This report by Heriot Watt University compares the service life, ownership cost and environmental impact of windows using timber, modified timber, aluminium-clad timber and PVCu 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 in each of the four frame materials, taking into account the relative durability of the materials and their maintenance requirements. Applying a factor analysis, an expected service life for wood window frames made to the WWA specification of between 56 and 65 years is reported against a base case for PVC‐u of between 25 and 35 years.