Many existing rating systems such as LEED, CASBEE and ITACA have been developed to evaluate the performance of buildings from a sustainability perspective. These systems however vary in their assessment of sustainability since each system has been designed to evaluate several criteria based on its respective local context. None of the existing rating systems considers energy in its assessment nor proposes a definitive guideline for sustainability assessment of Heritage Buildings (HBs). This limits the processes of decision-making when considering the best renovation alternatives for HBs. Currently, 1092 locations are considered as heritage in the world, of which 77% and 23% are categorized as cultural and natural heritage locations, respectively  (Giovine, 2019). 
The objectives of this research work were to: i) identify and study the criteria, factors and indicators that impact the sustainability of HBs; ii) design a hybrid, multi-criteria sustainability-rating model and scale for HBs; iii) determine sustainability-based energy savings for the life cycle phases of HBs; and iv) design an automated assessment tool and perform sensitivity analysis for the developed model. To accomplish the first objective, a literature review was done to analyze 12 major existing rating systems and identify the principal criteria, factors and indicators that affect sustainability of buildings. A questionnaire, targeted to experts in Canada and Saudi Arabia, was also prepared and used to determine the importance of the principal criteria, factors and indicators with respect to sustainability of HBs. To accomplish the second objective, sustainability model development for HBs involved the application of ‘fuzzy logic’ using Fuzzy TOPSIS to calculate the weights and indices of the model parameters. Information from the questionnaire was also useful to examine if the calculated weights reflect reality. To accomplish the third objective, expert responses, regarding the percent energy consumption in each of the six life cycle phases of HBs, were applied on real energy consumption and cost data from two case studies. The exact energy consumption and cost in each life cycle phase was then estimated and compared with simulated energy consumption and cost results from the newly developed model. To accomplish the fourth objective, a web-based graphical user interface was created to automate the assessment process. A webpage, built on Ruby on Rails (for the backend) and HTML, CSS and Javascript (for the front end), was built based on weight calculations. The proposed scale was also applied to obtain the sustainability rating for HBs. Sensitivity analysis for the sustainability model was performed using two case studies and multi-criteria decision-making methods such as Fuzzy TOPSIS, Simple Additive Weight SAW, Weighted Sum Model WSM, Weighted Product Model WPM, and Operational Competitiveness Rating Analysis OCRA. Two HBs, Murabba Palace (MP) and Grey Nuns (GN), were studied to determine weight values for each of the tested factors. 
Results showed that energy was the most important factor for both HBs in the case studies, with a sum indicator index of 1.623 and 1.891 for MP and GN, respectively. Water use was of the least importance, with a sum indicator index of 0.121 and 0.055 for MP and GN, respectively. Moreover, the total Sustainability Assessment for Heritage Buildings (SAHB) was 48% and 63% for MP and GN, respectively. This corresponds to Unsatisfied and Satisfied for MP and GN, respectively, based on the established scale for the newly developed model (Unsatisfied – < 49%, Pass – 50-59%, Satisfied – 60-69%, Bronze – 70-79%, Silver – 80-89%, and Gold - >90%). The newly developed sustainability model will be beneficial to decision-makers, HB specialists, engineers, architects, and project managers in their efforts to improve the sustainability of their HBs. It will also beneficial in the planning of rehabilitation projects for HBs.