The thermal inertia of a material is related to three key properties of that material. These are thermal conductivity, k, density, ρ, and heat capacity, cp.

The thermal inertia of a material characterizes the rate at which the surface temperature of that material will rise when it is exposed to heat. It governs how much heat energy will be transferred through the material to the back side in one case, or alternatively, if heat cannot transfer through the material, it indicates how quickly the surface temperature of the material might rise to a critical temperature for pyrolysis and/or ignition.

Materials such as brick and insulation have low thermal inertia; metals have high values. Wood can store a lot of heat but will transfer it slowly within itself. To see if a wooden door is hot, you would need to feel near the top of the door where there is more chance that heat might have transferred through the wood to the backside. If the wooden door has a metal handle or the door is metal, then you would expect this to be hot if there was a fire. This is because metal will not store as much heat near the surface and will more readily transfer it within itself.

Thermal inertia is very closely related to the concept of thermally thick and thermally thin solids.

"Thermally thin" does not necessarily relate to the thickness of the solid, but rather the time it takes to transfer heat from one side of a solid to another. When a thermally thin material is exposed to heat, the temperatures on the back and front sides of the object will increase in temperature due to the rapid transfer of heat through the material.

"Thermally thick" refers to a solid that absorbs heat energy more readily near the surface and does not transfer it as quickly through the material to the opposite side. Thermally thick materials, when exposed to a heat flux, will experience a substantial increase in temperature on the side facing the imposed heat flux, and the temperature on the back side will rise more slowly.