By Kelly Oakes

The world’s housing has something of an emissions problem.

Residential properties are responsible for between 1721% of energy-related carbon emissions globally. That figure covers everything from the electricity we use to power televisions and other electronic devices, as well as the fuel we use to heat our water and cook – but in cold climates the majority of a house’s energy budget goes on heating.

Add in the carbon emissions released in the manufacture of concrete, metal and other building materials along with the construction process itself, and it’s clear that housing has a considerable role to play if the world is to meet its ambition to reduce global carbon emissions to net zero by 2050.

It is an important target – one that is necessary to keep global warming to within the 1.5C increase that nations pledged to aim for in the United Nations’ Paris Agreement on climate change.

The good news is that we already have the knowledge and tools we need to reduce emissions from housing. It could mean that by 2045, as the mid-Century emissions deadline approaches, the way our houses are built, heated and powered could look very different. Not only could these innovations make our houses cheaper to run but also make them more comfortable to live in.

“We have a huge amount at our disposal that we can use to address the environmental footprint of the built environment,” says architect Maria Smith, director of sustainability and physics at engineering consultancy firm Buro Happold and editor of a report from the Royal Institute of British Architects (RIBA) on decarbonising the building sector. “We shouldn’t feel like we need to wait for breakthrough technologies in order to act.”

The first step is to reduce the amount of energy our houses use. The design principles used to achieve this are deceptively simple: make a house well-insulated so that heat from passive sources – the Sun, inhabitants’ body heat, even pets – stays indoors. The orientation of the house and the position of its windows can ensure it makes the most of the Sun’s rays, while shutters, overhangs, and even trees can keep houses cool in summer. In colder climates, only a small amount of additional heating should be needed in winter. Because an energy-efficient house needs to be relatively airtight in order to retain its heat, the final element is a ventilation system to circulate fresh air.

The leading building standard in this area was developed by the Passivhaus Institute in Germany and reduces space heating requirements by around 75% compared to a typical new build and can cut fuel bills by up to 90%.

Although the idea behind low energy housing was first conceived in the 1970s, it wasn’t until the early 1990s that German researchers built what would be regarded as the first passive house in Darmstadt. But the construction industry has been slow to adopt the model in many parts of the world.

In 2021, however, this is starting to change, with this way of building houses becoming more widespread. In 2015, the city of Brussels, Belgium, passed legislation making it mandatory for all new buildings (both residential and commercial) as well as all retrofits to meet tough passive house standards. And in 2019, a row of passive council houses in Norwich, UK, was awarded RIBA’s Stirling Prize.

Shifting to lower carbon building materials is also necessary. Today around 10% of global energy-related emissions come from materials used in construction and maintenance of buildings, including housing. “Cement and iron and steel are energy intensive in both their production and also in their transport,” says Angel Hsu, professor of public policy and the environment at the University of North Carolina-Chapel Hill. “Building materials are critical in helping the housing sector reach net zero.”

Sustainable alternatives to emissions-heavy concrete and bricks already exist in 2021. Timber, for example, is a material that has been used to build houses for thousands of years. One study estimated that between 20-80 million tonnes of carbon dioxide emissions could be avoided globally through replacing concrete floors with timber and steel composite structures by 2050 – and that’s before even taking into account the carbon that the timber would lock away.

Materials vary depending on location and climate, but hemp, cork, bamboo and straw bales are other low carbon options. “Anything that has grown is just such a gift because it locks in the carbon, and then if you can then store that carbon in a building – fantastic, we’re helping,” says Smith.

However, the best material from an embodied carbon point-of-view is often the one you already have, say researchers. That means reusing as much as possible from existing buildings to avoid the emissions from creating new materials. Rather than simply demolishing an old building, it can instead be carefully deconstructed so its bricks, for example, can be saved and repurposed. Similarly, steel beams that can’t be used again should be melted down and turned into new ones. “We really have to make the best use of what we’ve got,” says Smith. “That’s a really central part of making the built environment more sustainable.”

As well as increasing standards for new buildings, retrofitting existing houses to be more energy-efficient is likely to be essential. Around half the building area that exists globally in 2021 will still be around in 2050, and in some countries, like the UK, the number of existing buildings expected to still be in use in 2050 is as high as 80%.

So, how do we turn our current houses into zero-carbon homes? Measures like draught-proofing, adding or improving insulation, and installing double or even triple-glazed windows can reduce a house’s energy requirements significantly. Existing houses can even be retrofitted to meet the rigorous Passivhaus standard for retrofits, known as Enerphit.

Paul Testa, an architect living in Sheffield with his wife and two daughters, renovated his 1970s semi-detached house to Enerphit levels. It was a significant undertaking in terms of both time and money, but the results have been transformative.

Before the retrofit, the house was typically cold and draughty. “It had heavy velvet curtains for a reason,” says Testa. “You opened the curtains in the morning and you got this blast of cold air.”

Since the work has been completed, the temperature hovers around 21C downstairs, and 19C upstairs, with only a little additional heating, he says. “You can sit next to the windows and you don’t feel any cold off them because they’re triple glazed,” says Testa. He compares shutting his front door to getting inside an expensive car: “You kind of seal out the world.”

The house still uses a gas boiler for the handful of radiators that remain, but the family’s heating bill has dropped to £300 a year, from around £120 a month before the renovations.

Once a house is as energy-efficient as possible, adding renewable energy is the final step.

Heat pumps powered by renewable electricity are a sustainable option. They work by extracting heat from the air, ground or water – essentially acting like a fridge in reverse – and use four times less energy than gas boilers. Solar panels can provide the electricity needed to run the heat pump, and other household appliances, completing the zero-carbon set up.

In Copenhagen, a floating student housing development made out of old shipping containers draws heat from the surrounding seawater using a heat pump. Heat pumps are rising in popularity for more typical housing throughout Europe too, with the highest numbers sold per capita in 2020 in Norway and Finland, shortly followed by Estonia, Denmark, and Sweden.

In the most leaky homes, however, heat pumps will struggle to generate enough warmth to keep inhabitants toasty throughout the winter. Until those houses can be brought up to standard with insulation and other improvements, hydrogen – created by splitting water with electricity – is seen as an alternative option to natural gas. Crucially, though, the hydrogen must be made using renewable electricity – the “green” in “green hydrogen” – to make it zero-carbon. In 2021, the cheapest and most common form of hydrogen production globally uses natural gas, resulting in carbon emissions.

In Fife, Scotland, however, a green hydrogen project that uses electricity from offshore wind farms to produce the gas is set to heat 300 homes by 2023, giving an indication of how renewables could be used to heat homes in years to come.

But decarbonising housing isn’t just about reducing carbon emissions. With their consistent temperatures and abundant fresh air due to their sophisticated ventilation systems, energy-efficient homes are proving to be more comfortable places to live too.

“For me, one of the big reasons for doing the work is that the air quality in the house is so good,” says Testa. “We’ve not got any condensation that’s going to lead to dust mites or mould. It feels like a very healthy place to bring my daughters up.”