As an example, consider a composite wall of cross-sectional area A . The composite is made of an L 1 long cement plaster with a thermal coefficient k 1 and L 2 long paper faced fiber glass, with thermal coefficient k 2 . The left surface of the wall is at T i and exposed to air with a convective coefficient of h i . The right surface of the wall is at T o and exposed to air with convective coefficient h o .