Pillar Guide · Thermography

Thermal Imaging

A thermal camera measures surface temperature, not damp, heat loss or air leakage directly. Understanding what it can and cannot see is the difference between evidence and guesswork.

8 in-depth articles·By George Sora · Certified Passive House Designer

Infrared thermography is one of the most powerful — and most misunderstood — diagnostic tools in building performance. A calibrated camera maps the surface temperature of walls, ceilings and floors. From those temperature patterns, a trained surveyor infers heat loss, missing or slumped insulation, thermal bridges, air leakage paths and surfaces at risk of condensation. The skill is in the interpretation, not the colour picture.

But a thermal camera does not 'see heat loss', 'see damp' or 'see through walls'. It measures the infrared radiation leaving a surface and converts it to a temperature, subject to emissivity, reflected temperature and the conditions on the day. Understanding what it genuinely reveals — and the rigour required to make it reliable — is what separates evidence from a folder of pretty, meaningless images.

What this guide covers

Work through the cluster for the full picture: how thermography actually works (emissivity, reflected temperature, resolution), the temperature differential and conditions a valid survey needs, and the four main applications — insulation defects, air leakage (with the blower door), condensation and damp, and retrofit verification — plus the common mistakes that make a survey worthless and how to spot a bad one.

Featured Retrofit IQ Investigation

Loft & Kitchen Insulation / Draught-Proofing — Diagnostic Survey

Francis · Luton · 15 Dec 2025

“George carried out a blower door test, smoke testing and thermal imaging survey at our property, and we were extremely impressed with both his professionalism and his depth of knowledge. He took the time to explain everything clearly, usi…”

Read the full investigation All 3 thermal imaging investigations

From reading the building science to a measured investigation on a real London building.

In this guide

Read the Thermal Imaging cluster

How thermal imaging works — emissivity, reflected temperature and resolution.

How a thermal camera measures surface temperature, the role of emissivity and reflected apparent temperature, thermal vs spatial resolution, and the difference between a qualitative and a quantitative survey.

29 Jun 202611 min

The temperature differential — why a valid thermal survey needs a cold day.

Why thermography needs a sufficient inside-to-outside ΔT, how much is enough, the role of conditioning, weather, solar loading and timing, and the standards behind a valid survey.

30 Jun 202611 min

Thermal imaging — what it can (and can't) see.

Used properly, a FLIR survey is the single highest-value diagnostic tool we own. Used badly, it produces beautiful images that mean nothing.

2 Mar 20269 min

Thermal imaging for insulation defects — missing, slumped and bridged.

How thermal imaging reveals missing, slumped and gapped insulation, cavity-fill defects and cold bridging — reading the signatures, and why internal and external surveys complement each other.

1 Jul 202611 min

Thermal imaging for air leakage — finding draughts under depressurisation.

How thermal imaging detects air leakage when combined with a blower door: the cold 'fingers' of incoming air, how they differ from insulation defects, and why the two techniques belong together.

2 Jul 202610 min

Thermal imaging for condensation and damp — cold surfaces and dew point.

How thermography locates surfaces below the dew point, the evaporative-cooling signature of damp, distinguishing it from missing insulation, and why it's combined with humidity logging and moisture meters.

3 Jul 202611 min

Thermal imaging for retrofit verification — proving the work was done right.

Before-and-after surveys, checking insulation continuity and install quality, confirming thermal bridges are designed out, and providing the as-built evidence that closes the performance gap.

4 Jul 202611 min

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

Solar loading, insufficient ΔT, wrong emissivity, reflective surfaces, auto-scaled palettes, thermal-mass lag and over-claiming — plus how to tell a credible thermal survey from a worthless one.

5 Jul 202611 min

Also being written

Expanding this guide

We publish carefully — these topics are in production rather than thin placeholder pages.

Thermography qualifications and standards (Level 1/2, BS EN 13187)
Drone and external-elevation thermal surveys
Thermal imaging for flat roofs and moisture mapping
Quantitative thermography and fRsi measurement
Thermal imaging investigations — real defects found