You will rarely get better advice than from the Big Finn! There are many other building materials you can use better than cardboard that have reflective and insulating qualities, not to mention may be more fire resistant....mylar covered foamboard comes to mind right away! Take a walk thru a big building supply store and see if you get inspired by anything! They seem to come up with new insulating board all the time, and the insulation and sound deadening properties will help your grow stay stealthy too.