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” and is a value of this process. 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.

The physical dimension for the extent of heat transfer is the heat flow rate, that is the power which passes through a square metre of a surface which is perpendicular to a surface 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 the building component: this is the thermal transmittance 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.

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