What is thermal imaging? A guide for homes
Author: Domeny.ForSale editorial team Published
In brief: Thermal imaging examines surface temperature patterns using a camera that detects infrared radiation. In a home, it helps identify areas that may indicate heat loss, gaps in insulation or air leaks. However, it does not directly reveal the inside of walls or measure total energy losses on its own. The image must be assessed in light of the building’s construction and the survey conditions.
How does thermal imaging of a building work?
Surfaces emit infrared radiation, which a thermal imaging camera uses to estimate their temperature. It displays the result as a thermogram: an image in which colours represent different temperature ranges. The camera cannot see through a wall; it shows the temperature of the surface exposed to it.
In a heated home, temperature differences can reveal uneven heat flow. If part of a wall or other building element conducts heat more readily than the surrounding area, its inner surface may be cooler and its outer surface warmer. Such an area may be a thermal bridge: a place where heat flows more readily because of the construction or the materials used.
Temperature readings also depend on the properties of the surface. Emissivity—the ability of a material to emit thermal radiation—is important. Glass and shiny metal can strongly reflect radiation from their surroundings, making their images easy to misinterpret.
Where can thermal imaging reveal heat loss?
Thermal imaging is particularly useful for comparing similar parts of a building. It can identify areas that need closer inspection, but it does not always establish the cause of a temperature difference.
A home survey most commonly checks:
- Windows and doors — around frames, reveals and thresholds, where air leaks or gaps in insulation may occur.
- External walls — localised gaps in insulation, junctions between different materials and attachment points for structural elements.
- The loft and roof — gaps in insulation and junctions between roof slopes and walls.
- Balconies, ring beams and lintels — elements that may form thermal bridges.
- Junctions between walls and floors or ceilings — areas where insulation may be discontinuous.
For example, a cold strip beside a closed window may result from air entering, poor insulation at the junction between the frame and wall, or the construction of the window reveal. A thermogram alone is not enough to justify replacing the window. First, check the seals, the adjustment of the window hardware and how the window was installed.
When does a survey give reliable results?
Finding heat loss requires a clear, sustained temperature difference between indoors and outdoors. Surveys are therefore usually carried out during the heating season, with steady heating. The surveyor should specify the required temperature difference according to the survey method and the types of building elements being examined; around 10°C or more is often used.
Sunlight can warm the exterior walls and mask the true pattern of heat flow. Strong wind cools surfaces, while rain and moisture alter their temperature. An external survey is therefore often scheduled before sunrise or after enough time has passed since exposure to sunlight, in dry weather with little wind.
Before the visit, avoid sudden changes to the heating temperature or extensive airing of rooms. Discuss moving furniture and curtains away from the walls to be examined, as they obstruct inspection and affect local temperatures. Also tell the surveyor about recent rain, renovation work, dampness and heating problems.
How should you read the colours, and what cannot be inferred from an image?
Red does not automatically mean a defect, and blue does not mean good insulation. The colours depend on the chosen palette and temperature scale. Two images of the same wall can look completely different after the display settings are changed.
In winter, a warmer patch on an exterior wall may suggest increased heat flow from indoors. However, it may also result from sunlight, reflected radiation or a nearby heat source. On the inside, a patch that is cooler than similar surfaces may instead be cause for concern. Interpretation requires comparing temperatures and taking the building’s geometry and measurement conditions into account.
Thermal imaging alone cannot reliably translate a coloured patch into a specific amount of money lost on an energy bill. Nor does it conclusively confirm the presence of moisture or mould. It can identify a place for further investigation with a moisture meter, a visual inspection or another method.
How do you arrange a survey and use the results?
Before booking, define your goal: finding the cause of a cold room, checking insulation after renovation, or assessing areas that need repair. Ask whether the survey includes both internal and external inspections and what weather conditions are required. If air leaks are suspected, a building airtightness test may help. This uses a fan to create a pressure difference, making airflow easier to detect.
Ask for a report containing thermograms, corresponding conventional photographs, the locations of problems, measurement conditions and an interpretation. The report should distinguish an observed temperature difference from its suspected cause and identify what needs further verification.
Begin repairs by addressing a confirmed cause, not the most striking patch in the image. A cold window reveal may require improvements to the installation or insulation rather than new windows. A follow-up survey after the work will help assess whether the identified problem has been resolved.
See also: What is RAID?
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