Gina Johnson
Sleeping bag temperature ratings are a complex topic, and it's important to understand them before purchasing a sleeping bag. The temperature ratings are estimates based on lab testing, which may not always match real-world comfort. The lowest temperature you'll be sleeping in, as well as your personal preference for warmth, will determine the ideal sleeping bag for you. Sleeping bags are also rated based on the number of seasons they are suitable for, with one-season bags designed for summer camping and four-season bags designed for winter use. The fill power and type of insulation are other factors to consider when choosing a sleeping bag, as they affect warmth, weight, bulk, and cost. Understanding these factors will help you make an informed decision when buying a sleeping bag for your specific needs.
| Characteristics | Values |
|---|---|
| Temperature ratings | Estimates, not exact values; rounded to nearest 0 or 5; not always accurate due to variations in individuals and gear |
| ISO (or EN) rating | Not all bags have this rating, especially those designed for extreme cold or mild conditions; useful for comparing bags from different brands |
| Seasons | One-season bags are for summer; Two-season bags for spring, summer, and mild autumn; Three-season bags for spring, summer, autumn, and slightly colder nights; Four-season bags for winter and extreme cold; Five-season bags for expeditions or high-altitude use |
| Temperature ranges | One-season: above +5°C or 40°F; Two-season: 0 to +5°C or 30 to 40°F; Three-season: down to -5°C or 20°F; Four-season: down to -10°C or 15°F; Five-season: -10°C to -40°C or 15°F to -40°F |
| Individual factors | Consider your internal thermostat—whether you tend to feel cold or hot; comfort ratings are a good baseline for those who feel cold; lower limit ratings may be sufficient for those who sleep warm |
| Weight and bulk | Warmer bags tend to be heavier and bulkier; consider weight and compressibility for backpacking |
| Insulation type | Down insulation is longer-lasting but more expensive; synthetic insulation is cheaper, dries faster, and performs better when wet |
| Baffles | Horizontal baffles provide better warmth-to-weight ratio but are less ergonomic; vertical baffles are heavier due to mesh walls but can be sculpted |
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What You'll Learn
Sleeping bag temperature ratings are estimates
It is also important to understand the difference between "comfort" and "lower limit" ratings. A "comfort" rating refers to the temperature at which the average person would feel comfortable sleeping in the bag, while a "lower limit" rating indicates the temperature at which the average person would start to feel cold. When shopping for a sleeping bag, it is recommended to use "comfort" ratings as a baseline and choose a bag with a lower comfort rating than the coldest expected temperature. If you tend to feel warmer, you may refer to the "lower limit" ratings as a guide. Additionally, it is advised to give yourself a buffer of a few degrees Celsius to ensure comfort.
The season rating of a sleeping bag also influences its temperature rating. Sleeping bags are designed for different seasons, with one-season bags suited for summer, two-season bags for spring and summer, three-season bags for spring, summer, and autumn, and four-season bags for winter use. Five-season bags are also available for extreme cold conditions. The season rating corresponds to the expected temperature range during those seasons. For example, a four-season bag is designed for temperatures around -10°C to +15°F, making it too warm for milder seasons.
It is worth noting that not all sleeping bags have an ISO (International Organization for Standardization) or EN (European Norm) rating. These ratings provide standardised test methods and criteria to ensure consistency across different brands. However, they are not applicable to bags designed for extreme cold or children's bags. Brands may also choose not to obtain ISO testing for bags intended for mild conditions or casual use. In such cases, the temperature rating provided by the brand is likely an estimate rather than an ISO or EN-tested rating. Therefore, it should be used as a rough guideline rather than a direct comparison between brands.
Finally, other factors beyond temperature ratings should be considered when choosing a sleeping bag. These include the type of insulation (down or synthetic), the fill power or loftiness of the down, the baffle construction (horizontal or vertical), and the shape of the bag. For example, synthetic insulation is often more affordable, dries faster, and performs better in moist conditions than down insulation. Down fill power indicates how much air is trapped by the down, with higher fill power providing more warmth with less filling, resulting in a lighter bag. Horizontal baffles are generally warmer by weight, while vertical baffles allow for more ergonomic designs. Rectangular bags offer more space to move around, and weight and compressibility are essential considerations for backpacking trips.
