The interaction among the spectral characteristics of solar radiation, windows, and human skins may affect indoor thermal comfort. For analyzing such shortwave solar effects through windows on indoor thermal comfort, a more explicit method taking spectral features into account is indispensable. We built this new calculation methodology, called spectral integral method, upon a previous work that was mainly used to transfer the shortwave solar effect into the equivalent longwave effect to calculate indoor thermal comfort with the Predicted Mean Vote. Compared with the previous ones centered in the constant and simplified radiometric quantities, the uniqueness of this method is to provide the underlying radiometric calculation with the detailed and precise spectral contents and their variations of radiation sources, mediums, and receivers details. We adopted solar irradiance data from 8:00 to 18:00 with an interval of one hour in Denver, Colorado in a case study to verify the necessity of the spectral integral method relative to the constant method. The Predicted Mean Vote values with constant and spectral integral methods were then compared. The result of this work shows that using the spectral integral method could lead to quite different decisions of estimating indoor thermal comfort in some circumstances in terms of solar intensities and solar positions.