The application of antireflective coatings to the glass covers of solar thermal collectors, allows to increase the efficiency of the whole system. In this sense, sol-gel silica porous coatings have been deposited on borosilicate glass and solar transmittance values as high as 0.97 have been obtained. The densification process of the films has been studied and the optimal heat treatment conditions have been established. The durability of the materials used in solar systems is a key point since they should keep their initial properties during the operation life-time. In this work, the stability of the optical properties of the films after 2200 h at 50 degrees C under condensation exposure has been achieved, thanks to the application of a hydrophobic treatment to the coating surface. (C) 2008 Elsevier B.V. All rights reserved.