Quick Answer
Sleeping bag temperature ratings and sleeping bag seasons are both guides designed to assist with selecting the appropriate sleeping bag for the climate you require. Sleeping bag temperature ratings are calculated by testing bags to identify suitable temperature ranges a bag can insulate against, and are based on a standardized measure, ISO-23538-1. Alternatively, sleeping bag seasons are a general guide created to group bags based on seasonality, rather than precise temperature ratings.
Disclaimer: All information, content, and material on SlumberSeeds.Com is for informational purposes only and is not intended to serve as a substitute for the consultation, diagnosis, and/or medical treatment of a qualified medical professional.
How To Choose A Sleeping Bag
Choosing a sleeping bag can be a daunting process for first-time campers, trekkers and backpackers. You need to consider the temperature you comfortably sleep, the climate you will be sleeping in, the weight of your bag, sleeping bag dimensions, packed size (volume), your shelter, and how much you can reasonably afford to spend.
The primary focus of choosing a bag is for insulation, as the bag must keep you warm (and safe) above all else. Sleeping bag temperature ratings and sleeping bag season guides are designed to help you choose the appropriate bag for the climate you are exposed to. If you want to know more about how to choose a sleeping bag check out our article How To Choose A Sleeping Bag – 19 Types Of Sleeping Bags.
We have split this article into three parts;
- Warmth
- Sleeping Bag Seasons
- Sleeping Bag Temperature Ratings
Warmth covers sleeping bag comfort rating and the aspects that can affect your warmth. Sleeping bag seasons breaks down the general guidelines for categorizing a bag based on seasonality. Sleeping bag temperature ratings discuss the IS0-23537-1 rating and how it is used to determine a bags insulation.
What Effects Warmth When Sleeping Outdoors?
Sleeping bag temperature ratings are not the only thing you need to consider, as many factors will affect the warmth you feel while sleeping. The outside environment, your sleep system, and how warm you sleep, are all factors that contribute to your overall comfort and need to be taken into consideration when choosing a bag rating.
Outside Environment
The first thing that will affect warmth of sleep is the climate of the areas you are traveling to, or through. Not only will the actual temperature play a role, but also humidity levels, amount of wind, and rain.
Sleep System
What is a sleep system?
A sleep system consists of a sleeping bag, sleeping pad/mat, and shelter. A sleeping bag and mat work together to provide insulation around a sleeper, whereas shelter protects from rain and wind.
Sleeping Bag Comfort Rating
The sleeping bag comfort rating is the temperature a cold sleeper will most likely feel comfortable. As the temperature increases beyond this point, a sleeper will begin to feel overly warm, and as it decreases, the sleeper will begin to feel overly cold. The overall sleeping bag temperature rating is a great indication of the insulation you will get from a bag, however, a bag needs to be paired with a mat or pad to insulate heat escaping from the ground.
Sleeping Mat
A sleeping mat provides a sleeper with two things: cushioning for a more comfortable sleep, and insulation to keep warm. In addition to comfort and insulation, you will also want to consider the weight of the mat or pad when backpacking, trekking or hiking. One of the biggest challenges to warmth when sleeping outdoors is your sleeping bag compacting under your body weight, reducing the insulation provided beneath the user. A sleeping pads insulation or temperature rating equivalent is the R-Value. R-Value stands for resistance to heat flow and describes how well the pad will resist transferring heat from your body to the ground. The higher the R-Value, the more insulation a sleeping pad provides.
There are three types of sleeping pads;
- Air pads – Air pads like the Therm-a-Rest NeoAir Xlite are ultralight and provide great comfort and warmth, while being extremely light and small when packed.
- Self-inflating pads or open-cell foam pads – Self-inflating pads like the Stoik’d sleeping pad are lightweight and can protect in sub-zero temperatures.
- Closed-cell foam pads – Foam pads like the Redcamp closed cell pads are extremely durable although they are less comfortable than air pads.
In addition to the variation in sleeping pad types, some brands have pads that are specific to women, accounting for additional insulation requirements across the midsection and feet.
Shelter
The type of shelter you intend to use plays a role in the effectiveness and the insulation requirements of a sleeping bag. Although there is some debate around warmth, ventilation, and condensation when it comes to tents, at the very least they provide protection from wind and rain. The most common types of shelter you will find on your adventures include;
- Tent
- Tarp
- Swag
- Caravan
- Car
- Lean-to
- None
Temperature You Sleep
Warm Vs Cold Sleeper
A cold sleeper will require more layers of clothing and covering for a 40 °F night than a warm sleeper because they tend to produce less core body heat. Generally, women sleep colder than men and require a better insulating bag and pad. The reason being, women have slightly higher core temperatures than men and will, on average, feel colder.
