Ambient House is a building that maintains indoor temperature within a comfortable range by controlling gains and losses from ambient sources and utilizing thermal mass to moderate temperature changes when the sources are unavailable. Previously, necessary building characteristics were determined for passive solar as the heating source and ventilation as the cooling source in 11 US climate zones. (Sharp, M.K., 2012, “Indoor Comfort Achieved Exclusively from Ambient Sources Across US Climates,” ASME J. Sol. Energy Eng. 143, (6), p. 061005.) It was noted that in hot climates, such as Phoenix, AZ, there are long periods during which outdoor temperature is too warm for cooling, necessitating large thermal mass to avoid indoor overheating. In this article, thermal mass requirements are compared between sky radiation and nighttime ventilation cooling in all 16 US climate zones, including marine subzones 3C and 4C and very cold and subarctic zones 7 and 8. It is shown that sky radiation provides shorter intervals of cooling unavailability and allows much smaller thermal mass to achieve year-round indoor comfort in the hot climates of Las Vegas, Miami, New Orleans, and Phoenix, while it provides no significant benefits in cool climates, where thermal mass is dictated more by the need to slow the decrease in indoor temperature during cloudy periods in the winter. In Fairbanks, AK (zone 8), in particular, the lack of significant solar gains for almost 3 months during the winter requires large thermal mass to maintain indoor comfort. Minimal thermal mass is needed to meet the small summer cooling demand, and both sky and ventilation cooling are sufficient.