Examines the fundamental building heat loss calculations. Points out some anomalies in the traditional view of room thermal behaviour. Treats equations governing room heat loss taking account of the temperature of the walls and the environmental temperature. Justifies the use of the environmental temperature rather then the air temperature. Examines the heat loss due to ventilation. Discusses the use of the temperature ratio. Provides calculated examples which demonstrate that in general for most ordinary rooms the old method will do, but for large, well ventilated spaces it is advisable to use the new calculation method.