Other Factors
In addition to how cold or warm you generally sleep, factors that contribute to your sleep temperature and insulation needs include;
- The clothing you wear to sleep
- Food intake before bed
- General metabolism
- Dampness from sweaty clothing
Perception of hot and cold will also play a part in determining whether you are a cold or warm sleeper. This means two sleepers in identical climate conditions, same core body temperature (and all else being equal), may not feel just as comfortable, and one might be less tolerant of the cold.
Sleeping Bag Seasons
Sleeping bag seasons are a guide based on seasonality, range rather than precise temperature ratings. A sleeping bag season rating will give you an idea of which seasons a bag is most suited to. These guidelines vary widely and can be categorized as summer and winter, or season by number. Sleeping bag seasons are a starting point for buyers, rather than the actual temperature a particular bag can insulate against. Since sleeping bag seasons are not a formal rating, recommended season temperature ranges will vary based on the source of information.
Sleeping Bag Seasons: Summer
1 Season Sleeping Bags
1 season sleeping bags are intended for summer use, when temperatures at night are warm. Although lightweight and compact, they don’t contain much insulation, meaning they will not provide warmth on cooler nights. Since they have less insulation, 1 season sleeping bags are inexpensive compared to warmer winter bag types. Typically, the temperature range for bags that are categorized as 1 season is above 50°F / 10 °C.
2 Season Sleeping Bags
2 season sleeping bags are also intended for summer use, however, they can also be used in late spring and early autumn. They will help insulate on cooler nights, so they are much more versatile than the 1 season type. 2 season sleeping bags are best used for temperatures of 40 °F / 4 °C or higher.
Sleeping Bag Seasons: Winter
Winter sleeping bags are intended for use between late autumn and early spring. Like Summer sleeping bags, they too are broken into two categories: 3 & 4 seasons.
3 Season Sleeping Bags
3 season sleeping bags can handle the latter part of autumn, early spring, and possibly mild winters (depending on location) and are suited to temperatures 24 °F / – 5 °C and 40 °F / 5 °C. Although 3 season sleeping bags can handle cooler climates, they are generally not recommended for frosty conditions.
4 Season Sleeping Bags
4 Season sleeping bags are suited to frosty and snowy conditions, making them the best option for common winter temperatures. 4 season bags are best suited to hikers, campers and backpackers who experience temperatures 23°F / -5°C and lower. Unless you intend on venturing into extreme cold, sleeping bags within this category will satisfy most insulation requirements.
Sub Zero Sleeping Bags
Sub-zero sleeping bags are intended to be used for extreme weather conditions such as expeditions and mountaineering. On occasion, sub-zero bags can be referred to as 5 season sleeping bags, as the intended weather conditions aren’t typical of a winter season. Bags such as the Nemo Canon are a great example of a sub-zero sleeping bag, with a temperature rating of -40 °F / -40 °C that compresses to 16.5 – 18 liters (long + regular sizes).
What Does Sleeping Bag Temperature Rating mean?
Sleeping Bag Temperature Ratings Explained
Sleeping bag seasons are a guide that doesn’t align with actual temperatures, but have been created to group bags so users have a general range of bags to choose from. Sleeping bag temperature ratings, on the other hand, use a scientifically tested method of identifying suitable temperature ranges that a bag will be able to insulate against.
The testing involves the use of a thermal manikin for measuring thermal transfer through the sleeping bag. For this reason, individual sleeping bag temperature ratings are more accurate, as they provide you with the upper limit, comfort, lower limit, and extreme ratings. Sleeping bag manufacturers also use a manikin and a freezer to optimize their sleeping bags to achieve desired temperature ratings before sending them to be officially tested.
What Sleeping Bag Temperature Rating Should I Get?
Becoming familiar with the average low and the lowest possible temperatures in the areas you will be sleeping will help you determine which bags will, and will not, be suitable. Once you’re familiar with the climate, you can begin searching for bags that have a comfort rating within the range of your sleeping conditions. You would generally want to give yourself a slight buffer, as 10 °F extra will not cause a great deal of discomfort (you can always vent), whereas falling 10 °F short could lead to a cold night.
Sleeping bag temperature ratings are an accurate representation of the average level of insulation you will receive from a given temperature. However, there are numerous other factors to be taken into consideration, which affect how well we maintain body heat such as:
- Gender
- Age
- Weight
- Height
- Metabolic rate
For these reasons, sleeping bag temperature ratings are still a guide, but a guide you can use with confidence when accounting for your individual attributes, sleep system, and sleep preferences.
Technical
ISO 23537-1
The ISO 23537-1 is an international standard for sleeping bag temperature ratings. ISO stands for International Organization of Standardization, which is an independent consensus-based body that ensures products are safe, reliable and of good quality. The ISO 23537-1 sleeping bag temperature ratings are an upgrade (2016) from the older EN13537 ratings, which were established in 2005.
