In the UK, one third of CO2 emissions are currently caused by the domestic sector and two thirds of this value is due to space heating in homes. There are approximately 25 million houses in the UK that are currently below the building regulation standards for thermal insulation, and one third is classified as hard to treat (HTT) due to numerous reasons (owner preference, planning restrictions, etc). Internal wall insulation (IWI) appears a favourable option as many HTT houses in the UK are highly aesthetical and encapsulate a degree of cultural heritage. However, this solution is coupled with one major disadvantage of summer overheating because of low thermal mass of insulation. Use of Phase Change Materials (PCMs) has been well recognised as an effective way to resolve or mitigate the overheating issue. Design of PCMs-incorporated wall insulation should be based on the annual energy performance of a house. EnergyPlus software looks the most suitable tool for the design. In this study, a series of simulations were undertaken, on a simple HTT property, insulated with IWI incorporating PCMs with the melting temperatures of: 18°C, 23°C, and 26°C. Internal temperature was monitored during extreme conditions experience in summer periods. In addition to this the same building was also simulated when encapsulating: no insulation, EWI, and ordinary IWI. Results of these tests were compared and analysed to determine the potential benefits and limitations of incorporating PCM into an IWI system. The following paper summarises the findings of the investigation. All simulations were done using the EnergyPlus compiler couple with Google SketchUp as a graphical interface.