best heat insulator(powder/pellet form)

[fishtank]

iam looking to build a rocket stove and what to know what material to use as a heat insulator, i could just use sand but was hoping for something better (lighter weight and better heat insulation).
iam building 2 , a 1 gallon paint can, and other with a 5 gallon bucket

thanks

[albertaboy]

we use peralite, its a good insulator and very light. The garden stuff is very similiar. or maybe try the foam in a can.