$.$ Find U for a $15$ cm thick RCC roof slab plastered on both sides with $1$ cm thick cement

plaster and having a false ceiling made of two layers of $1\mathrm{.}2$ cm soft board with an air gap

of $2$ cm$.$ Consider k value for RCC roof slab $=1\mathrm{.}58$ W$/($mK$)$; k value for plaster $=0\mathrm{.}721$

W$/($mK$)$; k value for soft board $=0\mathrm{.}047$ W$/($mK$)$; Conductance value for air $=6\mathrm{.}22$

W$/($m$2$K