Healthy and affordable housing in New Zealand: the role of ventilation

Abstract

Are new houses in New Zealand that rely on natural forces for ventilation healthy places to live? It is well known that older houses in New Zealand are riddled with problematic indoor conditions, but have recent updates of the New Zealand Building Code addressed the issues? Answering this question is the original contribution to knowledge this thesis undertakes. A survey of fifteen free-running houses built after the year 2000 in the three climate zones of the New Zealand Building Code confirms the results of simulations: indoor air quality and thermal comfort are generally poor. Relying on natural forces for the provision of healthy homes in New Zealand climates is an inadequate undertaking. The way houses are ventilated was identified as crucial for the quality of the indoor environment. Simulations established that ventilation modes that rely on natural forces are systematically incapable of safeguarding good indoor air quality, in light of the typically low-rise houses and temperate climate – with one complete air exchange every two hours. Yet, increasing air change rates in the context of already severely underheated New Zealand houses, and a growing number of households in fuel poverty, is likely to exacerbate health problems caused by cold indoor environments, and therefore not a feasible strategy. Thermal comfort in all surveyed houses was below the minimum category of international standards for large fractions of the monitored period, with many houses frequently exhibiting indoor air temperatures below 16°C at high relative humidity. The air-leakiness of the building envelope could not be correlated with either air change rates or outcomes for the quality of indoor air. In contrast, it was found that with well-designed forced ventilation systems and an airtight building envelope, lower air change rates suffice to dilute indoor air contaminants to levels below concern. Installing and operating a forced air system however adds to the cost of housing. Optimised forced ventilation systems were therefore analysed regarding their cost effectiveness, also taking into account the quality of the building envelope. Annualised capital expenditure and operating cost were compared, with the aim of identifying the most affordable way of providing good indoor environmental quality in New Zealand houses. An efficient heat recovery ventilation system was found to be the most advantageous option. It is also economically viable under plausible assumptions for economic boundary conditions.