PROBLEM $2$:

A composite wall made of two materials consists of a $10$ cm thick layer of

brick $(k_b=0\mathrm{.}15\frac{W}{m}\frac{?}{K})$ and a layer of insulation $(k_i=0\mathrm{.}04\frac{W}{m}\frac{?}{K}).$ Knowing

that the surface temperature of the brick is $35C$ and that the contact

temperature between the brick and insulation material is $30C,$ determine

a$)$ the insulation thickness if the surface temperature of the insulation

must be lower than $20C,$

b$)$ the heat transfer value $k\frac{W}{m^2}$