It is critical to have an insulated riser. Temps inside run 1500 F + . You want to keep that heat traveling up to strike the top of your barrel and do a complete burn . An non insulated riser is going to shed its heat outward not up , your rmh will not draw well and you may end up with smoke back, plus excessive ash buildup . Just not a good idea.
Buy the builders guide or the dvd collection or both to understand how a rmh needs to be built to function properly.
Build it by the book the first time. You can always play with design changes later, outside ... after you have a perfectly working wonderful rmh in your living room.