Abstract: A method of validating whether a building or building portion has a design target heat loss coefficient is disclosed. According to the method, also known as the VeriTherm method, a plausible range of heat loss coefficients is determined in which an estimated measurement error does not exceed a combined sensor bias. An indication of whether the design target heat loss coefficient is validated is provided depending on whether or not the design target heat loss coefficient is inside the plausible range of heat loss coefficients. An apparatus may include modules adapted to perform the steps of the method. Further, a method of heating or cooling a building portion is disclosed. According to the method a power input to a building portion is determined in dependence on one or more of: a design target heat loss coefficient, a desired maximal internal to external temperature difference, a cut-off temperature, an intended period of measurement, and a heating/cooling period.