Limits & Ranges
Upper Limit
The upper limit sleeping bag temperature rating refers to the possibility of the sleeper becoming too hot when using the bag. This is the upper limit of the comfort range where the sleeper is partially uncovered and does not perspire too much. Exceeding the upper limit temperature will increasingly make sleeping too warm.
Comfort Range
Comfort range temperature rating refers to the temperatures between the upper limit and the comfort level. In this range, the average sleeper will feel increasingly warm and will require ventilation as the temperature increases.
Sleeping Bag Temperature Rating: Comfort
The sleeping bag comfort rating is the temperature in which your body will not exchange heat with the bag (thermal equilibrium). This essentially means that no air from outside the bag is making you cold, nor is your body passing heat to the bag. This is the lower limit of the comfort range, where a cold sleeper will feel comfortable.
Transition Range
Transition range sleeping bag temperature rating refers to the temperatures between the comfort and lower limit. A warm sleeper will likely feel warm in this range, while a cold sleeper will begin to feel cold without shivering.
Lower Limit
The lower limit sleeping bag temperature rating is defined as the temperature in which a sleeper, who is curled up, is not feeling cold. Individuals sleeping preferences and natural body temperature will vary and dictate whether they still feel sufficiently insulated, or slightly cold around this temperature rating. Generally, at the lower limit, a standard warm sleeper can expect to have a comfortable night’s sleep.
Sleeping Bag Temperature Rating: Risk Range
The risk range lies between the lower limit and extreme temperature. As temperatures decrease from the lower limit, the sleeper will feel increasingly cold. Approaching the lower extremes of the risk range will render the bag a survival utility, as the bag is not designed to be used for prolonged periods. In the risk range, the sleeper can expect to feel a strong sensation of cold.
Extreme
Extreme sleeping bag temperature rating is the point where the sleeping bag will no longer provide sufficient insulation. This temperature, and below, represents a point of danger, where there is a possibility of hypothermia and death. The bag is not designed to be relied on at these temperatures and is intended for emergency use only.
How To Determine The Rating?
Provided the manufacturer has had their bag independently tested, you will find the temperature rating on the bag, and bag specs section of a product listing. Not all manufacturers adhere to the ISO (EN) standards, as there is no legal obligation to do so.
What Does Sleeping Bag Temperature Ratings Mean For You?
Outside of providing you with an indication of individual insulation levels, the rating system allows users to more accurately compare sleeping bags. Since testing is (relatively) standardized, bags can be compared across brands so you are not subject to the standards of a single manufacturer.
Drawbacks Of The Sleeping Bag Temperature Rating System
The obvious drawback of the ISO/EN systems is the variance in the preferences and requirements of an individual. The system allows the manufacturer to say “at this temperature, this bag should give you a comfortable sleep.” Although the statement is true for the average person, you may not feel comfortable at the recommended temperature. In addition to the uniqueness of each individual, the lab testing environment will not be the same as the real environment at a given temperature.
Sleeping Bag Fill
There are two types of sleeping bag fills to choose from; down and synthetic. Down is usually derived from ducks and geese, and is considered superior to synthetic fill, as it has a better warmth-to-weight ratio. Goose down is considered a higher quality down than duck, as it generally comes from older and larger birds. Synthetic fill, although heavier, has a handful of advantages over down such as; price, hypoallergenic, better insulation when wet and dries quicker.
Fill Power
Fill Power, or Fill Power rating, describes the amount of space a given weight of goose or duck down expands into at its maximum loft, creating insulation. The greater the Fill Power rating for a given temperature, the smaller the bag will compress when packed. For a closer look at the differences in down and Fill Power, check out our article Duck Down Vs Goose Down – Down Fill Power For Sleep & Outdoors.
Size & Weight
Sleeping bag temperature ratings will indicate the climates a bag will insulate against, but they don’t tell you much about the weight of a bag, or the size of the bag when packed. If two bags had a sleeping bag comfort rating of 30 °F, one was 550 Fill Power and the other 850 Fill Power, the 550 Fill Power bag would be heavier and larger when packed. In addition to fill, each additional feature a bag provides will increase its overall weight, so only selecting the most import features will help you travel lighter.
Final Word
Both sleeping bag temperature ratings and sleeping bag seasons are used as guides to select the ideal sleeping bag for the climate you require, and also to suit personal preferences. A person in a sleeping bag will feel comfortable somewhere between the upper limit and the lower limit of the sleeping bag temperature ratings. Exceeding the upper limit will lead to a hot and sweaty night, and is generally not recommended. Whilst exceeding the lower limit will increase the chances that the sleeper will suffer health damage, as a result of insufficient insulation.
