insulating it means the header cannot "breath". i read up something about it sometime ago, its about the header's material needs to breath, wrap it with fiberglass heat wrap or coat it with ceremic basically preventing it from happening.
sometime before all race car in nascar used to wrap their header, because they give that extra little power that might let them win the race because if heat dun escape mean energy won't excape. but then they find that it reduces their header's life by alot, and mind you thats not some mass marketed header, each of them is custom made, so they found out its not really feasible, and nascar's budget isn't as high as F1.
there's one thing you can spray on the header i think, will insulate heat partially, not as good as those heat wrap, but they won't kill your header. was reported to cool the header down to 300 degree C, compare to 800-900 c