Overheating buildings: learning from the past

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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).

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 helped by other features such as high ceilings, thick walls, and windows which are shuttered against the sun during the day and opened at night to help ventilation.

Traditional roofs complement a number of distinctive architectural features that together promote passive environmental control, including loggias, deep eaves, courtyards and sunshades. Surface colour and finish are also important in absorbing or reflecting heat.

In order to understand how a warming northern European region might utilise traditional (though even now threatened) indigenous building practices in the Mediterranean region, studies are needed on the behaviour of traditional unmodified and modified roofs (with membranes for example) and modern roofs, so we can quantify improvements in energy consumption and sustainability, and assess the effects of cooling on comfort. Studies of cool roofs and evaporative roofs are not new. However, there is scope for innovation by combining data from a variety of sources to help us to understand the performance of northern European historic building typologies and the effects of modern adaptations. These data sources include thermal (and moisture) analysis from satellite images; similar data from drones equipped with optical, thermal and near-IR cameras; and direct in situ measurements of air and surface temperature and the relative humidity of built fabric and interiors.

Another source of inspiration for how historic buildings in northern climates might adapt to warming conditions is how the British army adapted historically to the hot and humid climates in India. In the 18th century, most British soldiers in India were housed in barracks in the coastal forts at Chennai, Cuddalore, Kolkata, Thalassary and Mumbai. The barracks were divided into large, long rooms with large windows and high ceilings. In northern India, the barracks were equipped with hand operated fans known as punkahs but not in South India, where the barracks were located on the climatically hospitable coast.

However, the cooling qualities of the architecture were compromised by the tendency of soldiers to divide rooms with hanging sheets to create private spaces for married troops, and to clutter their verandahs with furniture. The interior of the barracks where the single soldiers lived were turned into airless, dimly lit spaces. In the 1840s in northern India special military hill stations began to be developed so that a large portion of the troops, perhaps two-thirds, could be sent up to the cool of the mountains each summer to protect them from heat and disease.

The 1863 report by the Royal Commission on the Sanitary State of the Army in India provides interesting observations and insights on indigenous knowledge of ventilation. It was observed that the indigenous population slept only on upper floors, so it advised that barracks (officers and married quarters in particular) should be built as detached bungalows, well raised above the ground to enable free passage of air underneath, and well ventilated from the roof.

Barracks buildings were to be arranged diagonally to catch the prevailing wind and not arranged in close squares.

It was observed that locals at night placed a mat windward of their bed to cut off immediate draughts on their bodies. With the need for ventilation, uncomfortable air currents had to be managed using Venetian slats in doors and windows, louvres in the roof and clerestory windows along central raised aisles with ventilators in the ridge. In addition, sufficient space was allowed between one door or window and the next to eliminate draught exposure.

It was recognised that arrangements to maximise natural ventilation had their own disadvantages: variable wind, rain and dust ingress. It was on these matters that fresh air inside barracks depended. One solution was ‘to raise the walls three feet and to put on what was termed a Bengal roof, by which a current of air was admitted all round, and wire gauze was placed inside and outside the ventilators, beyond the reach of the men’. In other words, fresh air was admitted under the eaves and every effort was made to reduce tampering with the system.

It is important to connect with knowledge like this across the globe, even if it is not universally applicable.

So what are we in time to do? We can act in two ways: at a technical level and policy level. We must learn from the past, make effective use of historical and scientific data and look for ideas outside our immediate environment. To do what we need to do well and with courage is to evaluate critically, provide advice and guidance and participate in the development of standards to ensure what we get is fit for purpose.

This article originally appeared as ‘Climate change: learning from the past’ in the Institute of Historic Building Conservation’s (IHBC’s) Yearbook 2023. It was written by May Cassar CBE, Professor of Sustainable Heritage at UCL’s Institute for Sustainable Heritage.