A preformed insulation mat is provided for use between the inner and outer end cone housing of a pollution control device. The preformed insulation includes a frustum-shaped mat. The mat is formed of primarily heat moldable, silica based glass fibers containing Al2O3 and is molded at a selected temperature and for a selected time duration to heat set the fibers, whereby the frustum-shaped mat is seamless and self-supporting, requiring no shape no shape retaining elements or chemical binders.