A comprehensive energy efficiency improvement of existing residentialbuildings is urgently required to meet the EU's CO2 emission reduction targets.The EU's Fit for 55 project aims to reduce CO2 emissions by 55% by2030 compared to 1990 levels. The requirements for new buildings are continuouslybeing tightened, and by 2028, all new buildings must be climateneutralwith increased installation of solar panels and tighter building envelopesto reduce energy consumption. According to EU regulations, all newbuildings must achieve an energy class C (69–90 kWh/m²) or better and includesolar panels. While there are uncertainties regarding the requirementsfor existing buildings, the goal is to achieve emission-free buildings by 2050.This project studies a multifamily house from 1993, with an area of 422 m²,heated via district heating and equipped with an exhaust air system. Thebuilding currently has an energy class F, the second worst, with an energy usageof 207 kWh/m², making it an interesting subject for studying ways to reduceenergy consumption, CO2 emissions, and economic costs.The purpose of this report is to align with the EU's goal of reducing energyconsumption through energy efficiency improvements and to demonstrate thebenefits of investing in upgrades for an old house. The specific aim is to mapthe building's energy system and propose theoretical improvements focusingon ventilation and building envelope enhancements to achieve energy classC.A theoretical model is used to simulate the effects of switching to an FTX(heat recovery ventilation) system and improving the building envelope byreplacing windows and doors and adding roof insulation. Validation is conductedusing data from purchased energy and electricity for the house, as wellas preliminary studies from similar projects.The results show that the building becomes more energy-efficient with theproposed improvements, but ventilation is the only economically viablemeasure. Ventilation accounts for the largest portion of heat loss in the originalstate, 47%, and with heat recovery, energy use drops by a third, making itan investment that pays off after ten years. Other improvements reduce primaryenergy value and energy needs but are not financially profitable. Combinedimprovements result in a primary energy value of 143 kWh/m², achievingenergy class E.The study demonstrates that while certain energy efficiency measures, particularlyventilation system improvements, can significantly reduce energy consumptionand be economically viable, other upgrades may not be financiallybeneficial. However, combined measures still result in a notable improvementin energy efficiency, moving the building from energy class F to E.