Sleeping Bag - Temperature Ranges

Temperature Predictions

EN ISO 23537 The standard temperatures that designate the range of utility for a sleeping bag system.

Comfort temperature (Tcomf): Lower limit the comfort range down to which a sleeping bag user with a relaxed posture such as lying on the back is globally in thermal equilibrium and not feeling cold (related to standard woman and in standard conditions of use). This is the temperature range where a “standard” woman is comfortable. According to the EN/ISO standards, she is “not feeling cold,” in a “relaxed posture.”

Limit temperature (Tlim): Lower limit at which a sleeping bag user with a curled-up body posture is globally in thermal equilibrium and not feeling cold (related to standard man and in standard conditions of use). (Note: The temperature is lower because the metabolic rate used in the equation for a man is higher than that used in the equations based on a woman’s physiology.) Here, a standard man is “in a situation of fighting against cold (posture is curled up inside the sleeping bag), but in thermal equilibrium” and not shivering. That means that somewhere within this range is likely the performance limit of your bag.

Extreme temperature (Text): Lower extreme temperature where the risk of health damage by hypothermia occurs (related to a standard woman with a curled up posture in a situation of high cold stress with shivering). This is a point of danger that can lead to death. Per the EN/ISO standard’s language: “In this range, a strong sensation of cold has to be expected. There is risk of health damage by hypothermia. A sleeping bag should only be used in this range in an emergency.”

Season Ratings

The explanations for sleeping bags season ratings ere quite simple. Sleeping bags are sold with 1,2,3,4 or 5 season ratings and this refers to the minimum temperatures in which the bag can comfortably be used (hence the alternative term of 'comfort rating'). Note: where there is no EN endorsement, these ratings are a guideline only and will vary considerably from manufacturer to manufacturer.

  • One-season sleeping bags are basic and generally used indoors or in tropical climates. Think kids on a sleep-over or a balmy night in a hammock!
  • Two-season sleeping bags are commonly called summer bags. Typically used in summer but depending on your base layers and the weather, you may be able to use it toward the end of Spring and early Autumn. Typically sold as a bag which can be used in temperatures down to 5 degrees centigrade.
  • Three-season sleeping bags are the most common choice for experienced campers (e.g. our MUMMY) Suitable for all but winter camping these sleeping bags are designed to keep you comfortable in temperatures down to around 0 degrees centigrade.
  • Four-season sleeping bags, as the name suggests, are designed for those who will be camping all year round or are likely to be in harsher environments such as camping at altitude in the mountains. Temperature range for four-season bags is around the -10 degrees centigrade mark.
  • Five-season sleeping bags are regarded as 'Technical Sleeping Bags' and are not really designed for the general market. They are more usually associated with expedition standard camping requirements as they are suitable for temperatures down as low as -40 degrees centigrade.

Other Considerations.....
You don't need to be a scientist to know that people react very differently to the cold – what is chilly to one person would be unbearable for another. Season ratings are a great starting point but they are not an exact science. This is even more obvious when you realise that season ratings vary considerably between brands with the result that one make of two-season bag may actually be a warmer buy than another's three-season product.
Also consider the following: those of a slighter build will not only have less body fat to keep them warm but will also leave more empty space in their sleeping bag which affects insulation and warmth. Young people are less prone to the cold than older people. Fitness levels will also dictate to some degree how much you feel the cold – usually the fitter you are the warmer you will be.
Typically men feel the cold less than women.

Where your tent is pitched and the environment generally may affect how warm you will feel – a tent pitched at altitude, somewhere exposed to the elements or in a damp place will always make you feel colder. Your sleeping mat – no matter how super-duper your sleeping bag, you are going to get cold if you don't insulate yourself properly from the ground.
There is perhaps some degree of trial and error with regard to knowing which season sleeping bag will suit you best. Don't be too concerned about how hot you might get - sleeping bags can be unzipped or discarded altogether and getting cooler is always going to be easier than getting warm enough! Consider when and how you will be camping along with your own tolerance of the cold and then opt for the best quality sleeping bag your money can buy. As you snuggle down into your cosy sleeping bag on your next camping trip then you will be glad you invested both the time and money to find the perfect sleeping bag for you.