Too hot to handle

Next Friday, the DTI will take delivery of a report into the likely impact of climate change on the UK’s existing buildings. It will inform developers, designers, contractors and housebuilders that they are going to have to make changes to their products. It warns that from 2020 onwards, homes built to comply with current Building Regulations will become uncomfortably warm in the summer. And by 2080, a naturally ventilated 1960s office will be hotter than a Cairo street in July.

The study is one of the first to make a quantitative assessment of the effect of climate change on the built environment. It was commissioned by the DTI and based on research led by Arup. “The proposal was to look at the potential impact of climate change on the current building stock and then to look at what could be done to minimise that impact,” says Gavin Davies, associate director at Arup Research + Development, and one of the report’s authors.

Arup used computer modelling to find out how a building’s internal temperature is influenced by changes in climate. Using data from the UK Climate Impacts Programme, the team modified existing climate data to reflect increases in external temperature over the next 80 years (see “How the climate will change”, overleaf). An upper limit of 28°C was chosen as the maximum acceptable internal temperature; this is the point at which people “start to feel ‘hot’ and look for some level of cooling relief”, according to Davies.

This data was then fed into a thermal modelling programme. “We tracked the impact of heat created by people, the sun and equipment by taking a year’s weather data and running it through the dynamic thermal model,” says Jake Hacker, Arup’s project leader for the research.

The results show that global warming will make many offices unusable between June and August.

The worst performing building in the report is a 1960s naturally ventilated office; it predicts by 2080 temperatures in these offices will hit 39°C, three degrees hotter than an average July day in Cairo. Natural ventilation is used in 70% of UK business premises.

The results also call into question current strategies of low energy office design. Even the temperature in a state-of-the-art, low energy naturally ventilated office block is predicted to reach 35°C on some days in 2080.

Global warming will force architects and engineers to rethink one of the fundamental principles of low-energy building design – the use of night cooling. Many low-energy buildings rely on cool night air to remove heat stored in the building’s fabric during the day. However, Arup’s climate projections suggest that this will become less effective as night air becomes warmer.

The obvious way to mitigate rising temperature is to fit more air-conditioning to buildings. However, this solution is certain to make the problem worse. As Hacker says: “Air-conditioning systems emit greenhouse gases that contribute to global warming in the long term.” In any case, Part L of the Building Regulations were amended in 2002 to limit how much extra air-conditioning can be added. These restriction are likely to be increased as concern over global warming grows.

Hacker believes designers should be looking to develop buildings that combine mechanical with passive systems to produce what engineers call “mixed mode” ventilation. In such buildings, mechanical cooling and ventilation are used only when temperatures rise beyond set levels. He says: “Air-conditioning needs to be combined with passive cooling systems to provide a greener and more cost-effective solution”.

It is not just the future of naturally ventilated buildings that has been called into question. Many air-conditioned buildings will struggle to cope with climate change. Most cooling systems have been designed with reference to historical weather data. Remember last year, when temperatures at Gravesend in Kent reached a sticky 38.1°C, the hottest day ever recorded in Britain? If Arup’s computer predictions are realised, air-conditioning systems will need their cooling capacity to be increased substantially to maintain a comfortable working environment.

Overheated office workers are unlikely to find refuge in their homes. Arup predicts that by 2080, the temperature in a lightweight home could reach a sweltering 40°C. “The existing building stock has not been developed for future climate changes,” says Davies.

Arup’s findings also suggest that buildings with high levels of thermal mass will be easier to adapt to cope with climate change, which calls into question the government’s commitment to timber-frame, modular and other lightweight construction systems. “If you are looking at buildings designed for the long term, then lightweight construction is going to be very susceptible to climate change,” Davis has said.

By 2080, Arup says temperatures in London will reach those currently experienced in Marseille. Arup suggest that the best way to mitigate the effects of global warming is to consider adopting similar building designs to those found on the Mediterranean coast. “When the temperature inside a building hits 28°C, you cannot just open a window because it is hotter outside than inside. We need Mediterranean-style shutters, heavy walls, smaller windows and verandas to keep cool.

Arup’s recommendations for minimising the impact of climate change are as follows:

• Use smart building management to reduce heat gain, for example, by switching off lights and equipment when not required;
• Use blinds and shading to reduce solar gain;
• Minimise ventilation when the external temperature is hotter than the internal;
• Use photovoltaics or renewable energy to minimise emissions caused by cooling;
• Incorporate exposed materials with a high thermal mass inside a building.

