Borrall Wonnell wrote:
I've got sheets of 2.5" XPS (R-15). I'm considering placing a layer
on top of the existing blown-in insulation.

Any idea if this is a safe/sensible thing to do? The XPS will be
supported by trusses and won't compress the blown-in insulation.

Fire hazard?

Your blown in may already be compressed. It should be over the trusses
at least a foot deep.