With the on-going tightening of Part L of the Building Regulations, increasing energy prices and a growing demand for more sustainable design, pressure is being put on airconditioned buildings from all directions. Even the speculative offi ce market, which has traditionally paid little attention to energy consumption, is beginning to re-evaluate its largely unquestioned use of air-conditioning. At the heart of low-energy design is the building fabric and the way in which it interacts with the internal and external environment. In this respect, the high level of thermal mass provided by concrete is playing an increasingly important role in ensuring comfortable internal conditions in offi ces and other types of building. The use of concrete to provide passive cooling can achieve signifi cant savings in terms of capital and operating costs through avoiding or minimising the need for air-conditioning. The basic approach is to expose the soffi t of fl oor slabs, which can then absorb heat gains during warm weather and stabilise the internal temperature. Typically, the cool night air is then used to ventilate the building and remove the accumulated heat from the slab in readiness for the following day. This cycle of heating and cooling using the thermal mass in the building fabric is often referred to as Fabric Energy Storage (FES) – a term used throughout this guide. Over the last decade, a growing number of prestigious owner/occupied offi ce developments have opted for concrete construction and FES cooling, which refl ect design briefs that call for a high-quality internal environment and low operating costs. These buildings largely follow the same design format, typifi ed by the UK headquarters of companies such as PowerGen, Canon and Toyota (see Case studies, Appendix A1, A2 and A3). In contrast to this, property developers and investors in the property market such as the large insurance providers have, until recently, not shown interest in using thermal mass, opting instead for a standard air-conditioned format that has traditionally been viewed as a low-risk option with a short payback. However, we are now starting to see evidence of a shift in this sector, which includes projects such as the National Trust HQ in Swindon (Figure 1), Plantation Place in London and Belvedere Court in London.2 Other examples of high-mass speculative offi ce developments include Number One, Leeds City Offi ce Park and the Addison Wesley Longman offi ce in Harlow.3 This refl ects a market that is beginning to pay more attention to the running costs of highly serviced buildings and the questionable longer-term popularity of such buildings in a country with an increasingly fi ckle energy supply. FES can do much to simplify building design and operation, however, it also brings with it specifi c design issues that are not present in more traditional offi ce design. These issues mostly arise from the use of exposed concrete soffi ts, which has implications for acoustics, lighting, routing services and the general design process. Information is provided in this guide on a range of design issues including system options, surface fi nish, integrating services, lighting, acoustics and system control. This is supported by numerous cases studies, which offer practical examples and feedback on how specifi c design issues have been tackled and the lessons that can be learned. It is intended that the guide will assist designers, architects and engineers considering a high thermal mass approach to cooling.