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The requirements on the heat insulation of building components are stated in ´the Heat Insulation Regulation´ according to DIN 4108 (German Standard).

Calculation of the heat transition coefficient

The heat transmission resistance
The heat transmission resistance can be assumed as follows according to DIN 4108, Part 4:

Thermal resistance
The thermal resistance is strictly proportional to the thickness of the building component and is calculated as follows:

s=thickness of the building component
λR=factor of the conductibility
(see table)

Heat transition resistance
The transition resistance of a building component is calculated by addition of the heat transmission resistances to the thermal resistance as follows:

Heat transition coefficient
The heat transition coefficient is equivalent to the reciprocal of the heat transition resistance and can be calculated as follows:

Calculation examples, Factors

The factors of conductibility of the applied materials are the basis of calculation as per the following table:

Soft Wood (Spruce)
Particle Board (FPY)
Hard Fibre Board
Soft Fibre Board
Tubeboard with
55% solid content
about.   450
about.   700
about. 1000
about.   300

about.   500


The heat transition coefficient ‘k’ of a door blank with a Tubeboard core can be calculated as follows:

Thickness of door blank = 40 mm = 0,04 m
Conductibility λR about. 0,12 W/mK
(calculated, as multilayered)

Heat transmission resistances


The k-value of a 40mm thick door blank made of wooden materials meets the minimum requirements on flat entrance doors according to DIN 4108, Part 1: