In order to obtain the political goals for national and international energy savings of reducing CO2 emissions and avoid climate changes, renovation projects for dwellings are becoming increasingly more important. This paper describes the results attained from an extensive renovation of a detached single-family house built in the early nineteen seventies. The old house was renovated in accordance to the best Danish low energy class (class 1). The project was followed by detailed measurements of the indoor environment (temperature, relative humidity and CO2 levels) and energy consumption for room heating and hot water supply. Measurements were carried out before and after the renovation in order to compare results and see whether the ambition for a 78% reduction in the energy consumption was reached. The paper will describe some of the most important factors regarding the energy renovation, which will include construction of a new façade for the house, test of air tightness before and after the energy renovation, and installation and use of ventilation system in an existing building.