This report provides guidance on the use of building fabric as a storage medium for heating and cooling energy transported by circulating air. Particular attention is paid to the use of concrete slabs for storage of free cooling available from low ambient overnight air temperature. This work includes an introduction to the concept of fabric energy storage and a discussion of design considerations. Information is provided on the general performance of the systems in terms of maintaining comfortable temperatures in the occupied space, incorporating the expected variation with key parameters including - thermal mass, thermal linking between the fabric and the circulating air, ventilation rate and climate. The information provided is based on case studies and simulation modeling. Capital and energy consumption/cost data is also provided.