Building Physics

A thermal insulation product limits the heat transfer between its two sides. But how does it work? What is a λ value?
Heat transfer
There are three methods of heat transfer: Radiation, convection and conduction. The main property of insulation materials is their lack of heat conduction.
How does insulation work?
Effective insulation material insulates well - meaning it conducts heat poorly - because it stops air (or gas) from circulating. PU insulation materials such as PUR and PIR consist of fine 3D foam structures that form gas-filled cells. Thanks to the very fine structure, the heat has to penetrate a multitude of heat-impeding barriers before reaching the other side of the material.
Thermal conductivity λ [W/mK]
Material Thermal conductivity λ [W/mK]
Softwood  0.13
Concrete (plain)  1.4
Steel  50
How does insulation work?
An insulation layer in the structure keeps the inner surface from becoming too cold. That is why the space cools down more slowly and less heating is required.


Heat resistance R [m²K/W]
The less warmth passes through a product, the better its heat resistance (R). Heat resistance is a product property: it depends on the thickness (the thicker, the better) and the λ value (the lower, the better). *R = d / λ
R-value Material  Lambda Thickness in metres
 4,5  Wood  0,130  0,59
 4,5  Rockwool  0,037  0,17
 4,5  PIR  0,022  0,10
Composition of the construction
Most constructions consist of multiple layers. The overall R value can be calculated by adding up the heat resistance values of the individual layers. (Rtot = R1+R2+R3+R4). If a layer is heterogeneous (e.g. a timber frame wall with insulation or trussed rafters) then a specific calculation of the overall R value of the heterogeneous layer is necessary. (Rtot = R1+R2+R3+R4).


U value or heat transfer coefficient [W/m²K]
The heat transfer coefficient or U value is used to compare different constructions. This coefficient takes into account the overall heat resistance and the heat transition coefficients (Rsi and Rse), making it possible to also compare roofs with floors and walls. The lower the U value, the less transfer and, therefore, the less heat loss. U = 1 / (Rsi + Rtot + Rse)
The ABC of insulation
  • Λ = thermal conductivity (material property)
    The amount of heat that passes through the insulation material. The lower the U value, the less heat loss and the better the insulation properties. The Lambda value is like the material’s DNA.
  • E = energy performance (building property)
    The amount of energy required to live in a building. The lower the E level, the more energy-efficient the building.
  • K = thermal loss (building property)
    The amount of heat a building loses via the building envelope. The lower the K level, the less heat loss and the better the insulation properties.
  • R = thermal resistance (product feature)
    The amount of heat the material stops (in relation to its thickness). The higher the R value, the less heat loss and the better the insulation properties.
  • U = thermal transmittance (construction property)
    The amount of heat that passes through a construction (e.g. the roof). The lower the U value, the less heat loss and the better the insulation properties.



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