Thermal Performance Implications of a Housing Warrant of Fitness

A housing Warrant of Fitness (WoF) has been independently proposed by three organisations as a way of improving the quality of rental housing in New Zealand and reducing housing related injuries and health problems.   The New Zealand Green Building Council and the Housing and Health Research Programme (University of Otago, Wellington) have jointly developed a WoF based on Homestar and the Healthy Housing Index (labelled the GH WoF in this document).   Housing New Zealand is in the process of developing a WoF for their properties (HNZ WoF) , it has been tested on a number of state houses but the results have not yet been fully released.   Both the GH WoF and the HNZ WoF target thermal and moisture performance amongst other minimum requirements for rental housing. The WoFs specify slightly different housing features as minimum acceptable standards. Both of the WoFs are structured as a checklist, with all items considered mandatory to pass the assessment.  This research investigated the impact of meeting the WoF criteria which impact thermal performance. A thermal model of a villa style house suitable for a family of 4 was modelled in the thermal simulation programme EnergyPlus with a minimum practicable level of thermal performance. A range of New Zealand input values were sought and a number of sensitivity analyses were carried out to validate the model’s performance. Combinations of features each meeting the different criteria of the WoFs (such as insulation, ventilation and curtains) were then added to this house.  The results of these models demonstrated that while adding these features will improve thermal performance, a simple pass/fail may be misleading because a house which fails may have equivalent thermal performance to a house which passes.  The pass/fail result does not acknowledge trade-offs between temperature, energy consumption and running cost. Neither WoF accounts for the impact of climate on the ability to achieve comfortable indoor conditions, which has a greater impact than passing or failing a WoF.  The criteria targeted at moisture performance were unable to be analysed as the moisture loads put into EnergyPlus could not be adequately verified.  The research showed that while both WoF could lead to improved thermal comfort in rental housing, meeting the GH WoF requirements would bring about greater thermal performance benefits, the HNZ WoF enables the affordability of heat to be considered when insulating is not feasible.