Integral Collector/Storage (ICS) solar water heating systems suffer substantial heat loss during periods of low insolation or at night. Methods to reduce aperture heat loss include moveable insulated lids/shutters, transparent insulating glazing materials and selective glazing/absorber coatings. All of these approaches involve trade-offs with reduction in performance and/or an increase in cost. A novel ICS vessel design to mitigate heat loss is proposed. An ICS vessel utilising an inner sleeve arrangement is shown to reduce heat loss by up to 20%. This paper examines four inner sleeve design configurations, several of which demonstrate an increase in the heal retention capability over existing vessels, and an optimised design is presented.