The energy upgrade of a building is the actions and measures undertaken to build a shield against the losses it has and make it less energy-intensive as much as possible. This means that the annual cost of the necessary heating and cooling of the building are minimized.
A building which is properly insulated, has much less energy consumption, as the frames are with thermal break, the glass panes are energy-saving, it has a full insulation on the roof and there is a new technology boiler- burner or even alternative heating methods, such as the fireplace and pellet stove, gas, electric heaters, etc.
This practically means, that a “green built” building saves a lot of money every year. The official document which demonstrates the energy rating of a building and which energy class it belongs in, is the Energy Performance Certificate.