Thermal Resistance (R-value)

Thermal resistance tells you how strongly a layer of material slows heat flow. It increases when the layer is thicker and decreases when the material conducts heat well, because heat has to travel farther through a poorer conductor. This comes from Fourier’s law of heat conduction: for a flat layer, the temperature drop across the material is proportional to its thickness and inversely proportional to its thermal conductivity.

Use this formula when comparing insulation materials, estimating how much a wall, pipe wrap, or panel resists heat transfer, or when building up the total R-value of several layers in HVAC and refrigeration systems. It is especially useful for steady-state conduction through a single uniform layer.

• Using the wrong units for thickness or conductivity, which gives an incorrect R-value
• Thinking a larger k means better insulation, when it actually means heat passes through more easily
• Confusing thermal resistance R-value with total heat-transfer rate, which are different quantities