How does the thermal insulation work

Insulation functionality

Nothing but (still) air


Insulation reduces the heat transfer through a component such as the wall to a minimum. The question is, why can't the wall do this on its own, but an insulation board glued on from the outside? The answer is surprisingly simple.

How an insulation works

The basic principle of insulation materials is always the same: They bind air in a small space. Whether in the polystyrene spheres of EPS (Styrofoam), between the fibers of flax, rock and glass wool or the pores of foamed glass - air is always retained and has an insulating effect. The insulation thus makes use of a physical property of the air: Air is a poor conductor of heat.

Bad heat conductor = good insulation material

Different materials can transmit heat to different degrees. Typically, high-density materials conduct heat better than low-density materials. The best example are metals, which in the form of pans or pots make the heat of the stove very useful.

The thermal conductivity is given in the physical unit W / (mK). Some examples of the thermal conductivity of different materials are:

  • Copper 399 W / (mK)
  • Iron 81 W / (mK)
  • Concrete 2.1 W / (mK)
  • Window glass 0.87 W / (mK)
  • Bricks 0.38 - 0.52 W / (mK)
  • Air 0.026 W / (mK)

Some gases have an even lower thermal conductivity than air and are therefore used as cell gas in special high-performance insulation materials. The thermal conductivity of insulation materials can generally be easily read from the thermal conduction group (WLG) to which they belong. An insulation material of WLG 35, for example, has a thermal conductivity of 0.035 W / (mK).

The thermal conductivity is determined by the thickness of the insulation

Due to their low thermal conductivity, insulation materials have a high effect even with thin layers. An example from the NRW Energy Agency makes this clear: In order to achieve the same insulation effect as 2 centimeters of insulation material, a wall made of solid concrete would have to be 105 centimeters thick.


Insulating effect of building materials, © picture: EnergieAgentur NRW

How thick an insulation must be, however, cannot simply be said. The thickness of the insulation always depends on the overall construction of a component. For every component of a house - wall, roof, basement ceiling - there are legal requirements on the amount of heat that can flow through. The heat transfer should not be confused with the thermal conductivity and is determined by the so-called heat transfer coefficient (U-value).

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Tags:Insulation background