Within the UK non-domestic building stock, offices built between 1940 and 1980, are especially in need of retrofit, they can suffer from high energy consumption and thermal discomfort. Many post-war offices will still be in use throughout the first half of this century. This paper evaluates retrofit strategies for post-war office buildings accounting for the improved energy efficiency, thermal comfort and hence productivity, and reduction of capital and running costs. The aim of the paper is seeking optimal, generic retrofit strategies to provide guidance to building owners, occupiers and other decision makers. Dynamic thermal modelling is used to compare retrofit outcomes for existing building standards (PartL2B) and higher standards (Passivhaus retrofit: EnerPHit). The effects of location and orientation and both current and future UK weather conditions (2050) are considered. Multiple combinations of heating and cooling strategies and retrofit measures are assessed. The analysis methodology uses a sophisticated comfort, productivity and cost assessment. An Overall Building Thermal Discomfort (OBTD) index is introduced which enriches the current CIBSE overheating criterion 1 by including the number of occupants. Productivity improvements as a result of better comfort are included in cost calculations. Cost benefits are calculated both for buildings used by the owner (CBO) and for buildings let to a tenant (CBT). On cost and energy grounds, UK building regulation compliant retrofit is optimal provided that passive summertime overheating controls, such as night ventilation, blinds and/or overhangs, are installed. The EnerPHit standard retrofit provides resilience as the climate warms provided summer cooling is available, for example through mixed-mode ventilation.
