R-value is a term referring to properties of materials:

• Insulation: R-value (insulation)
• Soils and aggregates: R-value (soils)

r-value, usually uncapitalized, is also term used in Computer science. See the article on Value (computer science).

In crystallography, the R-factor (also known as R-value), is a measure of the agreement between the crystallographic model and the diffraction data.