2.2.2. Thermal Phase Lag (ϕ)

The thermal phase lag determines how long it takes for excitation heat to go through an opaque material, and it represents the time difference between the moments when the maximum excitation and response heat fluxes occur (Figure 4b) [26].

According to the above definition, the instantaneous heat fluxes occurring on both sides of an opaque wall can be used to calculate decrement factor and thermal phase lag directly. The decrement factor in the article is defined by the ratio of the maximum conduction heat gain on the inside face to the maximum excitation heat on the outside face of an external wall.

(**a**) Decrement factor.

(**b**) Thermal phase lag.

**Figure 4.** Thermal response on a sinusoidal heat transfer through a wall [26].