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ISO and EN ratings
Sleeping bag temperature ratings are estimates, not exact measurements. They are determined by lab testing, which simulates how a sleeping bag should be used. A sleeping pad is placed under the bag, and a test dummy inside the bag is dressed in base layers. While this protocol ensures that all bag tests are done the same way, it cannot account for variations like differences in people's clothing and gear, body types, changing weather conditions, and so on. Therefore, a sleeping bag's comfort level in the outdoors will probably differ from its tested temperature rating.
The "EN" or "European Norm" 13537 was the first widely used international standard for measuring sleeping bag temperature ratings. In 2016, the International Standards Organization (ISO) published ISO 23537 to replace the EN standard and make it more consistent. The difference between EN and ISO ratings is not relevant for consumers, and most backpackers and retailers still refer to the EN rating of a bag, whether it’s measured under ISO or EN.
The ISO 23537 test does not cover other features that can affect warmth, such as hoods, neck collars, and zipper draft tubes. Real-world tests of bags with identical ratings but different draft-prevention features often discover these differences. Other design issues can also make a sleeping bag feel warmer or colder, including insulation movement and clumping, overstuffing, and compressing insulation.
The EN/ISO testing protocols are a standard for all sleeping bags. The ratings from this test allow users to reliably compare bags of similar ratings and safely choose a bag that suits their adventure. The test involves tucking a sensor-rich, heated manikin into a sleeping bag, placing them in a cold chamber on a basic foam mat. Each manikin wears a specific base layer, similar to what a camper might wear. As temperatures drop in the cold chamber, corresponding measurements are taken from the manikin. The test looks for key benchmarks like when the manikin’s heat accumulates in the sleeping bag, the range where its temperature remains relatively steady, the point at which heat begins to be lost, and when it is lost at a rate where continued use of the bag would put the user at risk.
The results are tallied, and the ranges of Comfort, Transition, and Risk are established. Published temperature ratings are those at the bottom of each range and are referred to as Comfort, Limit, and Extreme, respectively. It’s important to note that, unlike old sleeping bag ratings, which provided a single temperature rating, the EN/ISO sleeping bag rating is read in ranges of temperatures, reflecting the subjective nature of warmth for different body types. The numbers included along the continuum are points of reference between ranges, helping users estimate where they might find the sleeping bag to provide ample insulation.
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One, two, three, four and five-season sleeping bags
Sleeping bag temperature ratings are estimates based on lab testing. The tests simulate how a sleeping bag is used, with a sleeping pad placed under the bag and a test dummy inside dressed in base layers. While this ensures that all bag tests are done the same way, they cannot account for all variables, such as differences in clothing, body types, weather conditions, etc. Therefore, a sleeping bag's comfort level in the outdoors will probably differ from its tested temperature rating.
One-season sleeping bags are designed for warmer weather and mild conditions. Even though they are designed for warmer temperatures, some high-end one-season sleeping bags offer features found in higher-season bags, such as built-in mosquito nets and antibacterial protection.
Two-season sleeping bags are lightweight and ideal for backpacking and car camping. They are designed for warmer temperatures and typically have a temperature rating below the lowest nighttime temperature expected. They come in various shapes, including rectangular, semirectangular, mummy, and double, and offer features such as draft-blocking details, snug-fitting hoods, extra zippers, and stash pockets for added durability, warmth, and functionality.
Three-season sleeping bags are also suitable for backpacking and car camping and offer a variety of temperature ratings and shapes. They are designed to be lightweight and pack down small, making them efficient at keeping you warm. Similar to two-season bags, they come with features like draft-blocking, snug-fitting hoods, extra zippers, and stash pockets.
Four-season sleeping bags are designed for colder temperatures and offer a range of temperature ratings and shapes to accommodate different preferences and sleep styles. They are typically larger and have more insulation to provide extra warmth.
