Building Design Optimization: Integration of Thermal and Fire Performance

Abstract

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.The first task to address an integrated assessment approach was to identify sharing quantitative parameters relevant to both thermal efficiency and fire performance disciplines by analysing the theoretical thermal models available commonly applied to each field. A fire scenario is a temporal event, and thus its physics is based on a transient thermal approach where the thermal conductivity, the density and the specific heat are significant material properties for evaluation. However, most building components are designed on a thermal steady state approach according to prescriptive thermal design parameters like the thermal transmittance “U-value” derived from a steady state thermal model where thermal conductivity is the only relevant material property. A steady-state thermal model is most precise when daily temperature fluctuations remain within a narrow range allowing for detail associated with large seasonal temperature variation, such as north Europe. In geographical locations, such Australian regions with potentially larger daily (smaller seasonal) temperature variations, a steady state approach can introduce significant errors when assessing building thermal performance. This study analysed Australian weather data and evaluated both steady and transient state model’s parameters on case study Australian prefabricated building component systems. It was concluded that introducing transient model thermal parameters like the cyclic transmittance “u-value”, and the surface admittance “y-value” a more precise thermal performance evaluation is achieved. Furthermore, the definition of these parameters includes all relevant material properties combined in the form of the thermal inertia needed for material flammability assessment allowing for an integrated assessment approach. By using this approach, a quantitative procedure was developed to characterise insulating properties under both steady and transient approaches to building assembly’s designs using numerical models. A small-scale thermal test procedure was defined to analyse both transient and steady-state heat flow processes, allowing for effective numerical fitting of parameters that describe all internal heat flow processes. By using this simple experimental thermal test set-up, the contribution of each element of an assembly design alternative can be evaluated on its overall insulating capabilities including the effect of structural elements acting as thermal bridges and construction imperfections, thereby allowing optimised assessment. As a result, the quantification of the steady state U-value and the transient state u-value and y-value is achieved delivering average values. Also, information can be obtained from specific points in the building assembly to quantify the interaction amongst the components for a more detailed insulating capabilities assessment.Both u-value and y-value analytical definitions include material properties combined in the form of thermal inertia as the material flammability analytical expressions do. The integrated assessment method continues by including in these analytical expressions the numerical outputs at specific points of the building assembly. Thus, it is established a system of non-linear equations that delivers apparent thermal inertia values of these points that include the influence of the whole assembly. From these parameters, a flammability analysis can be performed for different fire scenarios. It was observed that higher energy efficiency u-values and y-values are linked to better material flammability characteristics that effectively contribute to the fire safety strategy. Similarly to the thermal performance analysis, a small scale fire test approach was defined to support this study where the building assembly tested is exposed to higher heat loads.This study analysed and tested two real-life prefab building assemblies supplied by industry partners that challenged the method due to their design complexity. The outcome is presented in this study for illustration and procedural validation. Also, conclusions delivered vital information to assist in their ongoing design improvement. From the analysis of building design alternatives by using the proposed integrated approach and the definition of particular design criteria for thermal efficiency and flammability characteristics, an optimal and balanced solution can be achieved.