Innovative glazing units with improved thermal and optical performance are important for energy saving in buildings. In this work, light diffusing and thermally insulating aerogel glazing units (AGUs) have been assembled by incorporating silica aerogel granules into the cavity of double glazing units. Experimental results indicate that the optical and thermal properties of AGUs are significantly affected by the particle size of the employed aerogel granules. With respect to a conventional double glazing, a 58% reduction in heat losses and a 38% reduction in light transmittance are achieved by AGUs with large aerogel granules (particle size 3-5 mm); for AGUs with small sized aerogel granules (particle size <0.5 mm), the reduction is 63% in heat losses, but 81% in light transmittance. Moreover, the durability of AGUs depends also on the particle size of the employed aerogel granules. The importance of the particle size issue calls for an optimization of not only the synthesis of aerogel granules, but also the assembly of high performance AGUs towards practical applications. (C) 2014 Elsevier Ltd. All rights reserved.