A curtain wall system can make several problems like a condensation. So many studies about a condensation resistance were already performed and these studies usually used the method of computer simulation. But the gap still exists between actual measurement and computer simulation. So in this study, simulations which applied several intensive boundary conditions and input data were performed on the window system and these results were compared with the results of actual measurements. When external radiation enclosure is applied in a boundary condition, i. e. comprehensive input, the gap between actual measurement and computer simulation was reduced to 1.2~2.0℃. That way of applying external radiation enclosure is a desirable method to estimate the thermal performance.