Thermal images look impressive and persuasive – at first glance. In fact, they are a precise tool, but only under very specific boundary conditions. This article explains when building thermography delivers reliable statements, which findings are typical and where providers with "impressive images" often overpromise.
What thermography measures – and what it does not
A thermal imaging camera does not measure temperature, but infrared radiation. From the measured radiation, it calculates a temperature under assumptions about emissivity and reflection. This makes four things visible on a building envelope:
- Thermal bridges – points at which the envelope systematically loses more heat (e.g. reinforced concrete ceilings in the external wall, roller shutter boxes).
- Air leakage – convection through joints, visible as "streaks" of warmer air.
- Damp – changes the thermal behaviour and reveals damp components during the warming or cooling phase.
- Defects in plant technology – heating pipes, underfloor heating, electrical sockets.
What thermography cannot do: calculate U-values (a heat flux measurement does that), produce an overall energy balance (an energy consultant does that under DIN V 18599), or reveal hidden damage inside components without surface indicators.
The mandatory boundary conditions
An external thermography under DIN EN 13187 requires:
- Indoor/outdoor temperature difference at least 15 kelvin, ideally 20 K. In practice: only during the heating period (in NRW typically November to March).
- Outdoor temperature < 5 degrees C, because at higher temperatures the sensor resolution becomes too weak relative to the gradient.
- Dry facades – precipitation distorts the measurement for 12–24 hours.
- Overcast sky or images before sunrise – direct sunlight heats surfaces unevenly and makes images unusable.
- Constant heating for at least 24 hours beforehand, with all internal doors open.
- Wind speed < 4 m/s (otherwise forced convection distorts surface temperatures).
Anyone offering external thermography in April disregards the standard. Images at this time of year are not technically defensible.
Typical findings on existing buildings in NRW
In my imaging work the following finding clusters appear regularly:
Thermal bridges through floor slabs
Reinforced concrete ceilings that pass without thermal insulation into the external wall show as horizontal warmer stripes on the facade. Consequence: mould risk in the internal corner of the ceiling, because the coldest internal surface is there.
Uninsulated roller shutter boxes
Common in 1960s–1980s buildings: masonry roller shutter boxes with minimal insulation. Heat loss per box can reach 5–8 W/K, in winter visible as the warmest point on the facade.
Window connection joints
Often on older windows: convection between the window frame and masonry, visible as a "feather stroke" of cold internal surfaces at the reveal junction.
Radiator niches with reduced wall thickness
In prefab housing and 1960s–1970s construction it is common: the external wall was cut out behind the radiator to set it flush. Remaining wall thickness sometimes only 12 cm – dramatic heat loss.
Damp in plinth areas (internal image)
Internal thermography at cooler outdoor temperatures reveals damp masonry: damp components have higher heat capacity, cool more slowly and appear warmer than dry neighbouring areas – or cooler, depending on the warming/cooling phase.
Three common misinterpretations
- "Bright spot = warm spot = heat loss" – in false-colour mode that is true, in true colour it is not necessarily so. Without a scale, an image is worthless.
- "Cold spot indoors = mould" – mould risk depends on surface temperature and indoor humidity. An image alone is not sufficient; the hygrometer reading belongs with it.
- "An image in spring shows the same as in winter" – no. The temperature difference is too small; the image can conceal positive findings.
Costs 2026
Market-standard fees for building thermography in NRW (as of May 2026, ranges):
- External thermography single/semi-detached (all four facades, report): 350 – 600 EUR gross
- External thermography MFH up to 6 units: 600 – 1,200 EUR gross
- Internal thermography single-family (focus on thermal bridges, mould suspicion): 400 – 700 EUR gross
- Leak detection heating/drinking water with thermography: 250 – 500 EUR gross
- Combination with blower-door test (differential pressure measurement): additional 400 – 700 EUR
In my private-client engagements I often combine thermography with the extended building condition analysis – camera images are then part of the substance report, not a separate service.
When thermography is economically worthwhile
The investment pays off particularly in four constellations:
- Before a house purchase – to make silent heat losses visible that do not appear on the heating bill.
- Before refurbishment planning – as a supplement to the iSFP to identify priority components.
- With suspected mould – to localise thermal bridges before mould forms.
- With suspected leakage – heating pipes, underfloor heating, drinking water.
Pure "curiosity thermography" of one's own home without a concrete refurbishment decision rarely pays off – the energy performance certificate is then sufficient as an orientation.
Bottom line
Thermography is a high-quality but condition-sensitive tool. A technically correct image in winter, with report and interpretation, is worth 350–600 EUR and can underpin five- to six-figure refurbishment decisions. An image in spring, on the other hand, is a pretty picture – nothing more. Used correctly, thermography is one building block of the building survey – and it is precisely this survey with sound groundwork assessment that is essential to estimate costs and risks sensibly.
Related articles:
- Buying a House in NRW: The 12 Hidden Defect Risks
- Roof Drone Inspection: Process, Law, Costs
- iSFP 2026: Refurbishment Roadmap, BAFA Subsidy and Bonus
Next step: Overview of technology and diagnostics or directly request a thermography appointment (season November to March).
Next step
Would you like this topic assessed for your property in concrete terms? I offer a free initial consultation — fee ranges, scope and available subsidies discussed openly.