R-value [h-ft^2-R/Btu]: that means square-foot·degree Fahrenheit·hour/British thermal unit or square-foot·degree rankine·hour/British thermal unit

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R is for Rankine. "hr-ft2-F/Btu" and "hr-ft2-R/Btu" can be used interchangeably.

The Rankine scale is the same as Fahrenheit, except it's based on absolute zero.

R = F + 459.7
So 50 F = 509.7 R
70 F = 529.7 R

The difference between these two temperatures is 20 F or 20 R. So for any metric based on temperature difference, F and R are interchangable. The same is true for Kelvin and Centigrade.