Whole Life Analysis of timber, modified timber and aluminium clad timber windows: Service Life Planning (SLP), Whole Life Costing (WLC) and Life Cycle Assessment (LCA)


This report compares the service life, ownership cost and environmental impact of windows using timber, modified timber, aluminium-clad timber and PVC-U frames. It uses defined methodologies to compare the Service Life Planning (SLP), Whole Life Cost (WLC) and Life Cycle Assessment (LCA) of a standard window (1230×1480 mm with one side-opening light) in each of the four frame materials, taking into account the relative durability of the materials and their maintenance requirements.

Service Life Planning (SLP) is a decision process which addresses the development of the service life of a building, constructed work, or in this case, a component. Its purpose is to give a structured response to establishing normal service life from a reference or estimated service life framework. The objective of SLP is to provide reasonable assurance that the estimated service life of a building or construction on a particular site, with appropriate maintenance, is at least as long as the design of that building. The results show timber frames to have an expected service life of between 56 and 65 years. Acetylated timber frames show an expected service life of 68-80 years, and timber frames, clad with profiled aluminium, 71-83 years.

Whole Life Cost (WLC) was assessed using a standard discounting method, Net Present Value (NPV), which allows the time value of money to be allowed for in the value of future payments or incomes. The NPV of purchase, installation, repair and maintenance costs were evaluated over building design lives of 60, 80 and 100 years and under mild, moderate and severe exposure conditions. Over a 60-
year design life, the results show that timber windows offer the lowest cost alternative for mild scenarios, while aluminium-clad and modified timber offer lower whole life costs for moderate and severe scenarios. Despite having the lowest capital cost, PVC-U windows were shown to have the highest whole life costs over 60 years in all scenarios.

Life Cycle Assessment (LCA) is an internationally recognised tool for assessing the environmental impact of products, processes and activities. It is a methodology for evaluating the environmental load of processes and products during their whole lifecycle and is one of various environmental management tools currently available for assessing impact and sustainability. LCA is used to assess the environmental impact of processing raw materials, manufacture of finished products and components, during construction, to transport materials and products to site, to maintain components, and to process materials at their end-of-life to recycle and/or dispose of materials. This report is conducted within ISO 14040 and PAS2050 guidelines and sets a new standard for the whole life cycle appraisal of timber windows. It considers a base case scenario plus 6 alternative scenarios which test the sensitivity of inventory data and boundary inclusions on Global Warming Potential (GWP) of the frame materials considered. This report finds that all timber based window frame materials are preferable to PVC-U alternatives in every scenario considered. Using the methods adopted in this report, recycling is found to be the optimum end of life treatment for timber based window frames. The report conclusions lean to supporting the aims of WRAP in pursuing greater waste segregation, and possible tighter restrictions on timber waste entering landfill sites. This report also demonstrates the significant sensitivity of GWP outputs to the sustainable and ethical sourcing of timber under FSC or equivalent standards.