In this study, we examined the effect of argon gas leakage on the insulation performance ofhigh-performance windows filled with argon gas according to energy-saving design standardsfor buildings. Field surveys, insulation tests per KS F 2278 at various argon gas-filling rates, andWINDOW 7.8 simulations yielded the following main results. First, we found an averagegas-filling rate of 49.9% in insulating glass for buildings constructed after the 2018 revision ofthe building energy code in the scope of this study. A strong correlation was found betweencompletion years and argon gas-filling rates. Second, the insulation performance test per KS F2278 indicated that decreasing argon gas-filling rates from 95% to 0% resulted in a 10.9%decrease in insulation for low-E double-glazed windows and a 28.8% decrease for low-Etriple-glazed windows. Third, for window specifications from the building energy code, WINDOW7.8 simulations showed an average insulation performance decrease of 11.6% for low-Edouble-glazed windows and decrease of 15.8% for low-E triple-glazed windows when the argongas-filling rate was decreased from 95% to 0%. This supports the KS F 2278 test results, withthe simulation indicating a greater decrement in insulation performance for triple-glazed windows.