Heat transfer is the transport of thermal energy due to a temperature difference across a thermodynamic system boundary (in the case of the passive house: building envelope). The energy transported in this way is known as “heat” and is a variable belonging to the process. The heat transfer is always in the direction from the warmer to the colder area. In other words, heat transport always strives to balance energy across system boundaries.

The physical quantity for the extent of heat transport is the heat flow, that is the power that flows through per square meter perpendicular to the surface [measured in W/m² (watts per square meter). As a rule, the heat flow (at least for small temperature differences) is proportional to the difference in temperatures. If divided by this temperature difference, a quantity is obtained which characterises the heat transfer capacity of the enveloping surface component: This is the heat transfer coefficient or U-value. Its unit of measurement is thus W/(m²K) (watts per square metre and per Kelvin), where 1 K temperature difference is exactly equal to 1 °C temperature difference.