Common Thermal Imaging Mistakes & How to Spot a Bad Survey

Common thermal imaging mistakes — and how to spot a bad survey.

Thermal imaging is easy to do and hard to do well. A modern camera produces confident, vivid images whether or not the survey is valid — which is precisely the danger. This article sets out the mistakes that make a thermal survey worthless, so you can recognise good practice and avoid paying for beautiful pictures that mean nothing.

Most bad thermal surveys aren't bad because of the camera — they're bad because of the conditions, the settings or the interpretation. The physics that makes thermography powerful also makes it easy to fool: get the temperature difference, the emissivity, the solar exposure or the scale wrong and you'll still get a striking image, just not a meaningful one. Here are the errors that matter, and how to spot them.

The conditions mistakes

Insufficient temperature differential — surveying with too little inside-to-outside difference, so defects never develop a thermal pattern. The commonest fatal error (see the ΔT article).
Solar loading — surveying a sunlit building, where stored solar heat re-radiates for hours and completely masks or mimics defects. External surveys should be before dawn or after dark, on overcast days.
Surveying a cold or just-heated building — not allowing time for the temperature patterns to establish through the fabric, especially in heavyweight masonry with thermal lag.
Wind and surface wetness — convective cooling and evaporation flatten and distort the patterns; recent rain or strong wind degrades the survey.

The settings and surface mistakes

Settings and surface errors

Mistake	Effect	Correct practice
Wrong emissivity setting	Measured temperatures shifted/false	Set ε for the actual surface (~0.9–0.95 typical)
Ignoring reflected temperature	Reflections read as real temperatures	Set reflected apparent temperature
Imaging shiny/metal/foil surfaces	Camera reads reflections, not the surface	Avoid, or interpret with extreme care
Auto-ranging palette not fixed	A 1 °C spread looks as dramatic as 20 °C	Set and state the temperature scale
Steep viewing angles on glass	Glass becomes reflective	Image as near-perpendicular as possible

The interpretation mistakes

Confusing the three cold things — calling every cold patch 'missing insulation' when it could be air leakage or damp. Without a blower door and moisture measurement, the attribution is a guess.
Mistaking normal construction for defects — reading the regular cool stripes of studs/joists (expected repeating thermal bridges) as faults.
Reading the palette literally — treating the false-colour 'hot/cold' as absolute when the scale is auto-ranged and the differences may be tiny.
Over-claiming — asserting 'damp', 'no insulation' or 'major heat loss' from a single image, with no measured temperatures, no conditions recorded and no corroborating method.

How to tell a good survey from a bad one

You don't need to be a thermographer to judge a survey. Look for these markers of competence:

The conditions are recorded — ΔT, weather, time, internal/external, emissivity and reflected temperature.
It was done in the right conditions — heating season, adequate ΔT, no solar loading; the surveyor explains why.
The temperature scale is fixed and shown on the images, not left auto-ranging.
Cold areas are attributed carefully — distinguishing insulation, air leakage and moisture, ideally corroborated by a blower door and moisture measurement.
Claims are proportionate and explained, not dramatic assertions from a single picture.
The surveyor holds a recognised thermography qualification (e.g. a Level 1/2 category certification) and understands building physics — not just camera operation.

Common questions from clients.

Someone offered to thermal-image my house on a sunny afternoon — is that OK?

No — that's one of the classic mistakes. On a warm, sunny day there's usually too little inside-to-outside temperature difference, and solar heat stored in the fabric corrupts the images, so the results aren't reliable however vivid they look. UK heat-loss thermography needs the heating season, an adequate ΔT, and no solar loading (overcast, or before dawn / after dark for external work).

How can I tell if a thermal survey report is any good?

Check that it records the conditions (internal/external temperatures and ΔT, weather, time, emissivity), that it was done in valid conditions, that the temperature scale is fixed and shown, that cold areas are attributed carefully (insulation vs air leakage vs damp, ideally corroborated), and that the claims are proportionate. A folder of dramatic auto-scaled images with no conditions and sweeping conclusions is a red flag.

Does the surveyor need a qualification?

For credible, defensible results, yes — building thermography requires both a recognised thermography certification (so the operator understands emissivity, reflected temperature, conditions and the camera) and building-physics knowledge (so they interpret patterns correctly and don't over-claim). Operating a camera is easy; producing a valid, correctly-interpreted survey is a skilled discipline.