Tools to Assess the Risk of Surface Mould Growth and Condensation from Timber Thermal Bridges

This thesis aimed to identify a reliable tool to assess the risk of internal surface condensation and mould growth (internal moisture) from timber thermal bridges with particular reference to New Zealand. The current method to minimise this risk, used in Acceptable Solution E3/AS1 of the New Zealand Building Code, requires a minimum R-value for building elements of R-1.5, calculated using the Isothermal Planes method. However, this method smooths out the thermal bridges and does not consider their impact on localised surface temperatures. Moreover, the percentage of timber is used to calculate the overall R-value, however, this has been known to differ between the designed and built. Therefore, the assessed risk of the design may not be reflected in the built house.

With more strict requirements for building performance coming into force, it is clear a more detailed and reliable method to assess the risk of internal moisture from thermal bridges is needed. The review of the literature identified a spectrum of tools, ranging from simple hand calculations to a 3D dynamic simulation tool. Factors such as the type of simulation, the number of dimensions and the simulation conditions increase the reliability of the results but also increase the complexity of the tool. This thesis aimed to assess how complex a simulation tool needs to be to produce reliable results when assessing the risk of internal moisture from thermal bridges while considering the time and experience.

The thermal bridges of a case study building were modelled in various representative simulation tools, including a comprehensive WUFI Plus model calibrated against measured data. Benchmarking methods were used to interpret the results and allow for a comparison of the risk of internal moisture across the tools. A tool was considered reliable if the risk of internal moisture was consistent with the results from the calibrated ‘truth’ model.

The results from the static tools, regardless of the number of dimensions, were inconsistent with the ‘truth’ model and the measured data. The results from the dynamic tools indicated at least a 2D tool is required, with the impact of geometrical thermal bridges having a greater impact on the risk than systematic or material thermal bridges. However, this sort of tool requires the internal climate to be estimated, measured, or calculated. Indoor climate assumptions, used to estimate the internal climate, are unreliable and measured data is usually unavailable for new buildings. A calculated climate requires the modeller to build two simulations of the building, at very different scales, which defeats the aim of this investigation. Therefore, it is more important for the risk of internal moisture to be assessed using a 1D Whole Building Simulation, which considers the wider context of the building, rather than a 2D or 3D Building Component Simulation, which focuses on a specific junction of the building. This type of simulation also has the benefit of considering the impact of the building’s geometry and orientation, internal loads, ventilation, and space conditioning, to calculate the energy demand. This allows designers to assess the whole building to target other important factors to reduce the risk, such as ventilation and passive/active heating.

Various tools are available for designers to use to assess the performance of construction details, however, a simulation tool is only as good as the information entered and the assumptions it makes. This thesis identified the factors that are important to ensure the assessed performance is reflected in built houses, resulting in buildings that are more energy efficient, comfortable, and healthy for occupants.