A composite slab has two layers of different materials with thermal conductivities k₁ and k₂. If each layer has the same thickness, then the equivalent thermal conductivity of the slab will be

Conductance = $$ = {\text{k}} \times \frac{{\text{A}}}{l}$$
Where A = slab area, $$l$$ = slab thickness and k = conductivity.
The equation for series conductance is :
$${\text{cond}}\left( {{\text{total}}} \right) = \frac{{{\text{con}}{{\text{d}}_1} \times {\text{con}}{{\text{d}}_2}}}{{{\text{con}}{{\text{d}}_1} + {\text{con}}{{\text{d}}_2}}}$$
Because the geometry of the slabs is the same:
$${\text{k}}\left( {{\text{total}}} \right) = \frac{{2\left( {{{\text{k}}_1} \times {{\text{k}}_2}} \right)}}{{{{\text{k}}_1} + {{\text{k}}_2}}}$$