Thermal conductivity coefficient (lambda) – what is it and why does it matter?

Choosing the right  building materials  is an important decision, both for a new facility and for the thermo-modernization of older buildings. One of the parameters affecting energy efficiency that is worth  considering is the thermal conductivity coefficient. What is the lambda coefficient (λ) and why is its value so important in construction?

Published: 22-01-2025

What is the thermal conductivity coefficient?

The lambda coefficient (λ) is a value describing the material’s ability to conduct heat. The lower the value of this parameter, the better  the thermal insulation properties of the material  and the reduction of heat loss. Examples of insulating materials that have a low thermal  coefficient include mineral wool and polystyrene, PIR thermal insulation boards or polyurethane foam.

It is worth remembering that thermal conductivity coefficients (λ) and heat transfer coefficient (U)  are two different parameters used in thermodynamics and construction, although they are related.

The basic difference:

  • Thermal conductivity coefficient (λ) refers to a single material and describes how a given material  conducts heat;
  • Heat transfer coefficient (U) applies to the entire partition and takes into account all layers and additional processes, such as thermal resistance on the surface.

Example: multilayer wall

For a wall consisting  of brick, mineral  wool and plaster:

  • Thermal conductivity coefficient (λ) will be different for brick, mineral wool and plaster.
  • Heat transfer coefficient (U) will be the result of summing up the thermal resistances of all layers of the wall and surface resistance.

Thermal imaging camera displaying an image of a house with a visualization of temperature distribution on the building's surface

How is the thermal conductivity coefficient  measured?

What is the unit of the thermal conductivity coefficient? The value of the lambda coefficient is determined in laboratories. It expresses the amount  of heat (in watts) that  flows through a material cross-section with  a thickness of 1 meter,  at a temperature difference of 1 kelvin (1 K). It is expressed in W/m·K (watts per meter times kelvin).

The measurement process involves  conducting tests under  controlled conditions, in which heat flow  through a material  sample is analyzed. Manufacturers provide  lambda  coefficient values in technical documentation, which makes it easier for investors  to compare  materials and choose the right  insulation solutions.

Why is the lambda coefficient  important in construction?

The value of the thermal conductivity coefficient helps determine how effectively materials reduce heat loss. How is this parameter used in construction?

Impact  on the building’s energy efficiency

A lower lambda  coefficient value means less heat  loss through walls, roofs or foundations. Buildings  with  better insulation properties consume  less energy  for heating,  which reduces bills and CO2 emissions.

Learn more about green building: https://www.products.pcc.eu/en/blog/ecological-construction/

Compliance with standards and regulations

According  to Polish standards,  the  heat  transfer coefficient (U) for  external  walls cannot exceed 0.20 W/(m²·K).  To achieve this, it is necessary to use materials  with  a suitably  low lambda coefficient.

Optimization of insulation thickness

Materials with  lower  lambda  coefficient  allow  to achieve high degree  of insulation  with  a thinner layer. An example could be the use of polyurethane foam with a lambda  of 0.025 W/m·K instead of polystyrene with a lambda of 0.040 W/m·K. In practice, this means saving space, which is important when insulating attics or foundations.

What to look for when analyzing the lambda coefficient?

Knowing  what the thermal conductivity coefficient is allows you to optimize operating costs. What to look for when searching for suitable insulation materials?

  • Declared value (λD). Manufacturers provide the declared value of the lambda coefficient under laboratory conditions. In practice, this value may change depending on the ambient temperature.
  • Durability and resistance to moisture. Some materials,  such as mineral  wool, may lose their insulation properties under the influence of moisture. It is worth choosing solutions  with additional protection against soaking.
  • Operating costs. Although materials with a lower lambda coefficient may be more expensive to purchase, their use translates  into energy savings in the long run.
  • Thermal bridges. It is worth paying attention to the elimination of thermal bridges, which increase the heat loss, regardless of the insulation materials  used.

Thermal conductivity coefficient. Summary

The selection of suitable insulation materials  and their correct application is a way to achieve maximum energy efficiency of the building. Regardless of whether it is about building a new house or modernizing an older  property, it is worth  choosing products with  a low lambda coefficient. This will not only allow you to meet building standards,  but also provide  savings in the long run.


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