A thermographic inspection of a building is not just a matter of when a technician and a drone are available - it depends fundamentally on when you carry it out. Poorly chosen conditions can turn the images into worthless pictures from which no conclusions can be drawn. Conversely, an inspection carried out at the right time and in suitable weather will reveal heat loss, insulation faults or water ingress with a precision that allows concrete decisions about repairs. This guide explains what determines the quality of a thermographic measurement and how to choose the ideal date.
Why the heating season is condition number one
A thermal camera measures surface temperature. For the image to show where heat is escaping, there must be a sufficient temperature difference between the inside and outside of the building - technically known as the temperature gradient. Without this difference, there is nothing to measure.
In practice this means one thing: the building must be heated, and it must be cold outside. Façade inspections therefore take place during what is known as the heating season, roughly from October to the end of March or early April. In these months it is common to achieve a gradient of 10-15°C or more, which is technically the minimum sufficient for reliable results.
What happens if the gradient is too small?
If it is 18°C outside and 21°C inside, a thermal camera will only capture the difference with difficulty. Areas with faulty insulation will indeed be slightly warmer, but the contrast will be so small that it is masked by the sensor’s natural noise. The result is ambiguous images from which no reliable conclusions can be drawn.
Recommended temperature gradient:
- Minimum: a 10°C difference between interior and exterior
- Ideal: 15°C or more
- Optimal (reveals even minor defects): 20°C or more
Summer is unsuitable for façade thermography
June, July, August - in these months a façade inspection is practically pointless. Buildings are not heated, and the temperature gradient is missing. What’s more, the sun heats different parts of the façade unevenly throughout the day, creating thermal artefacts in the images that have nothing to do with insulation faults.
Thermal imaging in summer makes no sense for:
- checking the insulation of external walls,
- detecting thermal bridges in window frames,
- assessing the quality of an ETICS façade system (an external thermal insulation composite system),
- looking for gaps in the building envelope.
Flat roofs are the exception - they have their own logic, covered below.
The role of weather: sun, rain and wind as disruptive factors
Even during the heating season, unsuitable weather can ruin the results. The three main factors are solar radiation, precipitation and wind.
Sun: the main enemy of accurate façade thermography
Direct sunlight heats the façade surface, causing areas exposed to the sun to show a higher surface temperature regardless of the actual state of the insulation. This effect is called solar loading, and it can effectively mask even significant heat loss.
How to avoid it:
- Measure in overcast weather, when the façade is not exposed to direct radiation.
- If it is clear, plan the measurement early in the morning or after dark, once the façade has had time to equalise with the ambient temperature.
- After a sunny day, wait at least 2-4 hours (sometimes longer) before starting the measurement.
Rain and moisture: distortion through evaporative cooling
A wet façade shows a lower surface temperature than its actual thermal condition would suggest - the water evaporates and cools the surface. This creates false cold zones and masks genuine thermal bridges.
General rule: after rainy weather, wait at least 24-48 hours until the façade has fully dried.
Wind: cools the surface and reduces the contrast in the image
Strong wind cools the façade surface evenly and reduces the contrast between well-insulated and poorly insulated areas. It also complicates safe drone flight. For drone-based thermal measurement, wind of up to roughly 5-7 m/s is suitable - about force three on the Beaufort scale.
Daytime versus night-time measurement
Thermal imaging is most often carried out in the evening or at night. The reasons are practical.
Advantages of night-time and evening measurement:
- The façade is no longer being heated by the sun.
- Temperatures are more stable - night air is more even than daytime air.
- The images do not suffer from solar reflections or patchy shadows.
Advantages of early-morning measurement before sunrise:
- The façade has equalised with the outside temperature overnight.
- The interior/exterior temperature difference tends to be greater in the morning, due to the overnight drop in outside temperature.
- Practical for winter months, when it gets light late.
Daytime measurement in clear, sunny weather is not recommended for façade inspections, for the reasons described above. The exception is when it is heavily overcast all day and the façade is not exposed to direct radiation.
A special case: detecting water ingress on flat roofs
Flat roofs work differently. Here we actually make use of the evaporation effect: after rain, water that has got in through a fault in the waterproofing collects beneath the surface or within the thermal insulation. This moisture is warmed by the sun during the day and evaporates more slowly at night than the surrounding dry areas - and that is exactly what the thermal camera shows.
For detecting water ingress on a flat roof, the following applies:
- Ideal time: clear nights after rain, or after a sunny day.
- The drone measures the roof in the evening or at night.
- Damp areas are warmer than their dry surroundings - the exact opposite of what you see on a façade.
This method works year-round and does not depend on the heating season.
Overview of conditions for thermal measurement
| Parameter | Ideal condition | Unsuitable condition |
|---|---|---|
| Temperature gradient | 15°C or more | Below 10°C |
| Time of year | October - April | June - August |
| Solar radiation | No direct radiation on the façade | Direct sunlight |
| Precipitation | At least 24-48 h without rain | Wet façade |
| Wind | Up to 5-7 m/s | Above 8 m/s |
| Time of day | Evening, night, early morning | Clear, sunny day |
How to plan the date of an inspection in practice
You do not need to watch the weather forecast yourself and adjust the date days in advance. When planning a thermographic inspection, we recommend the following approach:
- Book a date within the heating season, roughly October to April.
- We finalise the exact date according to the current forecast, looking for a suitable window of overcast, dry weather.
- The building should be heated normally to its usual operating temperature for at least 24 hours before the measurement.
- After a sunny day, we move the measurement to late evening or the following morning.
The good news: in the winter months, suitable conditions are relatively common. The overcast, mildly windy weather that people in the Czech Republic often complain about is, from a thermography point of view, exactly what we need.
The right date determines the value of the inspection
A thermographic inspection carried out under poor conditions can be a waste of money and time. A properly timed inspection, by contrast, delivers images from which problem areas can be clearly identified, documented in a report, and used to support a decision on repairs or a claim against the insulation contractor.
If you are not sure whether your planned date is suitable, or you want to discuss your building’s specific situation, get in touch with us. We will help you choose the best date, assess the current weather forecast, and reply within 24 hours. Just fill in the enquiry form - we carry out thermographic inspections all year round and are happy to advise, with no obligation.