One of the difficulties commonly experienced in embodied energy assessments using the process analysis approach, is how to account for the various steps in a multi-staged production sequence including the generation of by-products and waste. As well in most efforts to determine the energy embodied in products or materials the impact of whole life cycle energy is excluded. A systematic approach to address these two aspects is proposed in this paper. A generic algorithm for calculating the energy embodied in a product through the various steps of a multi-staged production process including a method to account for the generation of by-products and waste is presented. Another generic formula to estimate the impact of embodied energy over the life of a building, element or component including maintenance, repair and replacement is also described. An example case study demonstrates the application of these two models to analyse the energy requirements for manufacturing basic timber products involving the whole process from harvesting, transport, sawing, drying, planing and moulding into final timber products. The case study also investigates the amount of by-products and waste generated in the process. The results from the case study are compared to some of the more commonly quoted published values to highlight the very large variations in embodied energy values currently used for timber that exist.

• embodied energy