To the casual observer, there is something distinctly creepy about the silver van I'm crouching inside. On a pitch-black winter evening, we're crawling the streets of Reading, taking pictures of every home we pass.

Surrounded by computers in the back of the van, thermal surveyor Chris Brind points to a screen displaying a camera feed. Ghostly multicoloured images of houses flicker in front of him. "White will be really hot, the lowest temperature will be blue," he says. Snug in their homes on this cold, rainy evening, no one indoors has any idea that their houses are being inspected.

Fortunately for these unsuspecting homeowners, Brind and his van are here to help. The images - real-time snapshots of heat escaping from a building - will be familiar to anyone who has seen thermal pictures of public buildings, including City offices and the houses of parliament, taken by energy campaigners to show how much energy is leaking out. Brind's company, Heatseekers, has been working in partnership with 25 local authorities across Britain since last October. It hopes that by confronting homeowners with visual evidence of exactly how their buildings are wasting heat (and money) it will galvanise them into tackling the problem.

Energy efficiency in homes is an urgent, if unloved, issue: around a third of the UK's carbon emissions come from the energy needed to heat buildings, and a lot of that energy is wasted. Put simply, the Victorian, Edwardian and Georgian homes many of us live in are terrible at keeping in the heat. On cold nights, their uninsulated walls and lofts do a great job of warming the outside air, at the expense of the planet and our wallets: experts estimate that one pound in three spent on household energy bills is wasted.

This results in more CO2 in the atmosphere than all of Britain's flights and is equivalent to all emissions from cars. If we could get a lid on it, the climate benefits would be vast.

"In the UK alone, there are probably eight million properties that require insulation of some sort," says Keith Hewitson, director of Heatseekers. "If you're looking at getting cavity wall insulation and good quality roof insulation, to a depth of about 250mm, you could save £200 to £300 per year on fuel bills."

The difficulty is getting that message across to householders. Endless government schemes offering financial incentives to insulate homes have come and gone for decades, with limited results. Heatseekers thinks its images will reverse many people's inaction. "When they see an image of their property, they can see exactly what's escaping from their house," says Hewitson.

The surveys are carried out in the winter months, with shifts starting late in the evening when temperatures drop and householders crank up their heating. The colder the surrounding air, the more clearly warm walls show up on the pictures. Driving down a street at 10mph, surveyors can take energy snapshots of up to 1,000 homes an hour.

After the images have been recorded, energy advisers pinpoint homes they think need attention - usually ones with no insulation or those with patches that don't seem to be working - and call on them a few days later to offer a face-to-face consultation. Typically advisers will help the householders to arrange quotes and provide information on government-funded schemes such as Carbon Emissions Reduction Target (Cert), which can pay some or all of the costs for any work needed.

Part-time receptionist Kirsten Chapman from Leicestershire had her house scanned late last year before she was visited by a Heatseekers energy adviser. Chapman says it was immediately clear that her home was leaking energy. "It opened my eyes. Had I seen the pictures years ago, I probably would have taken steps to insulate sooner." Like much of Britain's housing stock, Chapman's semi-detached home, built in the 1960s without any insulation, needs loft and wall cavity insulation. The work will cost several hundred pounds but she is convinced that it won't be long before it pays for itself.

Thermal images can also reveal faulty seals around windows that might need replacing, chimneys and garage doors that are surreptitiously leaking heat, or show whether any double glazing that has been installed is doing its job properly.

In the three months since Heatseekers started its surveys it has grown from a one-van operation to a fleet of seven. Hewitson says councils have been lining up to use the company to survey their streets. "Currently we're working with local authorities from the Isle of Wight to the north-east. By the end of this year we should b e working with 30 to 40 local authorities."

Reading council was one of the first to ask Heatseekers to drive along its streets and, so far, 6,000 homes have been surveyed, paid for by Cert. "There's a direct benefit for local people, especially those on low incomes," says councillor Paul Gittings. He points out that the Cert scheme can cover the entire cost of insulation for those on benefits or a low income. In an attempt to tackle fuel poverty and climate change, the council aims to survey all their homes and then insulate 5,000 houses over three years - around half of the estimated need in the town.

Chapman says people approached with a thermal image of their homes should take the time to sit down with an energy adviser. "Don't dismiss it - it's easy to do that when people knock on your door, but when you see the images, it's an eye-opener".

• Grants are available for cavity wall and loft insulation. Go to energysavingtrust.org.uk/gid to search the Energy Saving Trust database for information relevant to your area, or call it on 0800 512 012.