Design for airtightness and moisture control in New Zealand housing

Abstract

The analysis of the existing housing stock in New Zealand confirms the prevalence of indoor environmental factors negatively affecting occupant well-being. Besides being energy-consuming, New Zealand homes are known for having cold, damp and uncomfortable interiors. Furthermore, the country has one of the highest incidences of asthma and respiratory related illnesses in the developed world as well as one of the highest rates of winter related deaths. Design issues and practical solutions for sustainable buildings in relation to construction details,ventilation and humidity control have been already investigated in several studies. This paper instead aims to demonstrate how comfortable and healthy houses are defined by measurable conditions that can be assessed by using specific tools, such as the simulation programme WUFI (Wärme und Feuchte instationär - Transient Heat and Moisture) and Blower Door tests. It has been proved that Indoor Air Quality of homes significantly affects comfort and health of their occupants. In particular, air leakages have been identified as one of the major causes of energy consumption and discomfort in the building industry. Indeed, airtightness is a crucial factor to ensure that insulation really does perform and that the building structure remains free of structural damage. Current building practice in New Zealand suffers from the lack of use of control tools to design and build energy efficient, durable and healthy buildings. This paper presents the calculative assessment through WUFI simulations of thermal and hygric processes in the external wall assemble of old, retrofitted and new buildings. In addition it compares results from previous airtightness measurements with those from Blower Door tests performed on some modern buildings, to investigate the levels of airtightness currently being achieved in New Zealand with common building practice. The proposed calculation and assessment methods can assist practitioners in providing a general evaluation of the hygrothermal suitability of particular building components. Used as a means of verification in this study, they served to demonstrate the inadequacy and potential risks associated with certain practices of building construction and renovation