Skip to content

In-situ U-value measurement on a PScoat wall: what it shows, what not

3 min read

On 6 and 7 September 2026, the U-value was recorded for almost twelve hours on a historic solid wall with a PScoat coating of around 0.5 mm. For a two-hour evaluation window, the supplier’s measurement report PSC-U-2026-001 gives a mean value of 0.140 W/(m²K). The report discloses the measurement set-up and boundary conditions.

Object measured and measurement set-up

Object measured historic solid external wall, coated in April 2025; measuring point in a normal wall area, about 1 m beside a window
Wall build-up, outside to inside PScoat coating, approx. 5 mm reinforcing mortar with mesh, cement or lime-cement render, three-layer solid brick masonry, inside cement render with acrylic wall paint
Coating approx. 0.5 mm; according to the report “Basic B + PSC Build (version 2024)”, approx. 18 l on approx. 35 m²
Measuring system Testo 635-2 with U-value kit, continuous recording
Period 6 September 2026, 20:57 to 7 September 2026, 08:54 (11 h 57 min), 42,978 evaluable data records
Boundary conditions Heating off, windows closed, no movement of people in the room after 23:00; room temperature at the start about 26 °C
Quantities measured Temperature of the inner wall surface, of the indoor air and of the outdoor air

What was measured

Evaluation window Mean Median Std. dev. Min. Max.
01:15–03:15 0.140 0.139 0.016 0.111 0.177
05:00 to end of measurement 0.195 0.183 0.044 0.105 0.293

All values in W/(m²K), source: table in the measurement report. The start of the recording is not included: the outdoor sensor was initially still inside the room, so the calculated U-value was far too high. The report treats this phase as a set-up phase and not as a result for the wall.

The figure therefore depends on the evaluation window chosen: 0.140 W/(m²K) in the first, 0.195 W/(m²K) in the second evaluation phase.

The derived lambda comparison value

From the U-value, the report back-calculates an equivalent thermal conductivity value for the coating. As the starting value for the wall it uses U = 1.60 W/(m²K), a “usual, technically plausible reference value”.

The worked example in the report uses U = 0.20 W/(m²K): total resistance 5.000 m²K/W, minus 0.625 m²K/W for the uncoated wall, leaves 4.375 m²K/W; divided by the thickness of 0.5 mm this gives about 0.000114 W/(m·K). The direct back-calculation yields 0.000077 W/(m·K) from the first and 0.000111 W/(m·K) from the second window. The report rounds to about 0.0001 W/(m·K) and makes clear that this value is not a material property determined in a laboratory.

In calculation terms, λ = d / ΔR: with the same measured additional resistance, the value for a layer twice as thick would be twice as high. Such a comparison value therefore always belongs to the thickness examined and the building element examined.

Assessment

The report documents one wall over almost twelve hours and presents itself as a technical evaluation; it does not replace an accredited laboratory test, certification or building-authority assessment. The report recommends that any transfer to other wall build-ups be checked technically. How in-situ measurements are to be classified in principle is described, for thermography, in the article What a thermal image proves – and what it does not.

Sources

Read more

More practical knowledge from the same subject area.