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The various types of insulation are rated based on their thermal resistance (i.e. how good they are at resisting heat flow). The two thermal resistance values you’ll encounter are the RSI and R-Value. The R-Value is an imperial system unit of measurement (ft2·°F·h/BTU) and the RSI Value is a metric system unit of measurement (m2·K/W). The higher the resistance value, the slower the rate of heat transfer through the insulating material.

Spray foam, blown-in, 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.

Did you see a building science or energy efficiency term you did not understand? Check out our glossary.