N.Harris : Air is the best insulator, look at any thing that insulates and it works by trapping air inside it.
Come out from the wall that needs protection approximately 2-4 inches, then use any light stiff non-flammable material, that will block the radiant
energy of the suspect heat source, think of it as standing between your heat source and the wall you want to protect, casting a shadow on it . Now put a radiant heat reflective foil barrier on the stand off wall.
This will solve 90%, of your problems, if you have heat high
enough to make a hand held behind the stand off wall hot, or just want to ere on the side of safety you can add insulation between your dead air space and your Radiant reflective barrier, - if you dont like the shinny silver foil THEN cover it with brick, tile or stone !
There is also eccentric placing of the drum, slightly off of center as bench marked by the placement of the Heat Riser!
With a 8'' Heat Riser wrapped with, or made from 3'' of insulation/refractory material it's outside diameter is 14 inches.
With the inside of the drum being a little over 22 inches the barrel can be off-set with the pinched side being made cooler and the 'opened' side hotter and radiating more heat from this side of the drum !
Off-setting like this can lower the amount of heat radiated toward the exposure to be protected ! As used and pictured in Ianto Evans' Great book
'Rocket Mass Heaters"
The eccentric placement of the barrel can also allow for easier crafting/placement of the receiving hole in the
Rocket stove Base for the exhausting gases destined to be turned at right angles and fed through the horizontal stove pipe to the thermal mass . - Pyro AL
