**Thermal conductivity, thermal diffusivity, and R-value**

As mentioned above, thermal *conductivity* tells us how fast (or how much) heat can transfer through a given material. A more formal definition would be the quantity of heat (Q) transmitted through a stationary material with a unit thickness (L) in a direction normal to a unit surface area (A) due to a temperature gradient (ΔT).

Thermal *diffusivity* may seem similar to thermal conductivity, but they are quite different. While the thermal conductivity tells us how much heat can move through a material, diffusivity provides us with a relationship between the ability of the material to conduct thermal energy and how much thermal energy it can store.

Thermal diffusivity can be seen as a material’s thermal inertia. In a material with high thermal diffusivity, the heat will move through the material rapidly because the material conducts heat fast relative to its thermal bulk.

R-value is closely related to thermal conductivity with a slight twist. R-value is a measure of thermal resistance and is commonly used in the building industry to measure insulative properties of materials (like the insulation between the walls of your home). It is the ratio of the temperature difference across the material relative to the heat flux. We can measure this property using our hot box instrument.