Consider a double$-$glazed window consisting of two $6$ mm thick layers of glass $($k $=0\mathrm{.}78$ W$.$m$-1$K$-1)$ separated by a $10$ mm wide stagnant air space $($k $=0\mathrm{.}026$ W$.$m$-1$K$-1).$ Thermal resistances coefficient of $0\mathrm{.}13$ m$2$K$1.$W$-1$ and $0\mathrm{.}04$ m$2$K$1.$W$-1$ can be assumed for the inner and outer glass surfaces, respectively. Calculate the U$-$value for the window.