The terms lambda value (𝝀), R-value and U-value are key parameters in the field of thermal insulation and building energy efficiency. They play an important role in the assessment of building materials and building constructions in terms of their thermal insulation. Find out more about the definitions, correlations and meanings of these three parameters.

The lambda value (𝝀) - thermal conductivity

The lambda value, also known as thermal conductivity, indicates how well or poorly a material conducts heat. The value is given in watts per metre and Kelvin (W/mK) and describes the amount of heat that flows through one metre of a material per second at a temperature difference of one Kelvin. The lower the lambda value, the better the material is able to insulate heat.

Meaning of the lambda value

A material with a low lambda value is considered a good thermal insulator as it only allows a small amount of heat flow. Materials such as glass wool, polystyrene or organic insulating materials typically have very low lambda values of 0.03 to 0.04 W/(m-K) as they reduce thermal transmittance. In contrast, materials such as metal or concrete have higher lambda values and conduct heat better.

The R-value - thermal resistance

The R-value indicates the thermal resistance of a material and thus describes its ability to resist heat. It is the exact opposite of the lambda value and is given in square metres of Kelvin per watt (m²K/W). The R-value is particularly useful for comparing different materials and assessing their insulating properties.

Meaning of the R-value:

The R-value depends on both the thermal conductivity (lambda value) and the thickness of the material. A higher R-value means that the material has a higher thermal resistance, i.e. better insulation. A higher R-value means higher thermal resistance and therefore better insulation. The R-value is often used when calculating the thermal insulation of walls, roofs and floors.

Application:

In construction practice, it is used to assess the thermal insulation of walls, roofs and floors. A high R-value is desirable in the construction of energy-efficient buildings, as it indicates good thermal insulation. In practice, the R-value is calculated for each layer of a building element such as a wall or roof in order to obtain as accurate an idea as possible of the overall insulation performance. In Switzerland, high R-values are required in particular for low-energy and passive houses in order to minimise energy consumption and increase thermal comfort.

The U-value - heat transfer coefficient

The U-value, also known as the heat transfer coefficient, indicates how much heat passes through a building element, such as a wall or roof. It is given in watts per square metre and Kelvin (W/m²K) and describes the amount of heat that escapes through the component per square metre and per degree of temperature difference between the inside and outside. The lower the U-value, the better insulated the component. The U-value of a building component is calculated from the R-values of the individual layers.

Significance of the U-value: A low U-value means that less heat escapes through the building component and the insulation is more efficient. This is particularly important when assessing the energy efficiency of buildings, as a low U-value results in a lower heating or cooling requirement. The U-value therefore makes it possible to assess the thermal insulation of an entire building element, for example a wall, a roof or a window.

Typical U-values for modern building envelopes in Switzerland are around 0.15 to 0.25 W/(m²-K) for walls and roofs. The U-value plays a particularly important role for windows, as these often represent weak points in the building envelope. The U-value of windows in modern buildings should be as low as possible, often in the range of 0.8 to 1.3 W/(m²-K), in order to minimise energy losses

Relationship between lambda, R and U value

In practice, all three values are required to assess the thermal properties of buildings. For example, an insulating material with a low lambda value can be used to increase the R-value of a building component, which in turn leads to a lower U-value and therefore better energy efficiency.

Standards and regulations in Switzerland

In Switzerland, there are regulations on the thermal insulation and energy consumption of buildings, which are governed by the cantonal building programme (GEAK) and the model regulations of the cantons in the energy sector (MuKEn). The aim of these standards is to reduce the energy consumption of new and renovated buildings and to achieve Switzerland's climate targets. The Swiss standard SIA 380/1, heating requirements, defines these requirements for the U-value of building components. Improving the U-value is one of the most effective measures for reducing energy consumption, particularly when renovating old buildings. Thick insulation layers or special thermal insulation windows are often used to improve the standard and significantly reduce energy consumption.

Conclusion

The lambda value, the R-value and the U-value are key indicators for assessing the thermal efficiency of buildings. In Switzerland, where there are high requirements for energy efficiency, these values are not just theoretical values, but an integral part of building planning and construction. The strict regulations in the various cantons and the building energy certificate system (GEAK) help to ensure that both new buildings and renovations fulfil high energy standards. This is a decisive step on the way to realising Switzerland's energy goals and reducing CO₂ emissions.

Knowledge transfer and training

The Sager Academy is a new platform that offers regular training courses on the building physics principles and energy standards covered in this article. Our practice-orientated training courses provide you with in-depth knowledge to successfully implement these requirements in construction planning. Contact us for further information and registration.