thick. The inside wall is to be covered by a layer of wood in order to reduce the rate of heat flow through the wall by $90$ percent. The inner surface of the wooden wall is maintained at $-10\backslash$deg C and the outer surface of the concrete wall is at $20\backslash$deg C$.$ The thermal conductivities of concrete and wood are $0\mathrm{.}80$ W$/$m $$ K $($concrete$)$ and $0\mathrm{.}040$ W$/$m $$ K $($wood$).$ What should be the thickness of the layer of wood?