It’s like X-ray vision – for heat loss. Last year, web engineer Jamie Matthews bought a thermal imaging camera. His home, in the southeast of England, was about to give up some of its secrets.

The colourful images on the device’s small screen soon revealed how his loft hatch was letting him down.

“That was really obvious on the camera,” he says, explaining that he could see cool areas on the hatch, where heat was apparently escaping.

Mr Mathews, who runs a software company that supplies the clean tech industry, also found some other small gaps in external walls and parts of his roof that appeared less well insulated than others.

“It’s good fun and it certainly shows up things that may not be immediately obvious,” he says.

While Mr Matthews was able to draught-proof some areas himself, he says he would take a more methodical approach if he were planning any major changes to his home. The web engineer later had a professional heat loss survey carried out when he installed a heat pump, for example.

That approach is sensible, according to building physics researchers who spoke to the BBC. There are signs that thermal cameras are becoming more popular, as homeowners seek to slash their heating bills and carbon emissions. Local councils are increasingly allowing residents to borrow the devices for free, for example.

But there is no substitute for professional advice if you are planning big works or interventions, experts say.

Thermal cameras work by using a sensor to detect infrared light, which we cannot see, but which we can sometimes feel as heat. Such devices aren’t cheap. Mr Matthews spent roughly £160 on his, though organisations including local councils and energy supplier Octopus Energy do loan them out to people free of charge so that they can gain a basic understanding of heat loss in their homes.

Wiltshire Council recently made 11 devices available to borrow from local libraries, as well as a brief guide on how to use them. There has been “huge uptake” says Cllr Ian Blair-Pilling, such that the council now has a waiting list for interested residents.

“Expert advice, combined with a measured approach to the findings from thermal imaging, will give residents the best chance of making informed decisions about energy-saving home improvements,” says Cllr Blair-Pilling.

Small interventions could help people keep their homes warm and reduce the risk of cold-related health conditions, he adds.

However, Jo Atkinson, senior consultant in buildings decarbonisation at Energy Systems Catapult, a research organisation, expresses concern over the growing availability of thermal cameras: “In my opinion, the risks are too great for making the wrong choices.”

It is possible to misinterpret thermal camera images, she says, noting that if a wall has been warmed by the sun, an image of that wall taken outside could make it look as though heat is escaping, when it is not. Reflective surfaces can also appear, misleadingly, to be leaking heat.

While using a thermal camera as a starting point for understanding your home is “absolutely fine”, according to Ronita Bardhan at the University of Cambridge, it is important to ask for independent expert advice when planning any renovation works.

Thermal camera imagery can make it look as though some areas of your home are losing a lot of heat, but this might simply be a result of how the camera has been calibrated, how the user holds it and moves it around, and which colour palette was chosen for the display.

Dr Bardhan adds that there is a risk of triggering “retrofit anxiety” in homeowners.

Plus, if people go on to alter their insulation, for example, such measures should be taken with great care as improper installation can cause damp and mould.

North Somerset Council provides thermal cameras to residents, on loan, and this scheme is also currently operating on a waiting list basis due to its popularity. The Council has published introductory advice on how to use the cameras, including an online video.

The Centre for Sustainable Energy (CSE), a charity, highlights a more detailed guide from the National House-Building Council, which describes how to use thermal cameras. However, a CSE spokeswoman adds that the organisation still recommends people seek expert advice.

Phil Steele, future technologies evangelist at Octopus Energy, agrees: “You have got to be careful.” Thermal cameras can, for instance, make the corners of rooms look very cold but this is simply a reflection of reduced air circulation in those areas, rather than an insulation problem, he explains.

Octopus Energy has around 500 cameras that it loans out to customers every winter. Last year, the firm distributed the devices roughly 3,800 times.

Thermal camera-wielding homeowners who have spoken to the BBC, such as Mr Matthews, say they have treated the imagery with caution.

Louise Green, a designer in London, says she read online training materials before using a thermal camera to search for points of heat loss in a house shortly after she and her partner moved in.

In her case, Miss Green rented the camera from Library of Things, a community-focused organisation that makes various items available for people to borrow. The device indicated loss of heat around windows and doors in Miss Green’s home. She and her partner were able to compare the results to their neighbour’s better-insulated property.

“It just confirmed what we knew already, but it was still interesting to see, you know, were there any other areas?” she explains.

Darrell Taylor at Flir, a firm that makes thermal cameras, says that members of the public can access some free training materials on his company’s website.

He adds that Flir aims to make thermal images as clear and easy to interpret as possible – including by merging optical imagery with thermal imagery, to add more detail to the final picture. And he suggests that, in the future, Flir cameras may offer machine learning-based instructions and advice as people use the devices.

Those who know how best to interpret thermal camera imagery can find extraordinary hidden problems in a building, though.

Dr Bardhan recently took a very high resolution camera to a viewing of a house she was considering buying. But the thermal images revealed cracks in a wall, which later proved to be signs of subsidence.

“That would probably not have been exposed unless I had gone in and seen it,” she says. Dr Bardhan decided not to make an offer on the property.