  The main perturbative contribution to the free energy of an electroweak
interface is due to the effective potential and the tree level kinetic term.
The derivative corrections are investigated with one-loop perturbation theory.
The action is treated in derivative, in heat kernel, and in a multi local
expansion. The massive contributions turn out to be well described by the
Z-factor. The massless mode, plagued by infrared problems, is numerically less
important. Its perturbatively reliable part can by calculated in derivative
expansion as well. A self consistent way to include the Z-factor in the formula
for the interface tension is presented.
