Temperature ratings are one of the most important factors to consider when purchasing a Sleeping Bag. Whilst you don’t want an uncomfortably cold night’s sleep, you equally want to avoid being too warm or carrying a Sleeping Bag too heavy for your trip.

There are various methods that help to determine the temperature ratings for a Sleeping Bag. However the European (EN) standard, known as EN 13537/ISO 23537, has now been adopted by the majority of Sleeping Bag manufactures, including Mountain Equipment.

The test takes place in an environmental chamber at fixed temperature. A heated manikin is placed inside the Sleeping Bag, and the Sleeping Bag rests on a specific foam mat. The manikin wears a thin base layer that’s typical for people to sleep in. Using a controlled environment and manikin reduces the risks and logistical complications of field testing on real people.

While we use the EN13537 / ISO23537 test to help inform the design and engineering of our Sleeping Bags, it is neither perfect nor relevant for all Sleeping Bags. Bags designed for extreme conditions are exempt, for example.

As such, whilst we quote EN13537 / ISO23537 results for the vast majority of our Sleeping Bags, for some specialist Bags (especially those which utilise very light, air permeable fabrics) the standard is a poor indicator of real-life thermal performance. Some of the factors to consider in relying on the EN13537 test Standard include…

Manikin Shape: The manikin used in the test is a specific shape, and if it compresses any part of the insulation around it then the Sleeping Bag performs poorly. The same effect would occur if a larger person attempted to use a Sleeping Bag that was too small for them. As a result our more tapered Alpine Fit bags sometimes have EN13537 / ISO23537 results that are slightly lower than the equivalent Mountain fit bag, even though for many users these bags will actually be more thermally efficient and consequently warmer.

Face Fabrics: When sleeping inside the shelter of a hut or tent the face fabric of a dry Sleeping Bag barely affects its warmth. This is because in these conditions there is very little air flow over the Sleeping Bag. However, the EN13537 / ISO23537 test is carried out with air moving at 0.3 m/s (about 1 mph) directly downwards onto the Sleeping Bag. This air movement is faster than users experience in most conditions, and is in a different direction. Thus, if a Sleeping Bag’s face fabric is air permeable (lets air through) then in the test it will tend to produce relatively poor results. For this reason, bags such as our Fireflash, Firelite, and Firefly Sleeping Bags perform much better than their EN13537 / ISO23537 ratings suggest unless used in very drafty situations.

All of our Sleeping Bags display EN13537 / ISO23537 temperature ratings except our very warmest Sleeping Bags, our Extreme Expedition Sleeping Bags and Glacier Expedition model, which fall outside the range of the test.

The EN/ISO tests produce three different temperature ratings: