Thermal resistance is the measurement that indicates a materials ability to insulate - there are a number of different ways to express this but, as they all measure the same factor, they are all related.
The starting point is the R-value. Around most of the world the R-value is expressed in 'SI' units and an R-value of 1 = 1 m2K/Watt. A material is said to have an R-value of 'X' when the temperature difference between the surface is 'X' degrees Celsius when the flow of heat through the material is equal to 1 Watt per square metre.
The exact meaning is perhaps not important when choosing a sleeping mat - its more the relative performance of each mat that you need to know. So all mats with an R-value quoted in SI untis can be compared directly with one another.
Some of our manufacturers quote insulation value using a 'TOG' rating - more commonly found to rate duvets a TOG of 12-14 is thought to be typical for a duvet suitable for winter us. For our purposes we need a conversion to the R-value expressed in SI units.
Happily the TOG value is based on the R-value and the conversion is that:
1 TOG = 0.1 R(SI) = 0.1 m2K/Watt
So a mat with a tog rating of 3 with have an R-value (SI) of 0.3
In the US R-values are expressed using imperial units of ft2F/Btu. The conversion is less neat than a TOG but still straightforward:
1 R-value (US) = 0.17622 R-value (SI)
Thus US quoted R-values are more than 5 times higher than the SI unit version.
Our Standard Approach
In order to compare mats with each other we have converted all manufacturers figures to the R-value (SI). Thus a typical low insulation mat will have an R-value of around 0.3, whereas the highly insulative Exped Downmats have values up to 1.4.