Heat transfer is the transfer of thermal energy across a thermodynamic system boundary (the building envelope in the case of a Passive House building) as a result of a temperature difference. The energy transferred in this way is referred to as “heat”. The direction of heat transfer is always from a warmer area towards a colder area, in other words: heat transfer always strives for an energy balance across system boundaries. There are three mechanism of heat transfer: 1) by heat conduction, 2) by heat radiation and 3) by convection ¹⁾. Often people are surprised to hear that air movement isn't the most important transfer mechanism for the heat losses of buildings: Envelope components (like a window or a roof) are quite airtight - but a lot of heat is transferred by heat conduction.

The mechanism of heat conduction is easy to understand in the kinetic model of heat: the hotter a material, the more violently the molecules in the material vibrate. If neighboring molecule vibrate less (are “cooler”), there is a tendency to transfer a part of the vibration.

The physical dimension for the extent of heat transfer is the heat flow rate, that is the power which passes through one unit of area of a surface perpendicular to it measured in W/m² (Watts per square metre). As a rule the heat flow rate (at least with small temperature differences) is proportional to the difference between the temperatures. If divided by the temperature difference, the result will be a value which characterises the heat transfer capacity of the envelope surface of this building component: this is the thermal transmittance coefficient or U-value. This is measured in W/(m²K) (Watts per square metre per Kelvin), whereby a temperature difference of 1 K is exactly the same as a temperature difference of 1 °C.

The resistance against the transfer of heat is growing, the more material (number of molecules which have to transfer heat to their neighbors) there is beetween the hot and the cold side. Thus the U-value is indirect proportional to the thickness of an insulating layer.

The differences in the tendency to transfer heat are quite high between different materials. Air, as long as it is not moving, has a very low thermal conductivity (~0.026 W/(mK)); metals are conduction heat very well: aluminum in the range of 230 W/(mK) - almost 10 000 times that of the air. Insulation materials are nothing more than very lightweight structures, which contain a lot of air (98 to 99,9%) using the low conductivity of gases.

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