A thermal image of a building is a picture of heat flowing through its envelope. Weather sets that flow: a large inside-outside temperature difference makes defects visible, sun on a wall stores heat that masks them, wind strips heat from the outer surface and flattens contrasts, and rain cools everything by evaporation. The best surveys are done before dawn in the heating season on a still, dry morning, when heat has been flowing steadily outward all night and nothing else is disturbing the surface.
The inside-outside difference
Insulation defects, thermal bridges and air leaks show because heat moves through them faster than through their surroundings. The rate is proportional to the temperature difference across the wall. Below about 10 °C difference, the surface temperature variations shrink toward the camera’s sensitivity limit and the image goes flat. Surveys from inside a heated house in winter, or inside an air-conditioned house on a hot day, give the difference; a mild spring afternoon gives nothing. See what NETD is.
Sun
Solar radiation heats external surfaces by hundreds of watts per square meter, far more than the tens of watts flowing through the wall from inside. Dark surfaces, south and west faces and anything in direct sun show patterns that are about sunlight and paint color, not construction. The stored heat persists for hours after the sun moves off. External surveys need overcast conditions or the pre-dawn hours; internal surveys are affected less but sunlit internal walls and floors near windows still mislead.
Wind
Wind increases the heat transfer from the outer surface to the air, cooling the whole surface toward air temperature and reducing the contrast between well-insulated and poorly insulated areas. It also drives air leakage, which can help an internal survey (cold streaks at leaks become stronger) and hinder an external one. Light wind, under about 3 m/s, is the usual limit for external work.
Rain and moisture
Wet external surfaces evaporate and cool uniformly, hiding differences. They also change emissivity. Allow a day of dry weather before an external survey. Indoors, high humidity slows evaporation from damp areas and weakens the moisture signature; see can thermal imaging find water leaks.
Clear skies and radiative cooling
On a clear night, surfaces facing the sky radiate heat to space and cool below air temperature. Roofs and upward-facing surfaces can show frost patterns and apparent cold spots that reflect sky exposure rather than insulation. Overcast nights are more uniform.
Conditions summary
| Survey | Best conditions | Avoid |
|---|---|---|
| Insulation, internal | Heating on for hours; ≥10 °C difference; pre-dawn or overcast | Sunlit walls; recently opened windows |
| Insulation, external | Pre-dawn, overcast, still, dry; ≥10 °C difference | Sun in the last few hours; wind; rain |
| Air leakage | Cold outside, house depressurized (extract fans or blower door) | Warm still days |
| Moisture | Dry indoor air; after a temperature change | Humid rooms; surfaces in sun |
| Flat roofs for wet insulation | Evening after a sunny day, as wet areas retain heat | Overnight rain |
For what a good image looks like once conditions are right, see what makes a good thermal image. Cameras are compared in best thermal cameras for home inspection.
Recommended Tools
Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.
Frequently Asked Questions
What is the best time to do a thermal survey of a house?
Early morning in the heating season, before sunrise, after the heating has run overnight, with little wind and no rain in the previous day.
Can I survey in summer?
From inside with air conditioning running and a hot day outside, yes; the heat flow reverses and defects show as warm patches. Without a temperature difference of about 10 °C or more, little shows.
Why does sunlight ruin the image?
Sun heats the outer surface unevenly by color and orientation, and that stored heat masks the small differences from insulation and air leaks for hours afterward.
Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.
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