Heat Transfer Coefficient of a Building: A Constant with Limited Variability or Dynamically Variable?

The heat transfer coefficient or the HTC is an industry-standard indicator of building energy performance. It has been predicated on an assumption that it is of a constant value, and several different methods have been developed to measure and calculate the HTC as a constant. Whilst there are limited variations of results obtained from these different methods, none of these methods consider a possibility that the HTC could be dynamically variable. Our experimental work shows that the HTC is not a constant. Experimental evidence base from our environmental chambers, which contain detached houses, and in which ambient air temperature can be controlled between -20 °C and +40 °C, with additional relative humidity control and with weather rigs that can introduce solar radiation, rain and snow, shows that the HTC is dynamically variable. Analysis of data from fully instrumented and monitored houses in a combination with calibrated simulation models and data processing scripts based on genetic algorithm optimization provide experimental evidence of dynamic variability of the HTC. This research increases the understanding of building physics properties and has a potential to change the way the heat transfer coefficient is used in building performance analysis.