Currently, 75% of European houses are energy inefficient, which means that much of the energy needed is simply lost .
There are several passive and active solutions to improve energy efficiency in buildings.
Passive solutions typically refer to solutions that reduce the amount of energy lost. They include façade isolation, roof isolation, insulation of heating tubes, energy-efficient windows, structural air tightness on doors and windows and other materials improving energy efficiency. The windows can have properties for thermal insulation or prevention of excess solar gain, avoiding overheating. In STARDUST, MySMARTLife and EU-GUGLE, both active and passive solutions were implemented.
Active solutions refer to solutions that monitor and control the use of energy, or even solutions that can generate energy.
The active solutions considered can range from energy management and monitoring solutions, to renewable energy production – such as solar thermal panels, shared photovoltaic systems, and ground source heat pumps. In some cases, it also includes energy storages like batteries or thermal storages . The active solutions can be also service-based, in which case the owner of the building is leasing the solution  . Also, thermal storages, thermally activated building structures and phase change materials can be a part of the solution   .
The solutions can be implemented in new buildings or in renovation of existing buildings [e.g. MySMARTLife - Nantes, Houseful - El Mestres]. Typically, the energy saving actions include improvement of thermal insulation and ventilation energy efficiency, as well as integration to low carbon energy sources like photovoltaic panels, ground source heat pumps, or integration into the local district network. The actions will reduce energy use, thus decreasing CO2 emissions. In addition, the actions will improve thermal comfort and air quality.
Source: Energy efficient building, active and passive solutions 
As renovations or implementation of active measures can prove a considerable financial burden, low-income households could benefit in the short-term from the adoption of temporary passive measures that can be implemented quickly and without the need for extensive technical expertise. Examples are radiator reflector panels that can be attached to the wall behind the radiator, insulation of tubes, water-saving showerheads, weather strips to close gaps between windows and walls, and hydronic balancing to optimize the distribution of water in the heating and cooling system to achieve maximum energy efficiency .
In commercial buildings, the heating, ventilation, and air conditioning (HVAC) systems are typically more advanced compared to those present in apartment buildings. In addition, in commercial buildings the cooling typically consumes a considerable amount of energy. Therefore, the choice of the most impactful measures for energy efficiency has to necessarily be informed by the typology of building.
Both passive and active solutions for energy efficiency are commercially available at building level. Some examples are:
Renovation of existing buildings with improved facade and window energy efficiency, ventilation heat recovery, photovoltaic panels, ground source heat pumps were demonstrated with commercially viable products in e.g. EU-GUGLE and in Houseful
Renovation of existing buildings with active room control was demonstrated in MySMARTLife
thermally activated building systems are commercially available