Here is an article about the EN 13537 norm. Some aspect will not be discuss as they are discussed elsewhere (see for example Mammut sleep well report).

I'll just discuss technical aspects and try to describe as precisely as possible how the get the temperature rating.

First let start with some physiology :

The EN test make some hypothesis about human physiology. The most important one for the test is the Heat production. The hypothesis is the following : a average man produce 38 watt per meter square of heat while sleeping. It's about 57 watt.

The model behind the limit confort temp (actually it is the same model fot other temp rating, only parameters are changing):

T_c=T_0-P*R

-T_c : is the limit confort temparature,

-T_0 : is the temperature at which an average man can sleep naked without any insulation. It is supposed to be 34 celsius (93.2 F) in the EN test. And it depends only on the physiology of the subject

-P : is the power output by meter square and depends only on the physiology of the subject. As I said it is supposed to be 38 in the EN test.

-R : is the thermal resistance which is related to dry heat loss (convection, conduction and radiative heat transfer. It doesn't take perspiration into account. So people that sweat more are forgotten in this number. It depends on the insulator as well as outer and inner shell. This number measure the insulating property of the sleeping bag. And is what Lab tester measure.

So a word about this model : If R=0 (no insulation) it gives T_c=T_0 which is logical.

T_0 and P are strongly dependant on the individual. You can measure your own P and T_0 and determine what is your confort limit temperature quite easily. I'll make a post about that later.

Now i rewrite the rule using the parameter in SI unit :

T_c=34-38*R.

I deduced this law in the following way. First T_c=T_0-R*P was the only formula that I could think of. I thought also about a quadratic correction term but by ploting EN test result on a graph I directly noticed it was linear. Then it was easy to determine the power output hypothesis. Then I compare the plot of this linear equation and compare with observed values and it fits perfectly.