The influence of air permeability and type of underlay on the hygrothermal performance of an inclined roof

The airtightness of inclined roofs is important in order to avoid hygrothermal problems and to guarantee the durability of the construction itself. Previous research in building physics showed that perfect airtightness of inclined roofs is difficult to achieve and maintain. In practice, air transport through the construction, i.e. in/exfiltration, cannot be avoided due to for example imperfections or bad workmanship. The heat and moisture conditions in the building component are strongly influenced by advection, i.e. air transport through the building component, resulting from air pressure differences between the indoor and outdoor environment.
Current models to predict heat, air and moisture (HAM) conditions in building components assume uniform boundary conditions, both for the temperature and relative humidity imposed at the internal and external surface of the building component. In such models, the heat and moisture fluxes due to advection are generally not considered. A more detailed description and prediction of the influence of the air transport through the building component on the HAM conditions in the building component would be desired.
In the work presented in this paper, the hygrothermal performance of an inclined roof is analyzed while taking into account the air transport through the construction. An airflow model is used to describe the advective transport through the building component. The airflow model is integrated into an existing HAM building component model. The analysis focuses on a case study which is based on a common roof design in Western Europe. Three configurations consisting of a vapour-retarding underlay foil (which was applied in the past), vapour-open underlay, and a fibre cement board underlay are investigated.