The short answer
The exterior envelope and roof of a hall are best surveyed in the heating season – roughly late October to March – at night, in calm air, on dry surfaces. The difference between indoor and outdoor temperature should be at least 10 K, ideally 15 K or more. These are not details: when they are not met, a thermal image can mislead, showing a defect that is not there or hiding one that is.
So we do not survey on a calendar date but on conditions: the night is chosen from the forecast, and if the weather does not deliver, we reschedule at no charge (up to 2 times). This article explains why these conditions matter and what they mean in a hall.
Temperature difference: how much is enough?
Heat flow through a structure is proportional to the temperature difference (delta-T). At a small delta-T, the difference between a defective and a sound surface shrinks to tenths of a kelvin and disappears in camera noise and in the differing emissivity of surfaces. At a larger delta-T the same defect becomes a spot several degrees warmer (or colder): easier to see, and more reliable to rank.
What is delta-T on an average heating night? In the calculations for our example facilities, the outdoor temperature on days below 12 °C averages 4.4 °C (Nyíregyháza, ERA5, 11 seasons). The resulting average delta-T:
- Warehouse, 16 °C
- 11.6 K
- Workshop, 18 °C
- 13.6 K
- Supermarket, 20 °C
- 15.6 K
A warehouse heated to 16 °C therefore only just clears the 10 K threshold on an average winter day; a reliable image needs a colder-than-average night. A building kept warmer (a workshop, a shop) offers more suitable nights. That is why the goal is not “any January night” but the night that meets the conditions.
Illustrative example – not real measurement data.
Sunshine: the wall remembers
In sunshine, facades and roofs warm up and the stored heat lasts for hours, especially in dark cladding, reinforced-concrete elements and roof membranes. This residual solar heat draws a second pattern over the heat-loss pattern in the thermal image. The difference between a sunlit and a shaded wall can exceed the difference at a gap or a thermal bridge.
The practical rule: we survey the exterior no earlier than about four hours after sunset, or before dawn. A long winter night leaves plenty of window; in the shoulder months it is shorter.
- Heating season: exterior facade and roof survey
- Start of season (late October): only on a cold enough night
- Summer: flat roofs, PV, electrical cabinets under load, cold rooms
Wind, precipitation, heating operation
Besides solar load and delta-T, we check three more factors before and during the survey:
| Condition | Requirement | Why it matters |
|---|---|---|
| Indoor–outdoor delta-T | at least 10 K, ideally 15 K or more | At a small difference the signal of a defect is lost in noise. |
| Sun | at least 4 h after sunset, or before dawn | Stored solar heat masks the heat-loss pattern. |
| Wind | below 5 m/s | Strong wind cools the surface and disperses the warm air leaving a gap. |
| Precipitation | no rain, dry surfaces | Evaporation cools the surface and moisture distorts the image. |
| Heating | stable operation for at least 24 h | In a transient state we do not see the real heat flow of the structure. |
Heating operation is a common question in halls. If you use night or weekend set-back, tell us when you request the quote: before the survey we agree how to keep the indoor temperature constant for at least a day. On the inside we also ask for the doors to be closed and for racking not to cover the walls.
What can be surveyed outside the heating season?
The “winter” rules apply to the exterior heat-loss survey. For other tasks, summer works fine:
- Flat roofs: at dusk after a sunny day the roof gives off heat; wet insulation cools more slowly and therefore stands out.
- Solar panels: hot cells and faulty connections show during operation in good irradiance. By drone as well, where airspace and permits allow.
- Electrical cabinets: in any season, but under load.
- Cold rooms and cellars: the temperature difference between the room and its surroundings provides the signal; seals and panel joints in a cold store show even when it is warm outside.
If the planned night is unsuitable
Sometimes the weather brings something else: fog instead of a clear sky, 8 m/s instead of calm air, or a night that does not get cold enough. In that case we do not survey in poor conditions, because a misleading thermal image costs more than a postponed night: money goes into the wrong repair, or the real defect stays hidden. We reschedule free of charge (up to 2 times); the exact rescheduling terms are set out in the quote.
When should you order?
Early winter is the best timing: the report is delivered within 10 working days of the survey, so you can still act in the rest of the season (doors, seals), and schedule roof and panel work for spring using the ranked list. If you order in the late-October to early-November window, there is a reserve for unsuitable nights. Towards the end of the season, in March, suitable nights become scarce.
The forint values – what the escaping heat costs and which item pays back quickly – can be explored in the payback calculator and on the method page. For warehouses and logistics halls see warehouse and logistics; for production halls manufacturing hall and workshop.