Development and experimental validation of a heat, air and moisture model for metal-clad roofs

• Heat, air and moisture (HAM) model developed for roofs • Suitable for roofs with interconnected roof and above-sheathing spaces • New model proposed to predict condensate runoff in simulations • Field tests run covering four ventilation configurations • Model successfully predicted measured ventilation rates, temperatures and humidity The need for accurate hygrothermal models of roofs is increasing, as design practices change to reduce emissions and adapt to the changing climate. However, existing models are not capable of simulating all relevant physical processes, and the pool of available validation data is still quite limited. The new heat, air and moisture (HAM) roof model developed in this study provides new capabilities, including: the ability to model several interconnected air spaces (e.g. roof and above-membrane spaces), a model to predict the onset and magnitude of condensate runoff, and the use of different external convective heat transfer coefficients to previous models. Experiments were run to generate validation data using a well-instrumented test building with gable roof, with four different configurations of ventilation openings. Extensive measurements were taken of temperature and humidity throughout the roof structure, ventilation rates using tracer-gas, and ambient weather conditions. Validation of the new model revealed relatively close agreement between simulated and measured temperatures (mean absolute error 1.0°C), humidity (mean absolute error 0.57 g / kg dry air), and ventilation rates (FAC2 76 % – 85 %). A parametric study also revealed: timesteps up to 30 min did not impact the accuracy of the model; an appropriate model for external convective heat transfer coefficients is identified; an airflow network allowing air movement between the roof and above-membrane spaces improved temperature predictions; and modelling the roof space as well-mixed produced the most accurate results. The developed HAM model provides a suitable platform for future simulation studies of metal-clad roofs.