Lambda 90/90 – simple explanation!

This article shows the relationship between the thermal conductivity and the Lambda 90/90 Value, which is the leading parameter for the CE-Declaration on insulating materials in Europe.

What is the Relationship Between the Thermal Conductivity and “Lambda 90/90“?

In order to sell insulating materials in Europe, you, the manufacturer, are required to demonstrate compliance with european product standards. One property of insulating materials is particularly important: Thermal Conductivity. This is given in Europe based on the Lambda 90/90 value. In our article you will learn how this value relates to the thermal conductivity and how it is determined with instruments from NETZSCH.

What Does Thermal Conductivity Mean?

Thermal conductivity (unit: W/(m· K)) is a measure for the amount of heat which is transported through a material of a certain thickness per unit of Kelvin temperature difference. A material with a thermal conductivity of 0.035 W/(m·K) and a thickness of 1 meter thus transports 0.035 W if the temperature difference from one side to another amounts to 1 K.

Why is Thermal Conductivity That Important?

In order to be able to compare materials with respect to their insulating effect independently of the component‘s thickness, knowledge of the thermal conductivity is key. Thermal conductivity is designated with the Greek letter λ (Lambda). It is a material parameter and therefore is not influenced by a component‘s thickness and/or geometry. The insulation property can be improved by increasing the component‘s thickness – and thus increasing the thermal resistance.
The picture shows the equation for the thermal resistance (R-Value)
Fig. 1: Equation for the thermal resistance (R-Value)
  • R = Thermal resistance (resistance of a component to heat transfer)
  • d = Thickness of the component
  • λ = Thermal conductivity
The lower the thermal conductivity or the thicker the insulation, the higher the component‘s resistance to heat transfer.

What Does “Lambda 90/90“ Mean?

The picture shows a typical bell curve for the distribution of thermal conductivities and the meaning of Lambda 90/90.
Fig. 2: One-sided confidence interval Lambda 90/90: A value, which ensures that 90% of your production has the declared value with a probability of 90%.
The Lambda 90/90 value is the thermal conductivity value that a manufacturer can guarantee for 90% of his output with a probability of 90%.

How is the Lambda 90/90 Value Measured?

Product standards for insulating materials prescribe routine thermal conductivity testing for factory production control. To this end, primarily heat-flow-meters (HFM) in accordance with ISO 8301 and ASTM C518 are used. This method guarantees a temperature gradient within a sample to be spanned between a hot and a cold plate. Due to this temperature gradient, a stationery heat flow is generated – the basis for calculation of the thermal conductivity.
The picture shows the working principle of a heat flow meter
Fig. 3: The basic setup of a heat flow meter
The picture shows the Furiers-Law: The basics for the thermal conductivity measurement
Fig 4: The Furiers-Law: The basics for the thermal conductivity measurement
By regularly checking the current production batches, a multitude of measurement values is yielded over a longer period of time. Based on these measurement values, the mean thermal conductivity and a standard deviation can be calculated. If 10 measurement results exist, which is the minimum required, then the confidence factor “k” can be determined in tabular form. This is multiplied with the standard deviation and added to the mean thermal conductivity. The more measurements that are carried out within factory production control, the lower the confidence factor “k” will be. This is because a greater amount of measurements means a higher probability that the measured values reflect the true distribution. Via frequent measurements and consistent production monitoring, a manufacturer can thus improse its declared values – or more accurately: The more sample a manufacturer measures during production, the better the Lambda 90/90 value will correspond to the actual production quality.

But Then Which Value is Ultimately Labeled on the Insulating Material?

Whenever any commercially available insulating product is acquired in Europe, it is always labelled with a CE marking. This shows that the product was brought to market in accordance with European standards. CE markings reflect compliance with the stipulations for factory production control. It includes determination of Lambda 90/90. This is simultaneously the basis for the thermal conductivity value printed on the label – the nominal Lambda value or the declared thermal conductivity λD. The nominal Lambda value is the result of rounding Lambda 90/90 up to the next higher 0.001 W/(m·K). In Europe, the thermal conductivity is generally based upon a mean value of 10°C.
The picture shows an example for a product label on a CE-conform polyurethane foam.
Fig. 5: How the declared thermal conductivity appears on the product-label
The producer is the entity responsible for determining the declared thermal conductivity or the nominal value of thermal conductivity for their product. An efficient way to determine Lambda 90/90 by daily measurements in your production environment is the newest Heat-Flow-Meter-Series (HFM) from NETZSCH:
The picture shows a Heat Flow Meter HFM 446 Lambda Medium
Fig. 6: Heat Flow Meter HFM 446 Lambda Medium
Notify of
Newest Most Voted
Inline Feedbacks
View all comments

[…] on your production samples can be calculated in terms of the statistical measurement value Lambda 90/90 by a single mousecklick. This is the basis for determining the […]

[…] is set and after a stationary heat flow is achieved using the Fourier equation. The statistical Lambda 90/90 value is calculated in a special report from a multitude of measurement values that are generated […]

[…] eines Dämmstoffes auf dem Etikett deklariert wird. Er basiert auf der statistischen Größe Lambda 90/90, die für einen Dämmstoff die Wärmeleitfähigkeit definiert, die innerhalb eines Zeitraumes von […]

[…] measuring the thermal conductivity and efficient determination of the CE-relevant statistical value Lambda 90/90 covers a wide range of materials, material thicknesses and sample dimensions. Especially for the […]

[…] you not only single values of thermal conductivity, it gives you the total statistics with the Lambda 90/90-Value, which is needed for determination of the nominal lambda values for […]

Thanks for your sharing. Thermal Resistance Tester, to determine the thermal resistance, thermal conductivity, and CLO value of various fabrics, quilting products, and other heat insulation materials. For more information, welcome to click on

Thanks for sharing this information on physiological effects of textiles – Measurement of thermal and water-vapour resistance under steady-state conditions (sweating guarded-hotplate test). I really welcome this helpful additional information for our readers.