# Passipedia EN

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basics:building_physics_-_basics:building_physics_-_heat:heat_transport

# Heat transfer 🌡️

Heat transfer is the transport of thermal energy beyond a system boundary (in the case of a Passive House building: the building envelope) as a result of a temperature difference. The thus transported energy is referred to as “heat” and is a quantity belonging to the process. Heat transfer always occurs in the direction from the warmer to the colder area. In other words: heat transfer always strives to balance energy across system boundaries.

In physics, the physical quantity for the heat transfer amount is the heat flux, measured in W/m² (watts per square metre), which flows through each square metre perpendicular to the surface. Generally, the heat flow is proportional to the difference in the temperatures (at least with small temperature differences). Dividing by this temperature difference results in a value which characterises the heat transfer capacity of the envelope surface assembly: this is the thermal transmittance (German only) or U-value. Its unit is therefore W/(m²K) (watts per square metre per kelvin [K]), whereby a temperature difference of 1 K is exactly the same as a temperature difference of 1 °C.

There are three different physical mechanisms for heat transfer:

• Thermal conduction (the process by which thermal energy is transferred from molecule to molecule without their changing their average location)
• Thermal radiation (also called temperature radiation: electromagnetic waves which are emitted by matter in heat oscillations. For the temperature range existing on Earth, thermal radiation lies in what is called the mid-infrared range, especially with wavelengths between 7 and 18 µm – which is a good deal more “long-wave” than visible light).
• Convection: This is the transport of thermal energy through a mass flow (e.g. a heated gas or a liquid). (A moving solid can also transfer heat. Like Maxwell, we then speak of “a hot body itself carrying its heat with it”.)