So what about the vermiculite insulation? It is true that it will be impossible to ensure that all sides of the liner are insulated evenly. As an earlier poster mentioned the flex liner will not be perfectly straight the whole way down. Even if it is straight it will only allow for a quarter inch of insulation on two sides as the inside diameter of my clay flue is 6". I can do the round for almost a thousand dollars less than oval. Do you think it is worth it to do oval so I can use insulation blanket?