How to remake our houses and cities for hotter weather

Times

3 days ago

Forget the price of Oasis tickets or the shrinking size of strawberries — the big debate of the British summer is how to adapt our homes and cities to cope with heatwaves.
As temperatures spike, 80 per cent of households report they're overheating, soaring from 18 per cent in 2011, according to a study from the University of East London.
Meanwhile our city streets turn into 'urban heat islands', where, according to the European Commission's Joint Research Centre, temperatures can now be 10-15C higher than rural surroundings.
The Savills City Heat Resilience Index assesses 30 global cities based on their risk of extreme heat and resilience to it. London is in the bottom half, at No 13, between Madrid (12) and New York (14). A trio of Nordic cities, where days over 30C are rare, Helsinki, Copenhagen and Stockholm fare best. Cairo comes last.
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'The heat creeps in through south-facing glass, across bare concrete pavements, into bedrooms that don't cool at night,' says Stefan Pitman, founder of the architects and chartered surveyors SPASE Design. 'Our homes, designed for damp and draughts, are becoming glasshouses. Cities and homes trap heat they can't release. We are far too reliant on mechanical cooling that isn't sustainable.'
Learning from places that have long lived with heat and embracing the modern principles of Passivhaus is the way forward, say experts. This means buildings thick walls, shaded courtyards and cross-breezes, and also controlling solar gain, insulating well and aiming for airtightness balanced with mechanical ventilation. The UK's urban planners look overseas for solutions, from the cool roofs now mandatory in Los Angeles, to the green urban walls of Colombia and the fountains of Paris.
'We need insulated windows for both winter and summer, so indoors keeps both warm and cool,' says the architect Marion Baeli, principal of sustainability and transformation at 10 Design. 'Architects think of a home like a Thermos flask, in winter retaining heat, in summer keeping heat out.'
Baeli and her husband, Robert Prewett, a fellow architect, have transformed their 1960s home in Sydenham, southeast London, on Passivhaus principles to maintain a steady indoor temperature, even during heatwaves. They have fitted triple glazing to help achieve this. Solar control coatings can help to mitigate heat as well.
'Passive solar design plays a key role [in keeping homes cool], but historic recommendations, such as large south-facing glazing, are being re-evaluated in light of current overheating risks,' says the architect Richard Dudzicki, director of RDA Architects.
External shading is gradually becoming a feature of new-builds. The building services engineering specialist Dan Watt, director of sustainability at the civil engineers Civic, highlights a Birmingham apartment development where horizontal brise soleil fins were incorporated for shading and heat reduction in summer, but allowing lower winter sunlight through.
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Deeper window reveals also help to reduce direct solar gain. Quick fixes include awnings and shutters. Internal blinds are another easy add. Baeli uses cloth dust sheets, strong hooks and bungee cord to fashion a makeshift blind for the hottest days: 'It works tremendously. I'm not ready to invest thousands on motorised blinds.'
Ventilation is key to keeping homes cool, says Ian Pritchett, a physicist and the co-founder of the sustainable developer Greencore Homes.
'The best way to let heat out is by opening doors and windows, if safe to do so, early in the morning and/or late at night to let hot air out and cool air in. Maximise impact by creating natural cross-ventilation, opening opposite windows to create a good flow of air.'
If you can, practise 'stack ventilation': 'This is where high-level windows or roof lights are opened as well as low-level windows. The hot air goes out of the upper windows while cooler air is drawn in through the lower windows.'
Struggling to sleep in a heatwave? You're not alone. A study by Loughborough University in partnership with the BRE (Building Research Establishment) found that 4.6 million English bedrooms — some 19 per cent of housing stock — overheat in summer.
'Thoughtful layout plays a big part. Where possible we'll sometimes place bedrooms on the cooler, shaded side of the house, or even downstairs where it naturally stays cooler,' says Venetia Rudebeck, co-founder at the interior design company Studio Vero.
You've got to be flexible if you're working from hom, says Baeli. 'In a heatwave you're not working where your desk is, you're working where it's coolest,' she says. 'In our house the ground floor has a lot of thermal mass, thanks to the party walls and tiles on the floor [it absorbs heat during the day, releasing it slowly at night]. I usually work at the top of the house, but when it's hot I migrate down to the kitchen table.'
Even the smallest balcony or garden can be effectively cooled if you choose container-friendly specimens — such as dwarf field maple or the compact Juneberry (Amelanchier × grandiflora 'Ballerina'), says Dean Meadows, principal arboriculturist and tree risk lead at the ecology consultancy Arbtech.
'Start by noting where the sun strikes your home and garden at different times. Once you know which areas need protection from intense sunlight, choose a deciduous tree whose mature canopy will intercept those rays and whose roots can spread without causing damage.'
Meadows recommends field maple or silver birch: 'These give dappled spring light and full shade through the summer, yet let winter sun through their bare branches.'
Thinking about overheating when planning home design should become second nature, says senior project architect George Yallop at the design studio De Rosee Sa, who introduced multiple cooling techniques in a recent project in Dulwich, southeast London.
