Thermal conductivity: definition and reading lambda

What does thermal conductivity measure?

Thermal conductivity measures how readily a material lets heat pass through it. It is written with the Greek letter lambda and expressed in watts per metre-kelvin, W/m.K.

The lower the value, the less the material conducts heat, and so the better it insulates. It is an intrinsic property: it depends neither on the thickness laid nor on the area, only on the nature of the material.

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How do you read a lambda?

A common insulant under a screed sits at a few hundredths of W/m.K. A screed mortar, by contrast, is counted in whole units: the gap between an insulant and a screed is not a matter of a few per cent, it is an order of magnitude.

That is why a screed, however thick, does not insulate. It stores heat and gives it back, which is useful with underfloor heating, but it does not stop heat escaping downwards. Only an insulant placed beneath does that.

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Lambda or thermal resistance: which to look at?

Lambda characterises the material, thermal resistance characterises the layer as built. The second is obtained by dividing the thickness by lambda, and it is what describes the real performance of the floor.

Comparing two insulants on lambda alone is therefore incomplete. A material with an excellent lambda laid thin can give a lower resistance than a more ordinary material laid thicker. In renovation, where available thickness is constrained, lambda does regain its importance: at equal height, it is what separates them.

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Why the stated lambda is not always the one you get

The value on a technical sheet is measured in a laboratory, under dry and stable conditions. On site, several factors degrade it.

Moisture is the main one: a damp insulant conducts heat markedly better, water being far more conductive than the air trapped in the material. Settlement and installation defects, open joints or rough cuts, also create paths where heat bypasses the insulation altogether. See our thermal insulation solutions.