Chapter 16, Problem 90GP

A Double-Paned Window An energy-efficient double-paned window consists of two panes of glass seen with thickness L₁ and thermal conductivity k₁, separated by a layer of air of thickness L₂ and thermal conductivity k₂. Show that the equilibrium rate of heat flow through this window per unit area, A, is

  $\frac{Q}{At}=\frac{(T_2-T_1)}{2L_1/k_1+L_2/k_2}$

In this expression, T₁ and T₂ are the temperatures on either side of the window.