SR343 Numerical simulation of ventilation in roof cavities (2016)

A numerical simulation tool is used to study the effects of installing additional ventilation channels to the roof cavities of typical New Zealand residential buildings. The simulation software was written specifically for this purpose and is based on a nodal model approach, where the building components are represented as nodal points with set properties, interacting with each other.

A standard gable-type roof as well as a skillion-type roof are used for the simulations. These two roof types are placed in four different geographical climate zones: Auckland, Taupo/Turangi, Wellington and Central Otago. In addition, we investigate the properties of an ideally ventilated roof cavity. Results for the summer and winter months and a range of wind speed conditions are given.

The findings are presented in terms of the probability of condensation conditions developing in the roof cavities. While absolute probabilities are stated in the report, they are mainly to be taken for comparative purposes. Additional ventilation channels are found to reduce the overall number of condensation events for both roof types in comparison to a roof without vents. In some cases, the length of a condensation event was also reduced. The skillion roof is found to be more susceptible to higher condensation event probabilities than the gable roof. It is demonstrated that reducing the moisture load in the living spaces by adequate ventilation downstairs can significantly reduce the condensation probabilities in the roof space upstairs.