Monolithic silica aerogel (aerogel) is a highly porous material with pore diameters in the range of 10 — 100 nm. The porosity is above 90%, which combined with the nanometre pore size makes the aerogel a highly insulating material with a thermal conductivity lower than of still air [1]. Further decrease in thermal conductivity can be achieved if evacuated to a rough vacuum, i. e. below approximately 50 hPa in which case the thermal conductivity in the pore gas is almost eliminated [2].

Beside the low thermal conductivity a high solar energy and daylight transmittance is achieved, which makes aerogel a very interesting material for use in highly energy efficient windows [3]. The compression strength of aerogel is sufficient to take up the atmospheric pressure if evacuated but the tensile strength is very low, which makes the material fragile, i. e. if in contact with liquid water the surface tension in the pores would demolish the aerogel structure. So, the application of aerogel for window glazing requires the aerogel to be protected against water and tensile stress. This can be achieved by placing the aerogel between two layers of glass and apply a gas and vapour tight rim seal. When evacuated to a rough vacuum only compression stresses will be present in the aerogel due to the external atmospheric pressure.

Figure 1 shows the advantage of aerogel windows relative to commercial available low energy glazing for which the reduction in U-value is achieved by multiple layers of glass and low emissive coatings — measures that all reduces the solar energy and daylight transmittance. But aerogel glazing has a solar energy transmittance equal to plain double glazing and at the same time has a heat loss coefficient equal to the best triple layered gas filled glazing units. Monolithic silica aerogel is the only known material that has this excellent combination of high solar and light transmittance and low thermal conductivity — material parameters that makes it possible to achieve a net energy gain during the heating season for north facing windows in a northern European climate as the Danish climate.

The utilization of the passive solar energy passing through the windows is an important factor in reducing the annual energy consumption for space heating in northern European countries and has been the background for the research and development projects HILIT

[4] and HILIT+ [5] financed in part by the European Commission. The objectives for both projects were to improve the aerogel elaboration process with respect to material properties (both thermal and optical) and process parameters (drying, duration, safety, etc.) and to develop final aerogel glazing prototypes with a total U-value lower than 0.6 W/m2K and a total solar energy transmittance above 75%.