A thermal imaging survey photographs the surface temperature of your walls, ceilings, floors and windows and turns it into a colour map. Cold patches on the inside in winter show where insulation is missing, where the structure bridges heat out, where air is leaking in and where insulation has got wet. It needs roughly a 10°C difference between inside and outside, so it is an October to April job, and it is part of our free heat loss survey. It cannot see through walls, measure a U-value or tell damp from cold on its own.
Key takeaways
- A thermal camera reads infrared radiation from a surface and shows it as colour. It sees surface temperature, nothing deeper.
- It needs about a 10°C inside to outside difference and a heated house, so the imaging part of our survey runs from October to April.
- What it finds: missing or slumped insulation, cold bridges at lintels and floor edges, air leaks round windows and hatches, and wet insulation.
- What it cannot do: see inside a cavity, give a U-value, work in sunshine, or tell you whether a cold patch is damp without a moisture meter.
- Our heat loss survey is free because it is how we scope the work. You get a written, prioritised plan with indicative costs, whether or not you go ahead.
- Imaging on its own is not a diagnosis. The camera shows where; the surveyor works out why.
How does a thermal imaging camera work?
Every surface gives off infrared radiation in proportion to its temperature. A thermal camera reads that radiation and displays it as a colour scale, usually warm surfaces in yellow and white and cold surfaces in blue and purple. The camera does not see heat loss directly. It sees the result of it: an inside surface colder than the surfaces round it because heat is leaving through it faster.
The 10°C rule
To show a clear pattern, the inside needs to be roughly 10°C warmer than the outside and to have been heated for several hours so the walls have warmed through. On a mild day with the heating off, an uninsulated wall and an insulated wall look the same. In London the imaging window runs from about October to April, and a cold morning before the sun reaches the walls is the best time of all. We survey all year; outside the heating season the surveyor checks the same things by hand.
What does a thermal imaging survey show?
Inside the house in winter, the camera shows the cold spots. From outside on a cold night it shows the warm ones, which is the same information from the other side. These are the patterns we look for.
| What the image shows | What it usually means | What fixes it |
|---|---|---|
| Cold band along the top of an upstairs wall or across part of a ceiling | Loft insulation missing, pushed aside or thinner than the rest | Loft top-up; see loft insulation |
| Cold lines at floor edges, window reveals, lintels and where the party wall meets the front wall | Cold bridges through the structure | Insulation carried into the reveals and across the junction; see cold bridging explained |
| Cold streak fanning out from a window edge, a socket, a loft hatch or the skirting | An air leak, cold air being drawn in | Draught proofing; see draught proofing cost |
| Cold patch colder than plain missing insulation, with a soft edge, near a roof or gutter | Wet insulation or a leak; water conducts heat and cools as it evaporates | Find the leak first, then replace the insulation |
What can a thermal camera not tell you?
Thermal imaging is sold with more confidence than it deserves. Knowing its limits is the difference between a useful survey and a set of pretty pictures.
- It cannot see through anything. It reads the surface it is pointed at. A cold patch tells you heat is leaving faster there, not what is behind the plaster. Whether that is a missing cavity fill, a solid section of wall or a leak is the surveyor's job, with a borescope where it matters.
- It does not give a U-value. That needs heat flux sensors over days or a calculation from known layers. The camera can rank one wall against another; it cannot put a number on either.
- Damp and cold look the same. A damp patch is cold because water conducts heat and evaporation cools the surface. A cold bridge is cold because the structure conducts heat. Both show blue. A moisture meter tells them apart.
- Sun, wind and rain ruin it. A wall that has had sun on it holds that warmth for hours; wind strips heat unevenly; rain cools everything to one temperature. Outside imaging is done on a cold, dull, still day or at night.
What else is in a heat loss survey?
The camera is one part of a longer visit. Our surveyor goes through the house in this order, because each step changes how the images are read.
- Construction. Solid or cavity walls, wall thickness at a reveal, age of the house, any extensions built differently; see cavity wall vs solid wall.
- Loft and floors. Insulation depth and coverage, eaves ventilation, the hatch; suspended or solid floor, void depth, air bricks, any damp or rot.
- Windows, doors and ventilation. Glazing, seals, trickle vents, extractor fans and whether they work, and any signs of condensation and mould.
- Heating. Boiler or heat pump, radiators, controls and flow temperature. A house planning a heat pump needs the fabric measured against the radiator sizes.
- Thermal imaging. In the heating season, inside and where useful outside, targeted at the questions the walk-round raised.
For a whole house project the same visit starts the PAS 2035 assessment our own retrofit assessor produces.
Why is our heat loss survey free?
Paid thermal imaging surveys exist, and for a dispute with a builder or an insurer an independent report has a place. Ours is free because it is how we scope the work: we cannot price a wall we have not measured, so the survey is the first step of every quote. The visit takes an hour or two, the surveyor is one of our own team, and there is no obligation at the end of it.
What do you get after the survey?
A written plan, usually within a few days, that lists the measures for the house in priority order with an indicative cost for each. Priority follows how much heat each fault loses, how cheap it is to fix and what has to come first, so draught proofing and loft top-up sit above solid wall insulation.
- The thermal images taken, annotated with what each one shows.
- Each recommended measure with an indicative price range from our quote book, the VAT rate that applies, and what it does for comfort, bills and the EPC.
- The order of works, and a fixed written quote for any measure you want to go ahead with.
Read where homes lose heat before the visit if you want to follow what the surveyor is looking for. Then book a free heat loss survey; if you want the thermal images, ask for a date between October and April.
Frequently asked questions
How much does a thermal imaging survey cost?
Ours is free as part of the heat loss survey, because it is how we scope and price insulation work. Independent paid surveys exist and suit a dispute or insurance claim; for deciding what to insulate, the free survey gives you the same images plus a costed plan.
When is the best time for a thermal imaging survey?
A cold day between October and April, with the heating on for several hours beforehand and ideally before the sun has reached the walls. The camera needs about a 10°C difference between inside and outside to show a clear pattern.
Can a thermal camera see through walls?
No. It reads the temperature of the surface it is pointed at. A cold patch shows heat is leaving faster there; working out what is behind the plaster is the surveyor's job, sometimes with a borescope.
Can thermal imaging detect damp?
It can show a cold patch that might be damp, because wet materials conduct heat and evaporation cools the surface. It cannot confirm it, because a cold bridge looks the same. A moisture meter on the same spot settles it, and our surveyor carries one.
What happens if I do not go ahead after the free survey?
Nothing. You keep the written plan and the images. Some customers do the cheap measures themselves and come back for the walls later.