offset leaks every where, you have to keep wood burn in flame, a little more COMPLETE burning in higher temperature, flavor is burnt to CO2.
In a vertical smoker, heat efficiency is so good, charcoal never burn in flame, temperature is much lower, much "in-complete" burning, releases too much flavor. If you discard gasket on door, or even open a little bit of door, you will have a big fire, more flavor components are burnt to CO2.
Maybe heat insulated vertical smoker is a stupid idea. single wall vertical may be worth of a try