One of the primary reasons for the application of cool materials is their energy and associated environmental impact on the built environment.  Cool materials are usually applied on the roof of buildings to reduce cooling energy demand.  The relative benefits of this reduction depend on the construction of the building, external weather conditions and use of the building.  This paper examines the impact from the application of a reflective paint on a flat roof in a naturally ventilated office building in the area of London, UK where the climate is moderate with high heating demand by buildings.  The environmental conditions (internal/external air and surface temperatures) of the building were monitored before and after the application of the cool roof during the summer.  It was found that internal temperatures were reduced after the application of the cool roof.  The building was modelled using TRNSYS and the model was calibrated successfully using the measurements.  A parametric analysis was carried out by varying the reflectivity and insulation of the roof and ventilation rate; the heating and cooling demand for a year was calculated using the Summer Design Year for London as the weather file.  It was found that cooling demand is significantly reduced, heating demand is increased and the total energy savings vary between 1 and 8.5% relative to an albedo of 0.1 for the same conditions.  In free floating (naturally ventilated) buildings summer comfort is improved but there is a penalty of increased heating energy during the winter.  Thermal comfort can be improved by an average of 2.5 °C (operative temperature difference for a change of 0.5 in albedo) but heating demand could be increased by 10% for a ventilation rate of 2 air changes per hour.  The results indicate that in the case of temperate climates the type, operation and thermal characteristics of the building should be considered carefully to determine potential benefits of the application of cool roof technology.  For the examined case-study, it was found that a roof reflectivity of 0.6–0.7 is the optimum value to achieve energy savings in a cooled office, improve summer internal thermal conditions in a non-cooled office (albeit with some heating energy penalty).  It indicates that it is a suitable strategy for refurbishment of existing offices to improve energy efficiency or internal environmental conditions in the summer and should be considered in the design of new offices together with other passive energy efficient strategies.