Buildings consume huge amounts of electrical energy in the Kingdom of Saudi Arabia, particularly during the summer months because of enormous air conditioning demands due to the very hot outdoor temperatures. Residential buildings consume more than half of the electricity in Saudi Arabia, with the air conditioning load making up 70% of the use. The main aim of this study is to evaluate the thermal performance of two residential buildings in Makkah, Saudi Arabia. The two buildings are six stories and have the same orientation, but the first building is thermally insulated, while the second building is not. To investigate indoor thermal performance of the two buildings, environmental site monitoring was undertaken in May 2019. The data provided consisted of indoor air temperature values that were recorded every fifteen minutes for a period of sixty-eight hours for two chosen rooms in each building. Analysis of site monitoring data was conducted, and the results obtained to have a better idea of the effectiveness of the existing building fabric characteristics, in particular external walls and roof, in relation to indoor thermal performance. Then this was calibrated with the simulated results taken from thermal analysis software (TAS) to validate them, and finally to quantify the cooling load in the case study buildings. The outcome illustrates similarity between the measured and simulated results and also indicates that thermal insulation can decrease the cooling load to 50%.