A temperature rating defines the warmth of almost every sleeping bag. This rating tells you the coldest temperature you can experience and still stay toasty inside your bag. A sleeping bag’s actual warmth will depend on the standard used to test it. Sleeping bag temperature ratings can be determined by several different standards, or by no standard at all.
The most common protocols to determine sleeping bag temperature ratings are the European Norm (EN) 13537 and the International Standards Organization (ISO) 23537 standards. These standards establish a way to determine how well an adult will stay warm as air temperature drops. EN/ISO ratings establish two temperature ratings: a comfort rating and a lower limit rating. However, many sleeping bags display a temperature rating but don’t specify which (if any) testing standard was used. If your sleeping bag wasn’t tested using an approved standard, you could (literally) be left in the cold.
EN and ISO Sleeping Bag Temperature Ratings
The EN 13537 standard was established in 2005 to make temperature ratings consistent between different sleeping bags. The EN 13537 standard has since been updated to the ISO 23537 standard. These standards establish repeatable methods for testing sleeping bags so that when you buy a 20o F bag you’re not shivering the second it gets below 40o F.
Basically, the temperature test involves putting a sensor-infusing manikin into a sleeping bag, which also contains lots of sensors. The sleeping bag and manikin are placed into a temperature-controlled chamber. Temperatures on the manikin and in the bag are monitored as the temperature in the chamber drops. Results from the test actually give four different temperature ratings. The comfort, lower limit, extreme and maximum temperatures. Usually, only the comfort and lower limit temperatures are reported by sleeping bag manufacturers.
Temperature ratings are determined scientifically, but they are still subject to differences of individuals. If you get cold easily while sleeping you might need a 0o F where someone else would be just fine with a 20o F bag.
The comfort temperature is the lowest temperature where a user in a ‘relaxed posture’ is ‘not feeling cold’. Generally, the comfort temperature is the lowest temperature where a warm sleeper would still be warm. Often, this temperature is used to rate women’s sleeping bags.
Lower Limit Temperature
The lower limit temperature is the lowest temperature where a ‘user with a curled up posture’ is ‘not feeling cold’. Generally, the lower limit temperature is the lowest temperature where a cold sleeper would still be warm. This temperature is most often used to rate sleeping bags.
The extreme temperature is the ‘temperature where the risk of health damage by hypothermia is possible’. Do not use a sleeping bag below this temperature unless there are no other options.
The maximum temperate is where a user in a ‘partially uncovered’ sleeping bag would not be sweating ‘too much’. Unzip the bag for more comfort when this temperature is exceeded.
Why You Should Only Get a Sleeping Bag That is EN or ISO Rated
Sleeping bags that are not EN/ISO tested will also have temperature ratings. Often, these bags do not specify the testing method. The comfortable temperature of a bag that is not EN/ISO rated could be 20-30o F higher than the bag’s rating. My wife and I once shivered for an entire night on the North Rim of the Grand Canyon in 0o F bags that were not EN/ISO rated. The outside temperature was 30o F.
Assume the temperature rating for a sleeping bag is incorrect if it was not tested to EN or ISO standards. Sleeping bags with EN/ISO ratings may cost a little more because it takes time and effort to conduct temperature tests. The extra money you spend will be worth it. Contact the manufacturer if you are unsure of the testing protocol used to rate a sleeping bag.