REVIEW AND ANALYSIS 35 CLIMATE CHANGE: LEARNING FROM THE PAST MAY CASSAR THE HISTORIC built environment sector first became conscious of the impact of climate change on archaeology, buildings, monuments and sites at the start of the millennium. Since then, academic and practitioner awareness has grown and grown as historic built environment specialists began recording change and monitoring the speed of the change. Concern has been expressed for losses of material heritage from a range of causes: rising sea levels, flooding, wind driven rain, extreme temperatures, drought and desertification. The focus of research has been on understanding the range of physical problems caused to the historic environment by a changing climate; more recently, the debate has shifted to solutions, with the voices of stakeholders and communities demanding to be heard, and for good reason. Going forward, our discussion needs to be both pragmatic and practical. In particular, what are we in time to do in the face of accelerating climate change? What can we learn from the Mediterranean region, already living through the rising temperatures that we are beginning to experience in northern Europe? What can we learn from traditional architecture and indigenous building knowledge about managing overheating? And how do we embed unfamiliar knowledge in what we know or have been doing? A good starting point is the roof, as its materials and design have a profound impact on the environmental performance of buildings, the comfort of occupants and, in dense traditional urban settings, whole communities. Roofs are the building elements most exposed to the sun and a primary source of overheating. In the Mediterranean, roofs often cover over 30 per cent of an urban area. Understanding and measuring the behaviour of different roof types, including traditional roofs, enables building professionals, owners and occupants to work with buildings in passive and sustainable ways. Cool roofs rely on the application of a reflective roof coating that limits heat gain in two ways: imparting a higher solar reflectance and a higher thermal emittance, also determined by the absorbance of the surface and underlying materials. Evaporative roofs utilise evaporative cooling, which is achieved through the intrinsic properties of porous materials to lower the surface temperature of a roof. Stored pore water from rain or high humidity at night evaporates during the day, maintaining a low surface temperature due to the cooling effect of vaporisation. Traditional, breathable roofs are particularly efficient in the hot, often very humid Mediterranean climates due to their use of evaporative or passive cooling. The layered roof structure is capable of absorbing and releasing moisture, even from condensation, heavy dew or rain, resulting in a cooling, evaporative effect on drying. Rooms on the upper floors of traditional buildings remain cooler and more comfortable in summer, greatly Traditional Mediterranean architecture is designed to reduce solar gain. Here the terracotta roof tiles of this chapel in the Elaphiti Islands, Croatia, provide evaporative cooling, the thick walls moderate the heat, and the trees and loggia provide shade. (Photo: Jonathan Taylor)
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