ISO 23537 is the common sleeping bag testing standard adopted and strictly enforced by the US since 2017 (Previously known as EN13537), so we decided to test our products according to the toughest standard in existence.  

Ratings are based on a person wearing one long underwear layer and a beanie, sleeping on an insulated surface. Everyone's body and sleep comfort differ, and conditions of use (posture, clothing, sleeping pad R-value, wind, humidity, etc.) affect total insulation, so ISO ratings are intended as a guideline to help you compare products, rather than a guarantee of warmth.

Here is a brief description of the temperature ratings:

Comfort: the temperature at which a standard woman wearing the Selk'bag can expect to sleep comfortably in a relaxed position.

Limit: the temperature at which a standard man wearing the Selk'bag can expect to sleep comfortably in a relaxed position. 

Extreme: the temperature at which a standard woman wearing the Selk'bag can remain for six hours without risk of hypothermia or death. 

If you would like to understand it better, our partner REI has made a great video explaining it really simple and very straight forward. Check it out here