This paper presents a novel method to determine building envelope thermal transmittance (known as U-values) and air infiltration rate by a combination of Energy modeling (DesignBuilder and Energy Plus), regression models and genetic algorithm at quasi-steady state conditions. DesignBuilder is used to develop the thermal model of an office building, including physical building models, materials specification, occupancy schedules, detailed HVAC system and components for energy simulation purposes. Specifically, the simulation was carried out in EnergyPlus at diverse U-values and air infiltration rates to produce a large datasets. Subsequently, the results were used to generate a linear regression model to evaluate the associations of thermal demands with U-values and air infiltration rate. Genetic algorithm was then applied to obtain a set of Cr-values and air infiltration rate with the minimum difference between field measurement and model prediction. The calibrated U-values and air infiltration rate were employed as inputs in EnergyPlus to model one workday heat consumption. When compared with thermal demand from measured data, the accuracy of the calibrated model improved significantly. (C) 2017 Elsevier B.V. All rights reserved.