Introduction
The building envelope (roof, external walls, windows, doors and floors) serves a number of functions - this chapter focuses on the thermal aspects, the insulation of the building. The climatic differences throughout Europe, for instance, are mirrored in the traditional buildings that range from uninsulated (usually quite heavy) constructions in the south (designed mainly for summer conditions) to fairly well insulated heavy or light constructions in the north, definitely designed primarily to comply with winter conditions. Improving the thermal properties of existing building envelope is, in many cases, one of the most logical solutions in order to reduce building energy consumption and in consequence this is one of the most important strategies in building retrofit.
The improvement of building envelope energy performance is often the result of an optimisation process and the choice of the correct strategies has to be the result of a general overview of all the necessary interventions.

In a building to be retrofitted, interventions usually involve windows, doors, walls and roofs. An unbalanced intervention between different components can lead to unsatisfactory results.
There are two key components to a super-insulated building shell: high levels of insulation with minimum thermal bridges and airtight constructions. High levels of insulation are accomplished by constructing a thicker than normal wall and filling it with an insulation material. However, simply adding more insulation does not turn a conventional assembly into a high-performance assembly. The wall system and junctions between building components have to be carefully designed to be airtight and avoid thermal bridges or discontinuities. As more insulation is added, the thermal discontinuities become more important.
Cost effectiveness of building envelope intervention is a critical issue: so, it should be remembered, for instance, that the first layer of insulation is the most effective and the law of diminishing returns dictates that each additional layer of insulation is less effective than the previous layer.
This chapter intends to make an overview of the technologies applicable in building envelope retrofit, looking at different components, by means of a correct use of the most recent technologies in the fields of windows, insulation, overcladding systems and doors.