  We study the relative zeta function for the couple of operators $A_0$ and
$A_\alpha$, where $A_0$ is the free unconstrained Laplacian in
$L^2(\mathbf{R}^d)$ ($d \geq 2$) and $A_\alpha$ is the singular perturbation of
$A_0$ associated to the presence of a delta interaction supported by a
hyperplane. In our setting the operatorial parameter $\alpha$, which is related
to the strength of the perturbation, is of the kind
$\alpha=\alpha(-\Delta_{\parallel})$, where $-\Delta_{\parallel}$ is the free
Laplacian in $L^2(\mathbf{R}^{d-1})$. Thus $\alpha$ may depend on the
components of the wave vector parallel to hyperplane; in this sense $A_\alpha$
describes a semitransparent hyperplane selecting transverse modes. As an
application we give an expression for the associated thermal Casimir energy.
Whenever $\alpha=\chi_{I}(-\Delta_{\parallel})$, where $\chi_{I}$ is the
characteristic function of an interval $I$, the thermal Casimir energy can be
explicitly computed.
