Understanding thermal resistance and the value of a heat flux meter on site
Thermal resistance: what you're actually measuring (R, U, real-world performance)
Thermal resistance R (in m².K/W) expresses a whole wall's ability to slow heat loss. The higher R is, the better the insulation. The U coefficient (in W/m².K) is its inverse. With a heat flux meter, you estimate the wall's or roof's real performance, factoring in the assembly, thermal bridges, and the condition of the insulation.
Why measure in situ: spotting the gap with "catalogue" values
"Catalogue" values come from standardised tests under controlled conditions. On site, insulation that has settled, is damp, discontinuous, or poorly installed can drag the result down. An in-situ measurement brings these hidden gaps to light and helps decide between a simple installation touch-up, treating a singular point, or reworking the whole solution.
When a heat flux meter is most useful (retrofits, defects, doubts about the insulation)
It's very useful in retrofit work, when the wall build-up is unknown (lining, cavity, ageing insulation), or in case of defects (condensation, mould, cold walls). It's also a good control tool after the work, to settle a doubt and secure your choices on site.
Choosing the right measuring equipment and preparing the surface
Heat flux meter and probes: simple criteria to avoid bad readings
Choose a heat flux meter with an up-to-date calibration certificate, a sensor area suited to the wall, and a logger capable of holding the same sampling rate for several days. On probes, aim for good accuracy, a stable fixing, and protection from radiation (probe kept in the shade, cable pressed flat). Before starting, run a 30-minute test reading to spot any drift.
Preparing the wall: surface condition, homogeneity, locating thermal bridges
An in-situ thermal resistance measurement doesn't like surprises. Place the sensor on a homogeneous zone: a sound, dry, clean wall, with no loose render, ducts, or sockets. Locate the thermal bridges using the drawings, a visual check, and a thermal camera if possible. Mark the sensor location precisely so you can repeat the measurement.
Conditions to aim for in 2026: temperature gap, stability, weather, and occupancy
Aim for a stable gap between indoor and outdoor temperatures of at least 10°C, with regular heating and few variations in use. Avoid direct sun on the façade, strong wind, and rain. In an occupied dwelling, note the schedules, airing, and any incidental heat gains (cooking, a stove) so you can interpret the data correctly.
Carrying out a heat flux meter measurement: a step-by-step protocol for reliable thermal resistance
Where to place the heat flux meter: representative zones and common placement mistakes
Place the sensor on a zone that's genuinely representative of the wall or ceiling. Aim for the "middle" of the wall, away from corners, floors, internal partition walls, shutter boxes, joinery, and service runs. Also avoid a zone heated by a radiator, a flue, or direct sunlight. Classic mistakes: sticking it right on a thermal bridge, leaving an air bubble under the sensor, or measuring on a damp surface.
Measurement duration and monitoring: reaching a near-steady state without spending days on it
For a usable thermal resistance reading, you need a sufficient and reasonably steady indoor/outdoor temperature gap. In practice, allow at least 72 hours, often 3 to 7 days. Track the rolling average estimate and stop once it stabilises, for example a small variation over 24 hours. Note any disturbances: heating boosts, forced ventilation, solar gains.
Calculating thermal resistance: using the readings (flux + ΔT) without getting it wrong
R is calculated simply. R = ΔT / q, with ΔT in K between the two faces measured at the same location and q in W/m² (the flux passing through). Work with averages over the stable period, excluding the first "warm-up" hours. Check the units and the sign of the flux, then convert to U if needed. U = 1/R. To know when to use a flat-rate, calculated, or measured U value, be clear about your entry method.
Checking the quality of results and interpreting the measured thermal resistance
Essential checks: consistency of the curves, drift, and uncertainty
Before interpreting a thermal resistance value, check for stable curves. The temperatures and the flux should converge over several day-night cycles, without a clear drift. Rely on the uncertainty figure given by the calculation or the software. If it becomes too high, or if outdoor conditions change too fast, the measurement isn't usable and needs to be extended or repeated.
Reading a result: when is the thermal resistance "good," "average," or "degraded"
Interpret using the relative gap between the measured R and the expected R (quote, product data sheet, calculation). In practice, a gap under 10 to 15% is usually reassuring. Between 15 and 30%, you're at an average level that needs confirming. Beyond 30%, or if R varies from one day to the next, performance is degraded.
Linking the measurement to on-site causes: moisture, installation defects, settling, air leaks
When R is low, trace it back to the on-site causes. Moisture increases losses. Installation defects create gaps and thermal bridges. Settling in the loft reduces the effective thickness. Air leaks add convection and skew the reading. A visual inspection, a thermal camera, and, if needed, a smoke test help settle it before opening anything up.
Putting the measurement to work for your jobs and your evidence (2026)
Turning the measurement into a decision: insulate, fix a defect, prioritise work items
A measurement (surface temperature, moisture, thermal resistance) is there to help you decide fast. Locate the cold zones and thermal bridges, then choose the most cost-effective action: loft, walls, floors, or adjusting a ventilation setting. Keep it simple. Deal with the biggest leaks first, then refine.
Presenting thermal resistance to the client: a clear, figures-based argument
Explain that the higher R is, the more the insulation slows losses. Show a figure before, a figure after, and what that changes for comfort. A common example around subsidies: CEE scheme sheets often require R ≥ 7 in unconverted lofts, R ≥ 6 on rafters, R ≥ 3.7 on walls. That becomes a concrete benchmark for validating the right material and thickness.
Recording your measurements: photos, markers, a simple report for job tracking
For your supporting documents, keep records. A photo of the markers (room, wall, date), a screenshot if you use a device, then a short report. Note the method, the values, and the location. Add 2 photos after the work (installation, product label). You end up with legible evidence for the client and for inspections.
Key figures
±10-15%
Accuracy
72h minimum
Measurement duration
€2,000-5,000
Heat flux meter cost
Frequently asked questions
In practice, allow at least 48 to 72 hours of logging, and more like 5 to 7 days if the weather is unstable or the temperature difference is small. The goal is to reach a near-steady-state regime with a stable indoor/outdoor ΔT (ideally ≥ 10°C). The heavier the wall (masonry), the more time it takes.
Louis Airy
COO of Argile
