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Calculation heat transfer coefficient

Heat transfer through a building component:
- Heat transfer from the air to the inner surface
- Heat transmission through the layers
- Heat transfer from the outer surface to the air.

Heat transfer

For heat transfer resistances RS inside/outside, the following applies according to DIN 4108-4 with the same air flow

RSi  =  RSe  =  0,13 m2 K / W

Heat transmittance

The thermal resistance R is directly proportional to the thickness of the component:

R  =  d / λ    [m² K / W]

    d  =  thickness of the component
    λ  =  Rated values of the thermal conductivity
            (see table)

For multi-layer components, the R values of the individual layers are added together:

R  =  R1 + R2 + R3 … + Rn  =  d1 / λ1 + d2 / λ2 + d3 / λ3 +  … + dn / λn  [m2 K / W]

Heat transfer coefficient

The thermal resistance RT of the entire component is determined by adding the heat transfer resistances to the thermal resistance:

RT =  RSi + R + RSe   [m2 K / W]

The heat tarnsfer coefficient U correspnds to the reciprocal of the thermals resistance:

U = 1 / RT = 1 / (RSi + R + RSe) [W / m2 K]

Calculation example, calculated values

The calculation is based on the design values of the thermal conductivity λ of the building materials used according to DIN EN 12524. The specification for the tubular chipboard follows internal measurements and is based on chipboard with a raw density of 300 kg/m3.

building material density λ
kg/m³ W/mK

coniferous wood
particle board (FPY)

hard fibre board
soft fibreboard
tubeboard with 55%
solid content
cork
aluminium

ca.   500
ca.   600
ca.   900
ca.   800
ca.   250

ca.   300
ca.   220
2700

0,13
0,14
0,18
0,18
0,07

0,10
0,04
237

Calculation of the heat transfer coefficient U of a door leaf with a tubular chipboard inlay:

- Door leaf thickness approx. 40 mm; of which
   tubular chipboard
   d = 33 mm, d = 3 mm per HDF deck
- Thermal resistance R  =  0,003 / 0,18 + 
   0,033 / 0,10 + 0,003 / 0,18  =  0,36  m2 K / W
- Heat transmission resistance
   RSi  =  RSe  =  0,13 m2 K / W
- Heat transition resistance
   RT  =  0,13 + 0,36 + 0,13  =  0,62 m2 K / W
- Heat transition coefficient
   U  = 1 / RT  =  1 / 0,62  =  1,60 W / m2 K

Result

The thermal insulation of door leaves in flat entrance doors is not specifically standardised. However, a comparison shows: A 40 mm thick door leaf of the aforementioned construction fulfils the minimum requirements for staircase walls of
R ≥ 0,25 m2 K / W (bzw. U ≤ 1,96 W / m2 K).