Definition: The R-value is a measure of the heat loss for insulation under specified conditions.
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The primary way heat transfer is slowed down by insulation is convection of the three modes of: conduction, convection, and radiation. A good insulator trap air which reduces convective heat loss leaving only conduction and radiation transfer. The primary role of insulation is to make a material that traps stagnant air.
Our Nordic Tubs use 2 lb. foam which is shown to provide an unbroken layer of insulation across all areas where it is applied.