Abstract Within a finished and a current EU project, research and development are being carried out on the use of monolithic silica aerogel as transparent insulation in windows. Results related to the window application will be presented here. At the thermal envelope of buildings, the window area is the weakest part with respect to heat loss, but at the same time, this area also provides advantages, e.g., solar energy gain. Glazing prototypes have been made of aerogel tiles of about 55 × 55 cm2 (fabricated within the EU projects). The tiles are evacuated rapidly and easily sealed between two glass panes and a specific rim seal. A heat-treatment phase (after the supercritical CO2 drying) of the aerogel is currently being developed in order to improve its optical quality. This step increases the solar transmittance by ∼6 percentage points. For glazing prototypes with an aerogel thickness of ∼15 mm, a center heat-loss coefficient of
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