Calculation heat transition coefficient

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

Heat transmission

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

RSi  =  RSe  =  0,13 m2 K / W

Thermal resistance

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 transition coefficient

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

RT =  RSi + R + RSe   [m2 K / W]

The heat transition coefficient U corresponds to the reciprocal of the heat transistion 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 transition 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 resistances
   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).


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