Radiometric vs non-radiometric (it matters)
A radiometric thermal image stores the actual temperature at every pixel. You can pull a spot reading of 41.2 °C from a hot solar cell after the fact. A non-radiometric image just shows relative temperature — hotter = brighter — with no absolute values.
For any compliance work (solar PV, insurance claims, building regs) you need radiometric. For a quick visual scan, non-radiometric is cheaper and often enough.
Our FLIR / Zenmuse H20T payloads are radiometric. Consumer thermal drones (the original Mavic 2 Enterprise Dual, for example) are non-radiometric and should not be used for compliance documentation.
Four things thermal drones detect well
Roof moisture ingress
Wet insulation holds heat after sundown longer than dry insulation. An evening thermal flyover (two hours after sunset) lights up wet patches as bright anomalies. This is the most reliable commercial-building use case and pairs naturally with drone roof inspection. Accuracy on a flat commercial roof: typically 90%+ true-positive on moisture anomalies >0.5 m².
Solar PV cell and string faults
Under load on a clear sunny day, failed cells or bypass diodes run 20 to 40 °C hotter than healthy neighbours. A thermal survey spots module faults, string disconnects, and hot-spot precursors before they propagate. IEC TS 62446-3 is the governing standard — specifies flight conditions (minimum 600 W/m² irradiance, clear sky), altitude, scan pattern, and reporting format. See our solar PV thermal inspection scope template for a compliant tender brief.
Cold bridging on building envelopes
Winter mornings after a cold night reveal thermal bridges at wall-floor junctions, balcony penetrations, and poorly-installed insulation. The heat loss pattern is diagnostic for retrofit scoping. Useful for BER improvement works and SEAI grant applications.
Industrial electrical and mechanical anomalies
Overheating bearings on rooftop plant, faulty electrical connections at pole-top transformers, anomalies in refrigeration plant condensers — a radiometric drone captures them from a safe stand-off distance.
Six things thermal drones cannot detect (despite claims)
Anything behind metal cladding
Metal reflects thermal radiation almost completely. The drone sees the cladding surface temperature, not what is behind it. Rain-screen aluminium, corrugated metal roofs, and metal cassette cladding all block the signal.
Leaks on dry days
The moisture-detection trick only works when the roof has recently been wet and is drying. Fly a roof after two weeks of dry weather and it will look uniformly healthy even if the insulation is soaked below a recent patch repair.
Structural cracks
Cracks do not have a thermal signature. A drone capturing a cracked concrete slab will image the crack only in visible wavelengths. For structural inspection you need high-resolution visible imagery, not thermal.
Very small defects
Ground sample distance on a thermal camera is ~4x worse than visible at the same altitude. A hairline crack is invisible. The smallest reliably-detected defect at 20 m standoff is typically 5 cm.
Under-floor heating performance
From above, a drone cannot see underfloor heating anomalies inside a building. That is a handheld interior thermal job.
Buried utilities or voids
Thermal imaging reads the ground surface, not what is below. GPR (ground-penetrating radar) is the right tool for subsurface.
When to commission
Thermal drone flights should be scheduled for specific conditions:
- Roof moisture: 1 to 3 hours after sunset on a clear night, after a recent wet period.
- Solar PV: midday, clear sky, minimum 600 W/m² irradiance, arrays under load.
- Building envelope heat loss: winter, after a cold night, ΔT of 10 °C or more between interior and exterior.
- Plant and electrical: under load, typically daytime operating hours.
Commissioning thermal work for the wrong time of day produces data that cannot be interpreted. Always include flight-condition specifications in the tender — or see our roof condition survey scope template for a ready-made brief.
Quick check: if a thermal inspection report does not list weather conditions, irradiance, and time of capture, its conclusions are not defensible. Still weighing up capture methods? See when a drone survey is the right choice.