Solar extinction coefficient is an important parameter in simulating the thermal and energy performances of glazing, but it is difficult to obtain the solar extinction coefficient of aerogel granules layer (SECAL) due to its nano-porous network structure. In this study, an in-field test room is built to measure the irradiance at both sides of granular nano-porous silica aerogel glazing system under different climatic conditions. The SECAL is estimated through an optical model and mathematical methodology from the measurement data. The optical model is proposed on the base of interface energy balance principle, in which the incident angle and scattering phenomena happening in the nano-porous silica aerogel are taken into account. Through the optical model, the irradiation transmittance is calculated by an assumed SECAL value and indoor irradiance is predicted. The SECAL value is estimated by finding the minimal error between the predicted and measured indoor irradiance. The SECAL values for clear and overcast sky are respectively obtained, and the values are 0.0392/mm and 0.0296/mm respectively. Sky clearness factor is used as the index of weather condition to select the SECAL values. The optical model and methodology is valid to determine SECAL and the optical model is accurate to calculate solar irradiance through aerogel glazing system. (C) 2019 Elsevier B.V. All rights reserved.