[Dix Dogfight]

The point with the islolation is to delay the parasitic heatloss to the surroundings.
The numbers we are interested in lies in the begining of the measurement = gradient of the slope. And not in the end where we reach equilbrium. The isolation makes th initial gradient appear for a longer time and therefore contributes to a better result.

If we, lets say, add a rad to the system it will reach equilibrium much faster and at a lower temp which gives us less data to correctly approximate the slope in the beginning of the measurement. A chiller is the total killer for this type of measurement because it regulates the water temp actively and that way destoys the gradient we are interested in.