Innovative glazing systems with silica aerogel in interspace were investigated for energy saving in buildings. Aerogel is a promising transparent insulating material, due to its low thermal conductivity (down to 0.010 W m(-1) K-1), high solar factor, high daylight transmittance and remarkable lightweight. Four samples were constructed with float and low-e glasses and granular or monolithic aerogel in interspace (14 mm thickness). The main optical characteristics of the samples were measured, in order to estimate the light transmittance tau(nu), the solar factor g and the colour rendering index R-a, in compliance with EN 410/2011. Finally, the thermal transmittance was calculated. The monolithic aerogel glazings showed the best performance with respect to granular systems, both for light transmittance (0.62 between two 4 mm float glasses) and thermal insulation (0.6 W m(-2) K-1). The solar factor was 0.74. Results showed a very promising behaviour of aerogel windows when compared to the windows normally used in Italy and in EU countries: monolithic aerogel between two 4 mm float glasses gave a 62% reduction in heat losses, with a 17% reduction in light transmittance when compared to a double glazing with a low-e layer; a high solar factor and colour rendering were also assured. (c) 2012 Elsevier Ltd. All rights reserved.