Be careful when examining the data sheets. Make sure you are comparing insulations at the same reporting temperatures. Also, it is important when comparing materials to obtain the insulation curve from the manufacturer at the lower pipe operating temperatures found in refrigeration systems as each materials ability to insulate is not linear. C Factor measure thermal conduction. It is the rate of heat flow through an actual thickness of material.
Thermal resistance is resistance to heat flow provided by a given thickness of material.
It is obtained by dividing the thickness of material by the K Factor. R Values change as the thickness of the insulating material changes. Primarily building building insulation products and plans are measured and specified by the material's R Factor. Make sure that appropriate safety factors from the manufacturer's data is properly representing the thermal values you need for design in your application.