Evolutionary genetic optimization algorithms (GA) have been used for thermal building optimization in the past. However, the results of these algorithms can differ significantly from each other because of random search and it is not guaranteed that the optimal solution is close to the global optimum. Furthermore, the use of these algorithms for non-expert users is limited. In this study, a hybrid single objective building optimization algorithm is introduced, which combines an evolutionary genetic algorithm with a modified simulated annealing algorithm. The goal of this paper is (1) to illustrate that the GA does not always provide solutions close to the global optimum and (2) to provide a building optimization method, which provides a higher reliability than what the GA alone can provide by using a relatively short computation time. Results: illustrate that the hybrid GA coupled with the modified SA provides solutions close to the global optimum in all of the test runs in this study. The proposed algorithm therefore provides more reliable results than the GA without the addition of the modified SA. (C) 2014 Elsevier B.V. All rights reserved.