We have been using Second Skin Spectrum in our builds on the roofs....it's light, it's a sound deadener and it has thermal insulation properties. GREAT stuff and easy to use as it can be brushed, rolled, sprayed, whatever you want. 3mm is ideal as a total thickness so we try to use Second Skins Damplifier Pro then about 1mm of Spectrum on top of it. It's worth doing. Put some butyl adhesive in between your roof supports and the roof skin while you are in there.
I can't say enough about how good Second Skin is for both products and tech advice. Those guys know how to do it.
After speaking to @c2dfj45 I went with Second Skin Spectrup and damplifier pro, what a difference! Every cruiser I own will get this at some point.