Thermal Conductivity in Practice
To evaluate the measured thermal conductivity of different materials correctly at different temperatures and humidity, we have to distinguish between
a) the nominal value of thermal conductivity and
b) the design value LambdaBW according to DIN 4108-4
Compared with the nominal values, the design values of thermal conductivity need to be increased to take into account
- the influence of temperature
- practical moisture content and
- the aging of the material
- Also the material is partially crushed in the installation process and thereby changes (worsens) its insulating properties.
Therefore only these increased values are used for Passive House calculations as they are closer to the real values.
For insulation, the nominal value is declared on the basis of measurements by the manufacturer, but the question is whether the above corrections are taken into account.
For sandwich elements e.g. are the design values Lambda (the coefficient "BW" to Lambda is usually omitted) are to be determined in each case by testing for the certification. The result will be assigned taking into account the charges, according to DIN 4108-4. This is the time of the thermal conductivity group WLG 025. The consistent size of the great computational value of the sandwich panels with polyurethane insulation core of λ = 0.025 W / mK is to be proven by ongoing third party inspection. Samples are taken and analyzed by an independent laboratory directly from the current production at the manufacturer. This ensures consistent quality is guaranteed.
The correction factor for nominal value to the design value is:
- + 5% if it has been tested by an independent laboratory,
- otherwise it is + 20%
