What is the difference between R-Value & RSI?

Q: What is the difference between R-Value & RSI?

Both are a measure of how well a material resists the passage of heat, R-Value is the imperial measurement and RSI is the metric equivalent. 

Spray foam insulation, blown-in insulation , and rigid-board type insulation are generally rated by their RSI Value per millimeter or R-Value per inch. Fiberglass and mineral-wool type insulation that is designed to fit into a framed cavity are generally rated by the size of the cavity that they are designed to fit into.

To convert an RSI Value to an R-Value, multiply the RSI Value by 5.678.

To convert an R-Value to an RSI Value, divide the R-Value by 5.678.