Constructed originally between 1666 and 1679, the Castle of Good Hope in Cape Town is one of South Africa's oldest surviving buildings and it is still in use. It is the seat of the military in the city and houses the Castle Military Museum and the William Fehr Collection of Cape Town’s Iziko Museums. Military personnel, museum administrators and tourism support staff are at work within its four-metre wide walls. However, inside the massive structure of the outer walls, there has been for many years a problem with the working environment. Despite the high thermal mass of the structure, the dark coloured roofing material has a high thermal conductivity and low insulating properties. This means the office space heats up through summer and becomes chilly in winter, once the inside walls have cooled to the ambient temperature.