So, in the middle of a climate emergency, why don't we just knock down the old ones and build new?
First, there is the upfront (embodied) carbon – over 50 tonnes are spewed into the atmosphere when building a new home, according to research that colleagues at Igloo have conducted. This is equal to 50 or more transatlantic flights, even in some of the newer, more fuel-efficient planes.
Because of its reduced need for heating, a new home might save maybe three tonnes of CO2 a year compared with an existing, older one, according to the latest government figures. But this implies that it will take 16 years for the carbon savings from a new energy-efficient home to compensate for the carbon costs of its construction, and the latest UK government carbon budget, a requirement of the 2008 Climate Change Act, says we need to cut our carbon emissions by 50 per cent in the next ten years, not increase them. Under the act, the government must set five-yearly carbon budgets towards a long-term goal of cutting greenhouse gas emissions, by 2050, by at least 80 per cent below 1990 levels.
Then there's the cost. A new home in most parts of the country might cost £100,000 to build for a saving of £500 a year in heating costs. That means it would take 200 years to achieve financial payback.
And then there are the practicalities – for example, there aren’t enough builders to rebuild existing homes as well as build new homes.
So maybe we could eco-retrofit the old homes. We could add insulation and take out the gas boiler and replace it with an air source heat pump. We have been doing this to social housing, and some courageous and principled pioneers have done it in their own homes. However, recent attempts to do this at scale to Enerphit (Passivhaus) standards have cost more than £50,000 a home, which implies a 100-year payback. And this approach would still require a lot of up-front carbon, because the polyurethane foams that are frequently used for insulation are highly energy-intensive to make.
However, there are some places where the economics of rebuilding are more attractive. Large old houses on big plots near to stations in suburbia can already be profitably replaced with more, smaller homes that are better suited to current smaller household sizes, more affordable, cheaper to heat and can be lived in with limited car use.
To make climate sense of this, we need to substantially reduce the upfront carbon used in the rebuilding. This means, today, building in wood (particularly fire-resistant, cross-laminated timber) and other natural materials, and, where we are demolishing first, reusing as much of the previous materials as possible.
We can already get the upfront carbon for a new home down to less than 25 tonnes without adding cost, which brings the carbon payback below ten years and makes sense in the context of our need to be zero carbon by 2030.
And we should have a 76 per cent low-carbon electricity grid by 2030, and a zero-carbon grid before 2050, so making building materials in this country could involve zero upfront carbon in the foreseeable future.
It's not time yet for large-scale demolition of energy-inefficient but otherwise structurally sound housing, but it's started and will accelerate. But will it come fast enough to save the planet?
Chris Brown is executive chairman of Igloo Regeneration