The UK building stock, being amongst the oldest in the developed world, is also one of the least energy efficient and accounts for approximately 45% of UK carbon emission. Energy use from housing alone was responsible for 13% of total UK carbon dioxide and greenhouse gas emissions in 2015. Therefore, achieving the national target of an 80% reduction in carbon emissions by 2050 against 1990 baseline conditions is highly dependent on the reduction of energy consumption in dwellings. The complexity of the problem of retro-fitting energy saving measures in the extensive and diverse aging housing stock is further compounded due to the number of 'hard to treat' properties that comprise over 40% of homes in the UK. In this article, the authors present an evaluation of the theoretical and experimental performances of a novel prototype external wall insulation system, developed to improve energy efficiency in 'hard to treat' housing. The system was designed to be primarily used to retrofit social housing, which comprises up to 18% of the current UK housing stock. A thorough testing regime was undertaken to test the suitability and effectiveness of the new product in the most common social housing construction typologies. This included: an investigation of the theoretical thermal performance of the prototype product through steady state modelling, a laboratory based prototype test, an analysis of empirical data collected from a cross section of social housing properties in Nottinghamshire, UK used to inform whole house dynamic modelling, and the development of dynamic simulations to assess the energy and carbon reduction impacts of the new product. The theoretical modelling suggested that the integration of the system resulted in thermal performance improvements for all construction types with space heating demand reduced by up to 42%. The results of the whole house dynamic modelling assessment also suggested that the addition of the system resulted in a reduction of heating energy demand of up to 49%. The prototyping testing shown that the system is easy to install requirement minimum building skills. The findings suggest that the new product not only meets the performance of existing external wall insulation systems, but also provides unique selling points with respect to easy installation and non reliance on weather conditions. The project finished with a pilot study when one house was retrofitted using the novel product. (C) 2017 Published by Elsevier B.V.