How are the temperature ratings worked out?
There are a number of standardised measurements for rating the warmth of sleeping bags, the most important being EN13537 in Europe and ASTM F1720 in the US. The minimum standard recommended by the European Outdoor Group (the outdoor trade organisation) is the labelling of temperature values as shown below:
- Comfort rating - Temperature at which a standard adult female can have a comfortable night’s sleep
- Lower limit - The lowest temperature at which a standard adult male is deemed able to have a comfortable night’s sleep
- Extreme - A survival rating only for a standard adult female. After 8 hours, hypothermia may start to start set in. Your purchasing decision should not be based on this!
The EN testing system is not perfect but does allow comparison of sleeping bags. It is not used by every sleeping bag manufacturer or for every sleeping bag in a range. For example, Marmot submit almost all their sleeping bags for EN testing except their very warm expedition bags as they do not believe the EN system accurately reflects the performance of those bags. The difficulty arises because the EN system tries to apply rigorous scientific procedures to a situation that, in real life, has many variables. So it uses a medium warmth mat in their laboratories. Which is fine but in the real world anybody sleeping at -30°C is going to be using a very warm mat (or mats). Somebody sleeping at +20°C may only need a very light mat for padding, not warmth. To be fair, it works for most bags, maybe not quite as well for bags at either far end of the warmth spectrum.
The Leeds University testing system (using British Standard BS4745 and ISO 5087-1) was in existence for many year before the EN system and is still used by some manufacturers. The test produces a TOG rating that can be converted to a temperature rating. In our experience, the Leeds test result lies somewhere between EN lower limit and comfort rating. In other words, treat it as generous by a few degrees compared to the EN lower limit.