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Thermal conductivity of thin coatings: how to read lambda values

4 min read

With a coating about one millimetre thick, a thermal protection argument hangs on one question: what does the lambda value refer to? The relationship is simple: thermal resistance is thickness divided by thermal conductivity, R = d / λ. With layers in the range of tenths of a millimetre, every assumption about thickness and measurement route therefore helps determine the result.

Three routes, three different statements

Route What is determined Follow-up question for the stated value
Guarded hot plate apparatus and heat flow meter apparatus (DIN EN 12667, DIN EN 12664) Thermal resistance of building materials and products Which sample, which thickness, which moisture condition?
Hot-box method Thermal resistance of the build-up, in the IBP test with insulation installed vertically Which build-up, which installation position?
In-situ measurement (ISO 9869) Thermal resistance and thermal transmittance on the building element How long, under which boundary conditions, which starting value?

The Baunetz Wissen overview of standards lists EN 12667 (products of high and medium thermal resistance) and EN 12664 (dry and moist products of medium and low thermal resistance) with the 2001-05 edition; the current version should be checked with the standards publisher.

That the measurement route shifts the result is shown by an investigation by the Fraunhofer Institute for Building Physics on IR-reflective insulation foils. With the hot-box method, in which the insulation is installed vertically, the value was twice as high as in the plate apparatus, in which the insulation is installed horizontally and compressed: 1.00 compared with 0.5 m²K/W. That is a different product, but the same lesson: method and installation position belong to the stated value.

System value instead of material property

For the PScoat system, the supplier does not state a material property under EN 12667 but an equivalent system value. The CE marking names the hot-box method for this and states that the values represent system performance values and do not correspond to the thermal conductivity of a material within the meaning of EN 12667 or EN 12664. On the evidence page, the supplier describes the value as a system value derived from the test-rig test for the tested layer thickness of 0.35 mm, which cannot be transferred linearly to other thicknesses.

The system manual goes further. The system performance values are not design values of thermal conductivity usable under building regulations, and since there is no material λ under EN 12667/12664, the system cannot simply be inserted into a U-value calculation. The measurement report PSC-U-2026-001 also supplies only a derived comparison value and makes clear that it is not a material property determined in a laboratory.

What a back-calculation delivers

An equivalent lambda value arises by determining the additional resistance of the layer from a measured U-value and dividing it by its thickness: λ = d / (1/Unew − 1/Uold). In the measurement report, U = 0.20 W/(m²K) at 0.5 mm gives a value of about 0.000114 W/(m·K), whereas the stable measurement window with U = 0.140 W/(m²K) gives about 0.000077 W/(m·K). A change in the U-value of 0.06 W/(m²K) thus shifts the result by about a third.

The value is therefore only as reliable as three input quantities: the measured end value, the assumed starting value of the wall and the thickness. The evidence page refers to the tested layer thickness of 0.35 mm, the measurement report to 0.5 mm; each value belongs to its thickness and its building element. A lambda without a thickness figure cannot be compared.

Five questions for a data sheet

  1. Material property or system value? Is the test standard stated, for example EN 12667 or EN 12664?
  2. Where does the value come from? From a test certificate, a hot-box test, a test rig or a field measurement?
  3. Which thickness and which build-up does it belong to? May the value be transferred to other thicknesses?
  4. Who measured, and when? Is the test report enclosed, or is it provided only “on request”?
  5. Is the value a design value for proofs? For the PScoat system, the manual expressly denies this.

The class information in the data sheet also belongs here; how to read it is explained in the article Reading the data sheet: what V and W classes really tell you.

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