Since Willis Carrier's invention of air conditioning in 1911, we traditionally think about building conditioning in terms of the heating and cooling of a building's indoor air. A better idea is the heating and cooling of a building's mass. The latter has been called the radiant method, of which a most attractive strain is the thermally activated building systems (TABS) proposed by Robert Meierhans in 1990s. In this paper, a resistor-capacitor (RC) model is built in Matlab/Simulink for studying the system requirement of a TABS-equipped building-room. Specifically, what is the requirement in the envelope thermal resistance and activated TABS thermal mass of the room so that it is able to keep the room's indoor operative temperature within the comfort range with its surroundings at neutral mean ambient temperature? Systematic simulations show that at neutral ambient temperature, the room's manageability requires the correct selection of thermal mass at normal value and thermal resistance within minimum envelope resistance range (MERR). With its surroundings at above neutral ambient temperature, the room with the required mass-envelope combination functions robustly, albeit with a slightly larger operative temperature variation. We introduce the term thermally manageable building, defined as BUILDINGS THAT CAN BE MANAGED WITH OFF-PEAK EQUIPMENT, EITHER MECHANICAL EQUIPMENT (e.g., a chiller) OR (natural energy gradient driven) LOW-POWER EQUIPMENT (e.g., a cooling tower). Simulation results also show that while the mean operative temperature level is maintained by cooling equipment (mechanical or low-power one), the operative temperature variation is primarily a function of a building's thermal mass and a building's envelope thermal resistance and, only to a small extent, a weak function of mean ambient temperature and the diurnal temperature amplitude. (C) 2012 Elsevier Ltd. All rights reserved.