$2.$ If the maximum U value of the external wall is recommended to be $1\mathrm{.}0$ W$/$m$2$oC$,$ calculate the thermal conductivity value of the building material required to be filled in the cavity of the wall so as to lower the U value of the wall to meet with the thermal comfort requirement. From Table $1,$ suggest a suitable material which can fulfill this purpose. $($Ans : wood$-$wool slabs is recommended$)$