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8Buildings account for around 40% of energy consumption in the UK. For over twenty years active thermal mass 9 systems have been a feature in low-energy buildings in northern Europe. By passing ventilation air, and utilising 10 night ventilation, through the hollow core structures efficient heating and cooling has been achieved. Despite 11 the success, such systems suffer from space overheating and efficiency losses during extended hot periods. 12Control strategies have largely mitigated this effect however low cost retrofit solutions that enhance the system 13 are of interest. This research therefore investigates the benefit of using innovative phase change material (PCM) 14 solutions to enhance thermal comfort and reduced energy usage of traditional active thermal mass systems. 15 A prototype PCM enhancement was constructed, with energy saving and comfort benefits investigated under 16 controlled laboratory conditions. In absolute terms the PCM solution offered an additional 12.5% energy storage 17 capacity and a 29% increase in active surface area, coupled with the existing sensible thermal mass. Under 18 laboratory conditions the PCM addition saved an additional 0.1 kWh per day, delayed AC onset by 1.2 hours and 19 offered an average 1.0°C reduction in room temperature during eight hours of fixed cooling load, contrasted 20 against the original active thermal mass system. 21 KEY WORDS 22Active thermal mass, phase change materials, thermal mass, free cooling, night cooling, energy-efficient cooling 23

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