Optical and visual experimental characterization of a glazing system with monolithic silica aerogel

Aerogel is a promising building envelope material that can be used to improve thermal and lighting performance. In monolithic form, aerogels allow visual interaction between the built and outdoor environments, unlike aerogels in granular form. However, the high production cost of large monoliths limits their use. In this work, monolithic silica aerogels were fabricated using tetramethyl orthosilicate as the precursor in a base-catalysed recipe, and a novel rapid supercritical extraction method. The method produces transparent aerogel monoliths in as little as six hours. A 30 × 30 cm glazing sample was constructed from nine 15-mm-thick aerogel monoliths sandwiched between two pieces of 4.7-mm-thick float glass. The glazing sample was characterized in terms of directional and hemispherical optical and visual properties. Measurements were made with an in-house-constructed spectrophotometer equipped with a 75-cm-diameter integrating sphere in the 380- to 2300- nm wavelength range with variable incidence angles. The light and direct solar transmittance are 0.69 and 0.62, while the light and direct solar reflectance are 0.25 and 0.19, respectively. The sample is also characterized by a high general colour rendering index (R a = 96), allowing indoor visual comfort. Gonio-photometric and directional spectral transmittance measurements show negligible visual distortion and colour variation at different observation angles. The light transmittance of the prototype is lower than that of glazing units present on the market, but about 10% higher than an equivalent unit with granular aerogel. This performance is promising for applications where view through a window is required. However, the size of the samples currently fabricated with the innovative process is limited to 14 × 14 cm and they are characterized by some optical defects that must be removed if the process is to be used for commercial product.