  We study the spin conductivity of the bulk states of three-dimensional
topological insulators within Kubo formalism. Spin Hall effect is the
generation of the spin current that is perpendicular to the applied voltage. In
the case of a three-dimensional topological insulator, applied voltage along
$x$ direction generates transverse spin currents along $y$ and $z$ directions
with comparable values. We found that a finite non-universal value of the spin
conductivity exists in the gapped region due to the inversion of bands.
Contribution to the spin conductivity from the vertex corrections enhances the
spin conductivity from the filled states. These findings explain the large spin
conductivity that has been observed in topological insulators.
