A minor confusion with calculation of the total thermal resistance
You should add some value to all of the following thermal resistances:


junction to case
case to heatsink
heatsink to ambient


The total thermal resistance is the sum of the stack.

The first is easily found from the component datasheet. The 3rd is found from heatsink's datasheet. The second depends on two different products and the used assembly method. It's presented in application notes or found by making measurements. An example of application note:

https://www.fairchildsemi.com/application-notes/AN/AN-4166.pdf

Thermal resistance from case to heatsink is increased if there's some insulation between the parts. The insulators usually have their own specified thermal resistance, which should be added to the stack. 

Rth from case to heatsink can be substantially reduced with thermal conducting grease. There are also thermal contact plates which do the job of the grease and are not intended to be insulators. See the linked application note!