(Also see "Energy Codes")

R-value is typically used as a measurement of the effectiveness of thermal insulating materials. Building and energy codes frequently specify a minimum R-value for roof assemblies and exterior walls, so R-value is something that you need to be aware of. Since R-value is so often used to calculate the amount and type of insulation required for a job, R-value is an important part of the product data provided by insulation manufacturers, and can normally be found on the technical data sheets at their websites. 

An insulating material’s resistance to conductive heat flow is measured or rated in terms of its thermal resistance, or R-value; the higher the R-value, the greater the effectiveness of the insulation. The R-value depends on the type of insulation, its thickness, and its density. When calculating the R-value of a multi-layered installation, add the R-values of the individual layers. Installing more insulation generally increases R-value.

Useful Links:

1. General: Insulation R-Value: a handy and extensive R-value chart that covers almost all insulation and other building materials is available toward the end of the R-value (insulation) Wikipedia page. You should, of course, see the manufacturer-provided technical data sheet for the R-value of specific products.