A New Zealand solution for designing healthy homes
"I want my research to be at the forefront of delivering healthy homes in New Zealand,” says Te Herenga Waka—Victoria University of Wellington PhD student Griffin Cherrill.
Griffin is aiming to support the creation of healthy homes by simulating the risk of mould growth and condensation during the design of timber-framed houses.
"I am attempting to identify a reliable tool that assesses the risk to healthy homes from thermal bridges and aligns with the New Zealand government's Health Homes Standards that commit to building warmer, dryer, and better-ventilated homes.”
In construction, a thermal bridge is a material with a higher thermal conductivity that passes through an insulation layer, such as a timber stud in an external wall. Griffin has discovered that 1-dimensional models used to calculate energy demand assume that surface temperatures are constant across an internal surface, but this is an oversimplification. Instead, increases in thermal conductivity due to thermal bridges in the building fabric lead to colder local surface temperatures and an increased risk of internal moisture.
“Using a 1-dimensional tool can produce results that are warmer than in reality, which could lead to the risk of internal moisture being overlooked,” Griffin says. “Therefore, the industry must identify tools to reliably assess the local risk of internal moisture if homes are to improve in these respects.”
Griffin hopes that the Ministry of Business, Innovation and Employment and building practitioners can use the tool he is working on to update the New Zealand Building Code and inform their home designs to serve New Zealanders better.
Griffin has also identified some issues with current standards used to reduce moisture in homes.
“The Clause E3/AS1 method used in the Building Code assumes that an overall R-value of at least R-1.5 will stop condensation and mould growth, but there is no evidence-based research of why this value has been chosen,” Griffin says.
Researchers have not been able to confirm whether these standards are actually enough to stop internal moisture in homes, he says. Also, previous research highlights that most timber-framed homes in New Zealand have a percentage of timber in the external walls that make it impossible to meet the minimum energy efficiency and internal moisture standards laid out in the NZ Building Code.
Griffin’s research uses the BRANZ Test House, located in Porirua, as a case study to identify critical thermal bridges where internal moisture is likely to occur. The single storey timber-framed house, with an attic and subfloor zone, has been the subject of building research for two years. He is using several tools, including heat and moisture modelling tool WUFI Plus, to document the risk of internal moisture depending on local climate, insulation levels, and percentage of timber used in construction.
To advance his research, Griffin is working closely with the Building Research Association of New Zealand (BRANZ), an independent organisation that researches, certifies, and tests building designs and materials in New Zealand in partnership with the industry and the central government, with BRANZ providing funding for his research. He hopes to discuss his ongoing work and building solutions to deliver warm and dry timber-framed homes in New Zealand with ministries and building practitioners and see his research in action in the New Zealand building sector.
Griffin Cherrill is a candidate in the Wellington Faculty of Architecture and Design Innovation under the supervision of Associate Professor Michael Donn, Dr Nigel Isaacs, and Stephen McNeil.
Contact Griffin to hear more about this research on griffin.cherrill@vuw.ac.nz.