit warmer than similarly rated men’s bags. The "comfort rating" indicates the lowest temperature at which a bag is expected to provide a comfortable night's sleep, catering to those who tend to sleep cold.
Metabolic rates are influenced by various factors, including gender, muscle-to-fat ratio, physical activity, and hormone function. Generally, men have faster metabolisms because they tend to have larger body sizes and greater muscle mass. Muscle tissue burns kilojoules rapidly, and individuals with more muscle mass produce more heat and cool down slower. This is relevant when considering sleeping bag temperature ratings, as people with greater muscle mass may require less insulation to maintain warmth.
It is important to note that lab-tested temperature ratings may not perfectly align with real-world comfort due to various variables that a lab cannot simulate. Therefore, when selecting a sleeping bag, individuals should consider their unique needs and preferences. Factors such as shape, size, fit, and insulation type also play a role in the overall warmth and comfort of a sleeping bag. Additionally, it is recommended to check the weather conditions and leave a safe margin between expected nighttime lows and the bag's rating.
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Season ratings: Bags are designed for specific seasons, ranging from three-season to five-season (high-altitude) bags
Sleeping bags are available in different season ratings, ranging from one-season to five-season bags. These ratings help users choose the right sleeping bag for their needs. The season ratings are based on the temperature range for which the bag is designed, with each season rating optimised for a specific temperature range.
Three-season sleeping bags are designed for colder nights in spring, summer, and autumn when the temperature might dip just below freezing. They offer a good balance between warmth, weight, and bulk, making them a popular choice for camping and backpacking trips.
Four-season sleeping bags are designed for winter use, suitable for frosty or snowy conditions with temperatures as low as -10°C or 15°F. They are optimised for cold conditions, often making them too warm for milder seasons.
The term "five-season" is used for sleeping bags designed for high-altitude expeditions and extremely cold temperatures. These bags are essential for climbers, explorers, and scientists in the world's highest mountain ranges and polar regions, with temperature ratings ranging from -10°C to -40°C (15°F to -40°F).
It is important to note that the season ratings are general guidelines, and the specific temperature ratings of sleeping bags can vary. Lab testing plays a crucial role in determining these ratings, but factors like clothing, gear, body type, weather conditions, and individual differences can affect the comfort level of the user. Therefore, it is recommended to consider your own preferences and the coldest anticipated conditions when choosing a sleeping bag.
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Frequently asked questions
Temperature ratings are estimates that indicate the range of temperatures a sleeping bag is suitable for.
The ISO 23537 test is the most widely recognised testing standard. It involves placing a heated or electronic mannequin, dressed in long underwear, inside a sleeping bag on a foam mat in a climate-controlled room. The amount of electrical energy needed to maintain the mannequin's temperature is measured as the room cools down.
The three main types of temperature ratings are comfort, limit, and extreme. The comfort rating is the temperature at which the "average woman" can sleep comfortably through the night. The limit rating is the temperature at which the "average man" can maintain thermal equilibrium in a curled-up position. The extreme rating is the temperature at which there is a risk of hypothermia.
Consider the lowest temperature you're likely to encounter, as well as your personal internal thermostat. If you often feel cold, opt for a bag with a lower comfort rating than the expected lowest temperature. If you sleep warm, you may rely on the lower limit rating, but it is recommended to have a buffer of a few degrees Celsius.
One-season sleeping bags are designed for summer camping, with night-time temperatures above 5° Celsius or 40° Fahrenheit. Two-season bags are for slightly cooler nights in spring and summer, with temperatures ranging from 0 to 5°C or 30 to 40°F. Three-season bags are for colder nights in spring, summer, and autumn, with temperatures down to -5°C or 20°F. Four-season bags are for winter use, with temperatures around -10°C or 15°F.
