With existing buildings, architects have much less opportunity to change most fabric components. Reducing outside air infiltration into the building by improving building envelope tightness is usually quite feasible. During re-roofing, extra insulation can typically be added with little difficulty. Windows and insulation can be upgraded during more significant building improvements and renovations.
Improvement in the thermal performance of the building envelope is a prerequisite for energy conservation in buildings. In Northern Europe, the superinsulation level for walls and roofs has almost been reached in new buildings and is increasingly prescribed in the building codes, and large scale retrofitting has been carried out. Further improvement can be achieved in windows. Improved frames and the use of aerogels or evacuated sealed units may offer windows with U-values of 0.4-0.5 W/m2K with high solar transparency, giving a positive seasonal heat balance for all orientations even in a northern climate.
Otherwise, the major trend for the future is multi-functional walls and roofs, including energy-efficient functions, such as passive/hybrid solar heating and cooling. Some examples include wall or roof elements in which a heat exchanger between fresh and exhaust air is incorporated, with solar preheating of the inlet air, or walls covered with transparent/translucent insulation - a solution applicable to new as well as existing buildings. Unlike opaque insulation, transparent insulation and passive/hybrid solutions change, and often dominate, the appearance of the building, thus setting up a new challenge for the building designers.