This study tests the feasibility of a more climate-friendly approach to space conditioning. This new strategy (called Ambient House) employs thermal mass in conjunction with ambient sources of heat and cold to maintain indoor temperature within a comfortable range with no auxiliary energy. Buildings in 11 US climate zones were simulated as first-order systems with envelope losses, internal heat gains, solar gains, and heat rejection by natural ventilation. Building similitude parameters were adjusted to maintain indoor temperature within a range of 20–25 °C. Building performance is defined by two mathematical parameters. First, the asymptotic temperature difference, which is a ratio of solar and internal heat gains to envelope losses and ventilation cooling, also incorporates control functions for solar heating and ventilation cooling. Second, the building time constant is the ratio of thermal capacitance to envelope losses and ventilation cooling. Temperature response was compared with that of buildings with the same characteristics, except low thermal mass. Without auxiliary heating and cooling and without active controls, low mass buildings reached temperatures as low as −3 °C and over 70 °C. With controls, extreme temperatures were still −3 to 40 °C. The Ambient House comfort temperature range was achievable with thermal mass within or near that of conventional construction in all but the hot climates of Miami, New Orleans and Phoenix. In these locations, large thermal mass and high ventilation cooling were required. This study confirms that it is feasible to thermally condition spaces exclusively with ambient sources in all the US climate zones.