The main objective pursued by a fire safety strategy defined for a building design is to achieve an appropriate safe egress time required by the occupants under any fire scenario. This time must be shorter than the fire growth rate, profoundly influenced by the material flammability characteristics of building components. As witnessed in recent fire scenarios, this egress time can be severely compromised by the flammability of innovative building assemblies that include light and combustible materials which are suitable for thermal insulation and provide an easy and affordable way to comply with strict energy efficiency building code requirements. This study analysed the potential conflict between the assessment of thermal insulation and flammability characteristics such as the onset of ignition through heat transfer fundamentals. It was concluded that material properties like the density needed to control flammability characteristics are not evaluated by the current energy efficiency approach adopted by most building policies. By addressing this issue, this study focussed on the development of an integrated assessment method to attain optimised design solutions from where the best insulating properties for a particular geographical location is achieved together with adequate fire safety performance. During my Doctor of Philosophy period, my knowledge and understanding of buildings have been highly enhanced thanks to the invaluable support, time and kind help of many people. As a result, I have been enriched personally, and indeed, the attention received encourages me to share and promote the constructive attitude that I received.