What is R-value?
The fetched Omni insulation page explains R-value as a material's resistance to heat flow. Higher R-values mean better thermal resistance, while the reciprocal U-value represents how easily heat passes through the assembly.
How to calculate the R-value of a barrier
Omni sums the contribution of every layer after multiplying each material's R-value per inch by its thickness:
Total R-Value = R1 x t1 + R2 x t2 + ... + Rn x tn
R-value and U-value
The source page also highlights that the RSI value is simply the SI-unit version of R-value, and that the U-value is the reciprocal of the total R-value. Lower U-values are preferable because they limit heat transfer.