minimum hight for a heat riser

I have a 2.5 foot tall barrel and i would like to use it as the barrel for my RMH, from what i have read it seems that the taller the heat riser the better the performance. I am only heating a small space so the exhaust run could be short.

so my question:

can you create a rockety burn with a very heavily insulated but only 2 foot tall heat riser?

and if so would this work better with a larger internal cross sectional area, perhaps 8in?

any answers would be appreciated

L

I don't know what the minimum is, but I should think that your heat riser can be rather taller than the barrel. that might allow even more insulation surrounding the portion of the riser that is below the barrel.