Energy security is a significant challenge facing Jordan and a burden on the Government due to the limited local energy resources. Ensuring the country’s sustainable development requires an effective energy reduction plan mainly on the existing building stock due to its high energy consumption rate, as residential and public buildings in Jordan consume around 20% of the total Energy consumption. Existing buildings provide great opportunities to reduce energy demand when being retrofitted following energy efficient approach, especially in Jordan as the annual construction work on existing buildings, that have been given licenses, has increased by approximately 46% from 2007 to 2017 while the annual newly-built areas have decreased by around 33%. Since people are already renovating existing buildings, building on them, or reusing them for a different function, this guide aims to promote and discuss the Energy Retrofit concept as well as the related issues with a special focus on the building envelope to optimize energy efficiency and thermal comfort. It also provides an understanding of the current situation of the main existing building envelope components, issues and practices, an assessment of the thermal performance of roofs, external walls, Fenestration, Partition walls between different units and slabs between different floors, and suggests retrofit solutions, which are internal or external insulation, with a calculation of the minimum thermal insulation thicknesses required to achieve the requirements of the Jordanian Energy Efficient Building Code for each building envelope component. The four main building envelope retrofit strategies, which are improving the thermal performance of the building’s ROOF and EXTERNAL WALLS by adding thermal insulation and improving the thermal performance of the building’s transparent surfaces by installing double-glazed WINDOWS with enhanced thermal performance for glass and frames, as well as install ing SHADING devices on the south facing windows are examined for two building typologies, commercial and residential, in two climate zones in Jordan (Amman and Aqaba) in order to show the potential of energy savings. The results have proved that when combining the four main proposed energy retrofit strategies, it reduces the total cooling and heating energy use for commercial buildings by 53% in Amman and 45% in Aqaba, For the residential typology, the total reduction percentage is around 50% in Amman and 47% in Aqaba.