Minimization of life cycle costs for building materials and operational energy consumption of a reference commercial office building is achieved through optimization of envelope design parameters by the use of integrated simulation and numerical optimization. EnergyPlus v6. 0 program is coupled with GenOpt v3. 0 generic optimization tool to automatically detect optimized values of thermal insulation thicknesses for external walls and roofs in addition to glazing unit types. A life cycle cost model is implemented into GenOpt v3. 0 program for the evaluation of the objective function after necessary post-processing of energy simulation outputs. A stochastic population-based optimization technique of Particle Swarm Optimization is utilized for the search of the parameter space. Annual total site energy consumption of the reference model is reduced by 33.3% with respect to initial case. Optimized envelope parameters can yield 28.7% life cycle cost reduction over a 25 years life span with a simple pay-back period of 4.2 years.