What Thermal Imaging Can and Cannot Tell You About Your Building

A thermal camera does not see moisture. It sees surface temperature.

That distinction explains both why thermal imaging is genuinely valuable on building investigations and why some thermal reports are essentially decorative. Understanding the mechanism tells a council what to ask for and what to be sceptical of.

How it actually works

An infrared camera measures radiation coming off a surface and converts it into a temperature image, with colour representing temperature. It does not see through walls. It does not detect water directly. Everything it says about the inside of an assembly is inferred from what the surface temperature is doing.

The inference works because of thermal mass. Wet material holds and releases heat differently from dry material. When the temperature changes, a wet area in a wall or roof warms up or cools down at a different rate than the dry material around it, and that difference shows at the surface as a temperature anomaly.

The camera detects the anomaly. A person interprets it as moisture. Those are two separate steps, and the second one is where the expertise lives.

The condition everything depends on

Because the method relies on differential heating and cooling, there has to be something driving a temperature change.

On an interior scan, that means a meaningful difference between inside and outside temperatures, so heat is flowing through the assembly. On an exterior scan it usually means scanning during a period when surfaces are cooling after being warmed, so the thermal mass difference expresses itself.

Without a temperature drive, there is nothing for the camera to distinguish. A building scanned on a mild overcast day with little difference between inside and outside produces a uniform, uninformative image no matter how much water is sitting in the wall.

This is the single most important thing for a building owner to understand, because it means a scan booked for convenience can be worthless while looking exactly like a scan that worked. The images will still be colourful. They will still look technical. They just will not mean anything.

A competent operator will schedule around conditions and will decline to scan when conditions do not support it. That willingness is a good indicator of whether you have the right person.

What it genuinely finds

Wet insulation, particularly in roofs. This is the strongest application. A flat roof scanned in the evening after a sunny day shows wet areas retaining heat longer than dry ones, and the pattern maps the extent of saturation across a large area quickly. For a strata deciding between a roof recover and a full tear-off, knowing how much of the insulation is wet changes the answer, as discussed in the guide on roof recover versus tear-off.

Air leakage paths. Air moving through a gap changes the temperature of surrounding surfaces in a distinctive pattern. Combined with a blower door test that pressurizes or depressurizes the building, the leakage paths become clearly visible because the airflow is amplified and consistent. This pairing is the most reliable use of the technology, and it directly addresses the air barrier continuity problem described in the post on air barrier versus vapour barrier.

Missing or displaced insulation. Gaps in insulation show as cold or warm areas depending on scan direction, and the pattern is usually geometric, following stud bays or panel edges, which distinguishes it from moisture.

Thermal bridges. Structural elements carrying heat through the assembly show clearly, which helps explain condensation appearing at specific locations. The mechanism is covered in the guide on thermal bridging and cold spots.

The extent of a known problem. Where a leak has been confirmed, thermal imaging maps how far the wet area extends, which is directly useful for scoping a repair.

What it cannot do

It cannot tell you where water is entering. It shows where water has ended up. Because water travels along assemblies before it appears, often a considerable distance, the wet area is frequently well away from the actual breach. This is the same problem described in the guide on why leaks show far from the source. Thermal imaging narrows the search area effectively, and then other methods find the entry point.

It cannot prove moisture. A temperature anomaly has several possible causes: moisture, missing insulation, air leakage, a thermal bridge, a pipe or duct behind the surface, a different material, or a surface with different emissivity. Some of these are distinguishable by pattern and context, and some are not. Confirmation requires a moisture meter reading or a test opening.

It cannot see through materials. It reads the surface. A cavity behind a well-insulated cladding panel may not communicate its condition to the outer surface at all.

It struggles with solar loading. On an exterior scan, sunlight warms surfaces unevenly depending on orientation, colour, and material, and that variation can mask real anomalies or create apparent ones. Scanning outside solar hours and understanding the facade layout are both necessary.

Telling a good report from a bad one

The camera produces a striking image regardless of whether the scan was valid. That makes the report the place to judge the work.

A useful report states the conditions: interior and exterior temperatures during the scan, recent weather including whether it had rained, time of day, and wind. Without these, the reader cannot judge whether conditions supported a meaningful scan.

It states the equipment and its resolution, and the distance and angle of the scans.

It locates each anomaly precisely enough to find again, with reference images alongside the thermal ones, because knowing what the surface actually looks like is essential to interpretation.

It interprets each anomaly with reasoning, explaining why the pattern suggests moisture rather than an insulation gap, or vice versa.

It states how findings were confirmed. Moisture meter readings, test openings, or correlation with known leak reports. Anomalies that were not confirmed should be identified as unconfirmed.

And it separates what is verified from what is hypothesis. A report that presents every anomaly as a confirmed leak is overselling what the method can deliver.

If a report is a folder of colourful images with brief captions and no conditions data, it has not produced findings. It has produced pictures.

Where it fits in an investigation

Thermal imaging works best as a screening step, not as the whole investigation.

The sequence that produces results usually starts with the reported symptoms and their timing, since a leak that appears only during wind-driven rain behaves differently from one that is always damp, as covered in the guide on storm leaks versus chronic leaks.

Thermal imaging then covers a large area quickly and identifies where the anomalies are, which is exactly what a large building needs, because opening the assembly everywhere is not feasible.

Moisture meter readings at the anomalies confirm which ones actually involve water.

Targeted investigation at the confirmed locations, whether test openings, water testing, or electronic leak detection, establishes the entry point. The available methods are set out in the guide on moisture testing methods.

The value of thermal imaging in that sequence is that it makes the invasive step targeted. It does not remove the invasive step. Buildings that try to use it as a complete diagnosis, avoiding all other testing, end up with a repair based on an inference, which is how money gets spent on the wrong thing.

The honest cost assessment

Thermal imaging earns its cost when it narrows a search that would otherwise be expensive. On a large facade or a big flat roof where the problem could be almost anywhere, it converts an unbounded investigation into a targeted one. On a roof, it can map the extent of wet insulation across the whole area in a way no other method matches for speed.

It earns less on a small building with a single obvious leak, where opening the assembly at the likely location costs less than commissioning a scan.

The mistake that wastes money is treating it as a magic step that avoids opening anything. It reduces how much you have to open, which is genuinely valuable, and that is a different claim from what some reports imply.

Ask about conditions before booking. Ask what confirmation method will be used. Ask for a report that states the temperature differential during the scan. Those three questions separate a survey that will tell you something from one that will produce a nice-looking folder.

Sources and further reading

• ASTM C1153, Standard Practice for Location of Wet Insulation in Roofing Systems Using Infrared Imaging, referenced 2026
• ASTM E1186, Standard Practices for Air Leakage Site Detection in Building Envelopes, referenced 2026
• ISO 6781, Thermal insulation: qualitative detection of thermal irregularities in building envelopes, infrared method, referenced 2026
• Canada Mortgage and Housing Corporation, guidance on building envelope investigation methods, referenced 2026
• Allweathercoating investigation records, Metro Vancouver, 2026