Pat,
We do not insulate the tunnel as in fill in the area / volume between the pipes and the aluminium but we do have a layer of insulation under the carpets.
This is sufficient.
I am aware of at least two customers that have injected foam into the cavity and this has worked well for them. An alternative is to wrap the two pipes with an insulation planket material similar to the engine heat shield that we supplied.
Best wishes,
Robert