When talking about the envelope of a building we consider all the elements that separate the interior from the exterior.
The first rule to minimize heat losses is to avoid thermal bridges as much as possible. To do so, the insulating layer has to be continuous. Moreover, it is essential to consider the heat transfer coefficient U of each building element.
In order to meet the Passive House standard requirements, a maximum U value has been defined for each element. Obviously, the values will depend of the type of climate. The figure opposite shows the values recommended for a Central European climate, for instance Germany.
For warmer climates such as Lisbon, the value of U for exterior walls can reach 0.50 W/m²K (0.15 W/m²K for colder climates such as Germany).