This case study evaluates the best energy efficiency measures of an existing two-story office building from the late 1960s located in Vancouver. Natural gas was used for heating and electricity was used for lighting, cooling and other needs. The building was simulated to match both metered data and bills. The energy model allowed identifying the parameters to reduce the energy consumption and mitigate the impact on CO2-eq emissions. On-site renewable energy supply was simulated. The return on investment (ROI) of the retrofit strategies (building envelope and renewable energy) was calculated to determine the profitability. From the parametric study, the insulation of the wall and roof, the airtightness and window replacement have the most impact on energy saving and allowed reducing 45% of the total annual energy consumed. These improvements can save more than 70 tons of CO2-eq per year from reducing the natural gas consumption. The return on investment of upgrading the building envelope was 7.7 years in Vancouver. Net zero energy building performance was possible with the addition of photovoltaic solar panel and solar heating to supply the total energy needs of the building, with an ROI of 11.6 years. If we changed the building location to Montreal, the same optimized building envelope reduces the energy consumption by 39%, and the energy saving increases to 56% when using the electric heating system usually already in place. Overall, building envelope upgrades are solutions to consider to improve energy saving in northern climate. Crown Copyright (C) 2018 Published by Elsevier B.V. All rights reserved.