What is this?

Man, I would be going for at least an inch AND adding a shield if anywhere close (not a black one); the radiaint heat from the headers when heated up is quite intense and if you are cruising slow around town for a long time on a hot day, the heat transfer would be a concern to me. Air flow will help some. Support is a must, IMO.

Pishta's comment on oil on a hot header are spot on; in our rally racing, the prime cause of fires is oil of some sort on a hot exhaust/header/turbo; they can be hard to put out and will start in an instant.