Tools to assess internal surface mould growth: dynamic vs static

The paper reports on research to identify a reliable tool to take account of thermal bridging. This will allow designers to evaluate the performance of timber-framed construction and the potential for internal surface mould growth. The Isothermal Planes method required by New Zealand Building Code Clause E3/AS1 to avoid internal moisture is too simplistic, therefore a more reliable tool is required. This paper compares the results from the static (moment in time) tool THERM and the dynamic tool WUFI 2D. Internal conditions were estimated by following Appendix A.1 of ISO 13788:2012 and calculated using a calibrated whole building simulation, WUFI Plus. The Temperature Factor and VTT Mould Growth Index were used to interpret the results from the static and dynamic tools, respectively. When ISO 13788:2012 is used to estimate the internal conditions, the risk of mould growth concluded from the static and dynamic tools is inconsistent with the measured data. The reason is that ISO 13788:2012 assumes internal relative humidity (RH) does not exceed 70% RH, yet mould growth commences at 80%. When internal conditions from the calibrated whole building simulation were used in WUFI 2D, the risk of mould growth was consistent with the measured data. Although using a dynamic tool over a static tool is preferred to simulate over time and account for the changing external climate, the results also highlight the importance of applying correct internal conditions, especially when assessing risk.