Griffin Cherrill

Tools to assess the risk of internal moisture from timber thermal bridges

Griffin photo

MBIE has proposed that by 2035, buildings in New Zealand will use as little energy as possible while maintaining a healthy indoor environmental quality (IEQ) which includes a reduced risk of internal moisture. ‘Internal moisture’ may lead to damage to the building fabric. It includes surface condensation and mould growth but excludes interstitial condensation. The current method used to assess this risk from NZBC Clause E3/AS1 requires a minimum R-1.5 R-value for timber-frame construction. The R-value is calculated using the isothermal-planes method, which averages timber and insulation, smoothing surface temperature fluctuations which cause localised internal moisture. Ryan et al. (2019) found 45 of 47 timber-frame houses had not the expected 18% but over 26%, and up to 57%, of the external wall area occupied by timber – the thermal bridge. With larger than expected amounts of timber in the wall, the achieved performance of timber-framed houses is below the minimum standard required by the NZBC, which leads to an increased risk of surface condensation and mould growth.

As the MBIE requirements will require higher performance construction, both the achieved performance and assessment tools need to be improved. A number of tools currently exist but a review of literature found the most reliable tool is one that can simulate in all three dimensions using dynamic conditions. But to use such a tool, the modeller requires large amounts of time and experience, which is unlikely to be found in a design construction office.

Industry needs a design-stage tool to assess the energy, IEQ and moisture performance of new construction methods to achieve the requirements outlined by MBIE. Currently, available tools require higher levels of experience and analysis time than are acceptable. Simpler tools exist, but their assumptions can negatively impact results. Therefore, this research aims to identify how detailed a tool needs to be to ensure results can be used to reliably assess the risk of internal moisture, while ensuring the tool does not require significant time and experience.


Dr Michael Donn from the School of Architecture, Dr Nigel Isaacs from the School of Architecture and Stephen McNeil, Senior Building Physicist from the Building Research Association of New Zealand (BRANZ).