Problem $5-4$ The top floor ceiling of a building $3036ft$ is constructed of $24in.$ joists on

$18$ in$.$ centers. On the underside is metal lath with sand aggregate plaster, $\frac{3}{4}$ in$.$ thick. On top

of the joists there are only scattered walking planks, but the space between the joists is filled

with rock wool. The air temperature at the ceiling in the room is $78F$ and the attic temperature

is $25F.$ Find the overall coefficient of heat transfer for the ceiling.