Buildings are a major end-user of European energy, representing 40% of energy consumption and 1,800 million tonnes of CO2 emissions. Improved energy efficiency within buildings is, therefore, a priority objective in enabling Europe to meet its Kyoto Protocol commitments. Whilst a desirable architectural feature enabling enhanced occupier comfort, windows are recognised as a key weak spot in the building envelope, resulting in heat losses during winter months due to inferior insulating properties, and excessive solar heat gain during summer months, resulting in energy-intensive air conditioning. Such window-related losses represent 10% of a building's energy consumption (4% of Europe’s total energy use).Whilst super-insulating window designs are commercially available, more limited progress has been made towards cost-effective solar heat gain control (SHGC) technologies. Existing solutions are primarily based on low-cost tinted films that permanently block a proportion of light entering the room (leading to room darkening and negative energy efficiency during winter months). Whilst dynamic (switchable) films have been developed, such materials demonstrate either inferior properties or are cost prohibitive.Integrating emerging materials science with a cost-efficient polymer-extrusion process, the SOLARGAIN project will develop an innovative low-cost all-polymeric switchable reflective SHGC film. The film will enable dynamic optical properties in response to the changing lighting, heating and cooling needs of the room, and thus improved natural lighting, occupier comfort and enhanced energy efficiency. The SHGC films will be incorporated within:• A highly-insulated sealed-window-unit design targeting > 90% window-related energy saving• A retrofit window-film structure targeting > 90% energy savings associated with room coolingThe window systems will integrate photovoltaic and sensor technologies, thereby enabling self-powering and intelligent activation. Achieving cost competitiveness with existing window systems, the SOLARGAIN technology will also demonstrate significant long-term cost savings through enhanced energy efficiency. SOLARGAIN will generate €40 million business growth within a 3 year period, creating 85 new jobs; and has the potential to benefit > 8,000 SMEs operating in wider market sectors.