Mark K. : By blocking the Heat
Energy radiating off of the barrel you are creating a bunch of trade offs! Your new surround will elevate the temperatures in the resulting pocket,
Reducing the draft created at the barrel. Much of that energy will be absurd in to the new surround to be re-radiated at a lower temperature ! With enough distance between
the barrel and the surround this is only a small problem !
However you are by creating that pocket of warm air decreasing the amount of radiant energy to heat objects/you! and trapping heat energy in air, where it ether must be
pushed with fans or Rise by convection to heat the ceiling !
If you do not mind the losses in efficiencies, this may not be a problem to you at all beyond a reduction in the horizontal distance you can flow your hot exhaust gases, and the
potential need for a Taller final vertical chimney ! Y.M.M.V. For the Good of the Craft ! Big AL