The analysis of the New Zealand (NZ) housing indicated that a notable percentage of dwellings is energy-inefficient, cold, mouldy, and damp which is likely to cause significant health-related issues. Research has suggested that this longstanding problem is rooted in potential systemic failures. However, apart from energy modelling, sustainability assessment possesses a minimal presence in the used BIM tools. Although the necessity to improve building hygrothermal performance is increasingly recognized, the information and mechanisms on how to efficiently employ hygrothermal modelling in the design process are still missing. This study offers a new perspective on understanding the relationship between BIM and sustainability. It introduces innovations through analysing hygrothermal relations in buildings and specifying requirements for integration of hygrothermal modelling into BIM. The necessary movement in the building industry needs to follow the socio-cultural development into the second-tier thinking levels towards an integral system. Therefore, this study presents an alternative perspective from which we look at the challenging task of how to build better and healthier houses. From this integral perspective, the design and construction process and its end-product, buildings, form open systems that interact with other systems and the environment. The holistic approach to sustainability pertains with the whole project by applying systems engineering methodologies to the design and construction process. This thesis gradually explores the research objectives from an individual to a collective level. An experimental and numerical study on real houses (individual level) delivers new data which demonstrate the fundamental importance of hygrothermal modelling during the design process. The collective level examines general requirements for incorporation of hygrothermal modelling into BIM and system approach to the design of houses. In an interdisciplinary and systematic approach grounded in integral thinking, this thesis focuses on possible BIM innovation and its implementation strategy. The hygrothermal modelling integration into BIM enables competent decisions about environmental impact of new buildings and retrofits to prevent unintended moisture related problems. Suggested solutions inevitably require a restructuration and continual transformation of the whole design and construction process to progressively eliminate design defects and leaky house syndrome in NZ. The proposed Complex Integral Design New Zealand (CIDNZ) system accommodates the capacity to integrate diverse perspectives in a unified and flexible framework. The findings of the influence of various materials and construction types on indoor relative humidity levels enhance the practical knowledge. The research results demonstrate the benefits of hygrothermal modelling and the decisive use of hygroscopic materials to sustainable design in NZ. This thesis contributes to knowledge firstly by instigating integral thinking into the design process. Secondly, by applicating the systems approach to buildings and complex construction processes that are seen as elements of environmental systems. The proposed CIDNZ focuses on people by respecting a broad spectrum of human needs. Consequently, the design process might shift the forefront from prevailing aim for cheap and fast built to long-term energy and cost-efficient, durable, and healthy buildings.