Representing Top-hung Windows in Thermal Models

The UK climate is moderately mild and natural ventilation by window opening and passive ‘trickle’ vents is very common. In this paper the natural ventilation rates provided by top-hung windows open to small angles in single-sided designs have been studied using a common whole building simulation tool. It was found that the amount of ventilation greatly depends on how the windows are represented. Such software tools usually assume that the opening area has the same vertical separation as the underlying window (i.e. it is modelled as an arrow slit). This is a reasonable approximation when windows are opened to a large angle, but not for small angles, when most of the opening will be at the base of the window. This work has shown that this can have substantial consequences for the simulated airflow into and out of a room. By modelling such openings more accurately, it has been found that the usual way of treating top-hung openings can give rise to an over-estimation of the ventilation rate at low wind speeds by a factor of five or more. The problem has been tracked to the over-estimation of the effectiveness of temperature-induced air exchange. This could have important consequences for meeting parts of the building regulations in the UK, for example minimum ventilation rates in schools. It is recommended that engineers use the representation developed here in future modelling work.