Thermal Resistance Value: This is a measurement of the ability of a material to resist heat transfer.

RSI: Metric Measurement
R-Value: Imperial Measurement

The R-value tells us how well a particular construction material insulates. The R in R-value stands for resistance. The higher the R-value, the better the insulation and the more energy you will save. An R-value applies to specific materials, not to systems. When talking about a system the combined value is referred to as an Effective R-value. Insulation materials are normally measured in R-value or RSI values, and a majority of the time you can find these values listed on the materials packaging. This is referred to as the Nominal R-value.