Sitting in his cool front room in Worcester Park, a suburb of sprawling south London, John Butler explains how it used to feel living in his 1930s home.
The damp caused cupboards to go mouldy and the wallpaper to peel. The interior walls could be cold to the touch in the winter, and opening windows in the summer would give him a sore throat from the pollution outside while doing little to cool down the house.
"It would be excruciatingly hot," said Butler, 60, who has lived in the house for nearly three decades.
But this summer he and his partner, Dawn Mulvaney, have been comfortable for the first time, after their home was one of 13 chosen for a local social-housing retrofit, to pilot a new approach from the Netherlands called "Energiesprong".
Instead of boosting a home's energy efficiency one improvement at a time - updating the heating system, replacing windows, insulating wall cavities - the building is fully updated within a few months, or even a few days.
Under the Energiesprong system - the name means "energy leap" in Dutch - insulated pre-made roof and wall panels are installed to fully envelope the original house, like a tea cosy.
That, combined with rooftop solar panels, new doors, triple-glazed windows and a low-carbon heat pump and ventilation system, brings the house close to achieving net-zero emissions, with the power it generates mostly covering what it uses.
After Butler's retrofit, "the bills went down straight away, the air was cleaner, the rooms were cooler," he said.
In the six days after the work was completed in July, the couple's energy bill came to 4 pounds (US$4.84), one-fifth of what they used to pay per week in the summer.
As homes across Britain brace for soaring energy costs this winter due to a global surge in fuel prices driven by the war in Ukraine, investing in more such retrofits could ease the burden for the poor - and set up Britain to meet its climate goals.
UK household energy costs are expected to surge 230% this winter compared to last year, according to analysts at Cornwall Insights - a meteoric hike many families will struggle to afford.
The financial squeeze facing millions of Britons spotlights the government's sluggish progress on upgrading homes to improve energy efficiency, a move climate experts and housing groups see as vital to reaching net-zero goals and lowering energy bills.
A June report from the independent Climate Change Committee said Britain had made headway in boosting its use of renewable energy and electric vehicles, but a lack of housing retrofits is one thing hampering progress towards its goal of net-zero carbon emissions by 2050.
The report said government schemes supported energy efficiency projects in more than 150,000 homes last year, but that needs to increase to 1 million per year by 2030.
Britain has approximately 29 million households.
"The first thing we should do isn't subsidise everyone's Teslas, but make sure those in the most vulnerable housing are able to affordably keep warm," said Alastair Harper, head of public affairs at housing charity Shelter.
"People are stuck in old, leaky buildings that weren't designed for the world we're in now," he said.
Social housing focus
As a way to get homes to net zero, whole-house upgrades can be prohibitively expensive.
But Emmelie Brownlee, head of communications at Energiesprong UK, the nonprofit company working to expand the approach, said the cost would come down as the supply chain develops.
The organisation aims for a cost of about 75,000 pounds (US$90,720) per retrofit, with Brownlee saying its method is still cheaper and more climate-efficient than single measures over the long term.
With the help of government grants, Butler's retrofit was paid for by Sutton Housing Partnership, which manages the council's housing. Now the retrofit is complete, Butler will pay them a weekly 10-pound fee to contribute to the cost.
In search of other options, many local governments have applied for the government's 3.8 billion-pound Social Housing Decarbonisation Fund (SHDF), which launched last year.
The SHDF takes a "fabric first" view that prioritises loft and wall insulation before replacing energy systems, a method some experts back as quick and affordable, estimating an uninsulated home loses about a quarter of its heat through its roof.
London Mayor Sadiq Khan aims to accelerate retrofit efforts in the capital by mobilising about 500 million pounds through green bonds.
"Social (housing) is the way to go because we can ramp up quite quickly," Khan said in an interview with the Thomson Reuters Foundation.
"Rather than going door to door... if you do social homes, you get an entire block," Khan said.
Insulating against fuel poverty
But social housing makes up less than one-fifth of homes in Britain.
Fuel poverty can be an even bigger problem in private rentals and owner-occupied properties where there have generally been fewer efforts to upgrade energy efficiency.
The Department for Business, Energy and Industrial Strategy (BEIS) launched the Green Homes Grant in 2020 to kick-start energy efficiency retrofits for private homes and rental units.
But a report from the Public Accounts Committee found the scheme, which spent only 20% of its original budget after many applications were rejected, will have helped upgrade only about 47,500 homes out of the 600,000 planned.
The report added that the department's "fragmented, stop-go" approach to retrofitting homes in general "has hindered stable, long-term progress".
A BEIS spokesperson said the department is taking these lessons into account when designing new policies, and has a commitment to "go further and faster" in investing 6.6 billion pounds in energy efficiency measures over this parliamentary term.
Nick Mabey, director of the E3G climate change think-tank, said the best way to scale up successful retrofit projects is to give more control to local governments.
"We're the most centralised country in the G20 for infrastructure investment and financing, and that means we struggle when it comes to things that require detailed, place-based investment and engagement," he said.
Over in Sutton, Butler worries there could be social unrest in the winter when many people are forced to choose between heating their homes and paying their rent, or feeding their children.
"Things just can't go on the way they have been," he said.