Miguel, What is the diameter of the barrel you plan to use? My 55-gallon drum rocket stove, which is surrounding the burn chimney, gets to over 500 degf and I have actually measured temperatures greater than the range in my little infra red laser temperature sensor. In such a small space you are designing, the size of the barrel and the heat output would drive you out of the space if allowed to get really hot. So, heating such a small space would only require a very small rocket stove, usually not recommended in articles that I have read which say that 6-8 inch piping can be used. Whatever you do, and whatever size of your rocket stove, make sure the walls in the corner are insulated with a metal "air wall" that has about 1 inch to 2 inches of clearance between the wall and the metal, also at the bottom of the air wall. This allows air in to circulate by convection, thereby keeping the building wall cooler. Also the wall behind the barrel should be fireproof. If the system is designed properly the exit chimney temperature will be quite low in use, UNLESS THERE IS A PROBLEM, the normal 100-120 degf exit temperature could be compromised causing a fire if you do not install the chimney feed through properly with sufficient clearance and non flammable materials to handle such emergency conditions [like a chimney fire, for example]. My rocket mass heater is in a non insulated, translucent plastic, greenhouse that is 10 feet X 20 feet and 9.5 feet high at the apex. The mass is a 17 foot X 2 foot high x 2 foot wide rock wall of sedimentary rock [bluestone] with two courses of 8 inch pipe and 2 tees for the return and clean outs. The chimney exits and is about 12 feet tall. Therefore the total pipe length is about 46 feet. It works OK even in February when we are starting seedlings. As further assurance to keep our plants warm, we use row cover and a 1KW electric heater at night with the thermostat turned to the lowest setting just above freezing. I have an air wall over my barrel and do not experience any overheating problems even at 500 degf.