However, it is not just buildings that will have to adapt to a Mediterranean climate. Arup suggest that climate change will also affect the way we work. The researchers propose relaxing dress codes, sending staff home during hot spells, introducing flexible working or longer summer holidays and siestas. They also suggest relocating offices to the north of England.

The challenge for construction is to produce buildings that can be adapted over their lifetime. Hacker says one possibility is that the government insists developers take this approach. “Statutory requirements and Building Regulations do not require that designers plan for climate changes, but the results of our study might change this when the codes are reviewed,” he says.

The full report, which covers houses, flats, offices and schools, will be published by the Chartered Institution of Building Services Engineers this autumn.

Domestic houses

A modern four-bedroom “executive” detached house built to Part L of the Building Regulations.

Low mass

Construction

• External walls: timber frame with timber cladding on the outside, insulated cavity and plasterboard lining
• Internal walls, timber stud and plasterboard
• Synthetic slate tiled roof
• Timber floors on upper storey, solid ground floor
• Carpeted throughout

The existing building stock has not been developed for future climate changes

Gavin Davies, Arup Research + Development

Performance
Arup also modelled a high thermal mass house as a comparison to the above, constructed using materials such as an external brick, insulated cavity, blockwork wall with precast concrete floors. The high-mass house performed better than the low-mass house, but there was very little difference between the number of hours the two houses exceeded 28°. However, the temperature in the low mass house increased well above the 28° limit, whereas the high mass construction moderated the internal temperature, keeping it closer to the 28° limit.

How houses could be adapted

• Reduce solar gains by increasing external shading such as louvres or shutters to the outside of the building
• Fit blinds to windows
• Control the ventilation to limit the amount of heat introduced into the house from hot outside air
• Enhance the amount of thermal mass in a building through using phase-change materials, which take advantage of latent heat
• Introduce domestic air-conditioning
• Install mechanical ventilation to cool the building at night

Offices

All three offices below are three-storey, have the same occupancy hours and density, are open plan, and have the same equipment and lighting loads.

1960s naturally ventilated office

Construction

• Poorly insulated leaky envelope
• Natural ventilation by opening
• Internal slatted blinds

Performance

• The 1960s building is the worst performing of the offices because its ventilation rate is controlled by opening and closing windows
• Low thermal mass
• No external shading
• The office fails to meet the present day BRE Office of the Future performance standards; by the 2080s, almost internal temperatures will be over 28°C for one-quarter of the year, and peak temperatures will approach 40°C

How the building could be adapted

• Upgrade the envelop and introduce a controllable ventilation system or install a mechanical ventilation system
• It is unlikely that any further upgrading would be viable without a total rebuild

Mixed-mode, mechanically ventilated office

Construction

• This office is a typical, well designed, low-energy building
• Highly insulated, airtight structure, built in 2002 to comply with Part L of the Building Regulations
• Openable windows for use in spring and autumn
• Mechanical ventilation through raised floor
• Extract ventilation through concrete coffered ceiling slabs, which also provide thermal mass

Performance

• The mixed mode office fails the BRE’s office of the future performance standard of not more than 1% of occupied hours over 28°C
• The peak temperature of 34°C is 6° lower than the peak experienced in the 1960s office because of better solar control, and controlled ventilation including night-time cooling
• From 2020s onwards, the office would require additional comfort cooling

How the building could be adapted

• Daytime ventilation cannot be increased without compromising office air quality, but night ventilation could be increased with the addition fan power
• Additional thermal mass would be difficult to retrofit but phase change materials – which use latent heat to increase cooling – could be a novel way to enhance thermal storage
• Chilled beams could be added, cooled, where possible, from ground water
• A cooling coil could be added to the ventilation system to condition the air

Advanced naturally ventilated, high mass office

Construction

• This is a state-of-the-art low energy office similar to “Building 13”, BRE’s low-energy office in Garston, Watford
• The building has high mass, exposed elements and concrete ceiling slabs
• Well insulated envelop
• External shading on windows
• Cross-ventilation and stack cooled ventilation towers

Performance

• Even this state-of-the-art office fails to meet BRE’s benchmark from 2020 onwards
• Despite its advanced specification, the peak temperature in this building is similar to the mixed mode building, although on average this building is cooler
• Good solar shading, controlled ventilation, night-time cooling and high levels of thermal mass help moderate internal temperatures
• Additional cooling would be needed from 2050

How the building could be adapted

• Night ventilation and shading are installed so additional cooling could be provided by the use of chilled beams