'The extension in our Garden House project was designed with passive cooling in mind,' Yallop explains. 'The extension itself is slightly elevated from the garden, enhancing airflow beneath and through the room, while full-width sliding doors allow the home to be completely opened once the sun has set.'
Yallop added a narrow, solar-coated rooflight to bring in light without overheating, while mature garden trees were retained for natural shade and cooling.
Being smarter about choice of building materials, is a straightforward way to address overheating, says Simon McWhirter, CEO of the UK Green Building Council: 'Each type of material is better or worse at controlling the speed at which heat passes through it. Lots of natural materials such as hemp or clay plasters are much better at slowing down the heat, so whether it's winter or summer, they are much better at keeping the heat in and keeping the heat out.'
For some homes, from flats with single-aspect windows to period properties that can't be significantly altered, it's air conditioning. 'In a new-build tower, with apartments, you have to accept that even with all the improvements you can think of, fitting air conditioning will be the only answer [to overheating],' says Baeli. 'However, I do know someone who lives in such an apartment and made an agreement with their opposite neighbour that they would leave their front doors and windows open to encourage a cool flow of air.'
Colombia's second-largest city, Medellín, has created an entire metropolis of shade with its Green Corridors project. This transformed roads and waterways into green cycling lanes and walkways connecting the city's parks, reducing temperatures in these areas by 3C.
Meanwhile, in central Manchester, McWhirter cites Mayfield, a 24-acre brownfield site transformed into a 'pocket park' as a stand-out UK example of how green spaces can help to keep urban temperatures cool and improve general wellbeing. 'Nature is good at doing the heavy lifting,' he says.
Also in Greater Manchester, at Stretford Kingsway, Trafford, part of a dual carriageway has been reclaimed into a green walkway and cycleway, with a shade-giving weeping willow tree and sustainable urban drainage.
Blue corridors are urban bodies of water — rivers, canals, docks — which provide respite from scorching heat. Helping to reduce the temperature of the air around them, they also provide a chance to cool off (subject to pollution levels of course) with wild swimming, paddleboarding and other water sports.
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One stand-out example is Eden Dock (formerly Middle Dock) in the concrete jungle of Canary Wharf, London, designed by the architectural partnership Howells and HTA. Surrounded by timber decking and lush planting, it connects workers and residents with surrounding water and nature.
'Urban trees act as natural air conditioners and serve as a critical nature-based solution to rising temperatures in towns and cities,' says Meadows. Trees work their magic in three ways: shading streets and buildings; evapotranspiration, in which trees draw water from the soil, releasing it as water vapour through their leaves to create a cooling effect; and by helping urban airflow, replacing warm, stagnant air with cooler air drawn from surrounding green spaces.
US cities such as Los Angeles and Phoenix are coating roads with a heat-reflecting white/grey coating that can reduce the surface temperature by up to 9C.
Standard asphalt creates a heat sink by absorbing huge amounts of sunlight then emitting it back into the air as heat. It's said that this super-thin coating works by reflecting sunlight away.
Cities across India, plus New York, Los Angeles, Washington and Toronto in Canada are already painting rooftops with white reflective paint.
It could work in London too. According to research published last year by University College London, this kind of approach is more successful at heat mitigation than green roofs, covered in vegetation. In London, it could lower outdoor urban temperatures by at least 1.2C.
Increasing public tree cover helps to keep cities cooler, but it's expensive and planting is limited by underground infrastructure, says Tom Fox, associate partner at We Made That, a research, urban design and architecture practice. He adds: 'This means awnings and shade structures are important to providing sufficient shade to have a meaningful impact on temperatures.'
In Seville, Spain, which in 2022 became the first city to start naming its heatwaves, Antonio Muñoz, the mayor at the time, ordered the installation of awnings across the city as part of a 'policy of shade', which also included planting 5,000 trees a year and building more public fountains.
Parisians benefit from a network of more than 800 'cool islands', accessible city spaces including parks, misting fountains, swimming pools and museums, providing relief from vicious heat. There's even a mapping app, Extrema, to help residents find their nearest chilled outpost.
'These developments are necessary because Europe is warming faster than the global average,' says Elena Rivilla-Lutterkort, Savills's head of sustainability in France. 'Paris is working under an assumption that an average increase of 4C is a likely scenario.'
In dense urban environments, heat emitting from buildings contributes to scorching outdoor temperatures. In Abu Dhabi, where the mercury can climb to more than 50C, architecture practice Aedas designed the twin 25-storey Al Bahar Towers using mashrabiya, an ancient Arabic cooling technique using honeycomb-like latticed screens.
These 1,000 unfolding hexagonal shades have built-in sensors, responding to the sun. The architects estimate this reduces solar gain by more than 50 per cent, diminishing the need for air conditioning.
Dubai also has district cooling. Water is collected and chilled at a centralised location and then piped into residential and commercial buildings. According to Savills, district cooling achieves energy-efficiency index ratings between five and ten times higher than conventional air-conditioning units.