Five-season sleeping bags, such as the Tyin MTI 5-Season, are designed for tough conditions and extremely cold regions. They are made with robust synthetic fibers that provide improved insulation and protection from the elements. These bags are ideal for activities like mountaineering and climbing in cold and damp environments.
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Down fill power
When buying a sleeping bag, it is important to consider the down fill power. Fill power measures the loft and warmth that down provides—essentially, it indicates overall quality. The higher the fill power, the better the warmth-to-weight ratio. A bag filled with high-loft down will be more compressible than a similar temperature bag with a lower loft/fill power. However, a higher fill power down doesn't necessarily mean it will be warmer; it just means that less fill power down is needed to achieve the same warmth.
The fill power of a sleeping bag is calculated by compressing a one-ounce sample of down and then allowing it to expand in a special cylinder. The total volume the down expands to (measured in cubic inches) makes up its fill power. This means that an ounce of 750+ fill power down would fill at least 750 cubic inches.
The type of down also matters. Goose down has been marketed as a 'premium' product, but quality duck down can be just as lofty and long-lasting. The design and construction of the sleeping bag are also important in maximizing warmth and comfort. Baffles in down sleeping bags are designed to control where the down is placed and to stop it from shifting around and creating cold spots. The type of baffle matters, too—box baffles, sewn-through construction, and overlapping baffles are all suitable for different temperatures.
The weight of the sleeping bag is another factor to consider. The majority of backpacking sleeping bags are filled with down because it is considerably warmer, lighter, and more packable than synthetic insulation. However, synthetic insulation is much cheaper and retains its ability to insulate when wet. If casual, low-mileage backpacking mixed with car camping is more your speed, using a sleeping bag that weighs well over two pounds is fine. These models usually cost a lot less due to less-than-premium materials, so they’re great for backpackers on a budget.
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Synthetic insulation
Synthetic sleeping bags have also made strides in closing the "weight and bulk gap" with natural goose down bags. For three-season use, the weight and bulk differences between a quality goose down bag and a quality synthetic bag are negligible. Synthetic bags today can compress down to a package barely larger than a goose down bag, while only weighing about 4-6 ounces more.
However, for winter use, goose down bags are dramatically smaller and lighter in weight. The extra insulation needed for winter synthetic sleeping bags increases bulk and weight. Synthetic insulation can also be heavy and hard to compress. Synthetic sleeping bags are also not as packable as goose down alternatives.
When buying a sleeping bag, it is important to consider the lowest temperature you are likely to be sleeping in, as well as your own internal thermostat. Synthetic sleeping bags are often given a season rating to indicate the time of year for which they are most suitable. For example, a one-season sleeping bag is designed for summer camping and backpacking, while a four-season sleeping bag is designed for winter use. Sleeping bag temperature ratings are also often given as estimates based on lab testing. However, real-world comfort may differ from the lab-tested temperature rating due to variables such as differences in clothing, gear, body types, weather conditions, and food.
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Frequently asked questions
The degrees on a sleeping bag indicate the temperature range for which the bag is designed. For example, a one-season sleeping bag is designed for temperatures above 5° Celsius/Centigrade or 40° Fahrenheit, while a four-season sleeping bag is designed for winter use when temperatures drop to around -10° Celsius or 15° Fahrenheit.
A comfort rating is the temperature at which a person who feels cold would be comfortable in the sleeping bag. A lower limit rating is the temperature at which a person who sleeps warm would be comfortable.
First, consider the lowest temperature you're likely to encounter. Then, think about your internal thermostat—do you tend to feel cold or run hot? If you often feel cold, opt for a bag with a comfort rating that is significantly lower than the coldest expected temperature. If you run hot, you may rely on the lower limit rating, but it's still recommended to have a buffer of a few degrees Celsius.
Synthetic insulation is more affordable and keeps you warm even when wet. It also dries faster than down insulation. However, down insulation lasts longer and provides better warmth with less filling, resulting in a lighter sleeping bag.
Season ratings indicate the time of year for which a sleeping bag is most suitable. A one-season sleeping bag is designed for summer camping, a two-season bag for spring and summer, and a three-season bag for spring, summer, and autumn when temperatures might dip below freezing.
