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?

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.
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.