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Heat conduction As soon as a temperature difference arises in any medium – whether it be in solid, liquid, or gaseous state – heat is transferred from the warmer to the cooler part. The principle behind heat conduction is the transfer of heat by the oscillation of particles and collisions between them. The ability of a certain material to transfer heat in this way depends on its structure and is described by the temperature-independent constant of ‘thermal conductivity’. The amount of heat transferred depends directly on the temperature difference. If the temperature difference between the interior and exterior surfaces of a wall doubles, the heat loss likewise doubles. Fig. 2.4: Even a very cold object emits heat radiation. The tree in front of the house is still clearly visible at -1 to -2 °C. Even the sky emits atmospheric counterradiation at -6 °C. The radiative heat emission of the chimney is greater than that of the rest of the building.
Convection Convection is heat transfer by the transport of particles. There are two types of convection: → free convection (particle flow with natural driving forces); → forced convection (caused by external mechanical force). A typical example of heat transfer by forced convection is the use of heated air in a mechanical ventilation system for heating a building.
2 . 1 . 3 . 2 Stationary heat conduction and heat storage The U-value is used for calculating the stationary heat conduction of a component (e.g. in the Passive House Planning Package [PHPP]). It describes the energy flow through a component per square metre and per degree temperature difference. The higher the U-value, the more heat can flow through the materials. A component with high thermal insulation has a low U-value. The U-value of a component depends on the thickness of its layers and their thermal conductivity.
Fig. 2.5: U-value calculation for an external wall in the Passive House Planning Package [PHPP]
© PHI
The U-value only describes a steady-state heat flow through a component, i.e. under constant temperature conditions. When walls heat up or cool down, dynamic processes such as heat storage effects also occur. Temporary heat storage in walls and floors can stabilise the temperature in a room and can