U-value

Measurement guide

How to measure the U-value yourself or with a device.

You can determine a building element's U-value in three ways – from a quick estimate to a standard-compliant measurement. For reliable values, heat-flux measurement in winter is recommended.

1. Calculate from the layer build-up

If the element's build-up and materials are known, calculation is the fastest route. Thickness and thermal conductivity (λ) of each layer give the U-value – without any measuring device.

To the U-value calculator

2. With a heat-flux meter (per ISO 9869)

The most accurate method on an existing element: a heat-flux sensor is attached to the wall on the inside, with one temperature sensor each measuring inside and outside. The U-value follows from the measured heat flux and the temperature difference.

  1. Attach the heat-flux plate on the inside at a representative, undisturbed spot (not at thermal bridges, corners or behind furniture).
  2. Connect the inside and outside temperature sensors; ensure a temperature difference of ideally at least 10–15 °C.
  3. Measure continuously over several days (usually 72 hours or more).
  4. The U-value is the average of heat-flux density divided by the temperature difference (U = q / ΔT).

3. Rough estimate from surface temperatures

For a rough assessment, an infrared thermometer and a room/outdoor thermometer are enough. From room-air, wall-surface and outdoor temperature, the U-value can be approximated. This does not replace a standard-compliant measurement but quickly reveals weak points.

Tips for reliable measurements

  • Measure during the heating season – the larger the inside/outside temperature difference, the more accurate.
  • Avoid direct sunlight, rain and wind during the measurement (a north-facing wall is best).
  • Choose a representative spot: open wall area rather than corners, window reveals or radiator niches.
  • Measure long enough and average over time to balance out day/night fluctuations.

We are happy to support you personally with the U-value measurement on your building. Technical values of the PSCoat systems are available in NANO.DOCS. Contact.