A very promising method is to apply monolithic silica aerogel as transparent insulation material in glazings for windows. The glazing consists of an aerogel tile between two glass panes and the unit sealed at the rim. The gas pressure within the glazing is below 50 - 100 hPa i.e. a rough vacuum.

The main project objectives are: 1) to make the pre industrial elaboration process evaluation for the chemical part (aerogel process) and the glazing assembly process of  aerogel window and 2) to estimate the energetic interest of such glazings.

Other objectives are:

- Flat aerogel sheets of about 60 x 60 x 2 cm^3 with a solar transmittance of 90% or more and the lowest heat conductivity ever reported for such material.

- The aerogel material exhibits only negligible image blur i.e. the optical quality is at the same level as ordinary glass panes.

- Prototypes of 60 x 60 cm^2 evacuated aerogel glazing (aerogel thickness of 20 mm) made by the proposed process and having a centre heat loss coefficient (U-value) below 0.4 W/m^2 K, an overall U-value below 0.5 W/m^2 K and a solar energy transmittance (g-value) above 75%. Finally, the lifetime of the glazing with respect to maintaining the gas pressure below 50- 100 hPa will be at least 30 years.

At the end of the project, it is expected that the following are developed: A super critically CO2 based process with recovery, that can give flat aerogel tiles of 60 cm square as well as a glazing assembly process, suited for an industrial production, for the same size.

It is expected that after the end of this project, about three to five years of further development with respect to scaling up the aerogel production process and the glazing assembly process will be needed before an industrial, full scale production of aerogel glazings windows can be started.

In principle, the aerogel process consists of a sol-gel process succeeded by a supercritical drying of the gel. The product is a transparent, highly porous, inorganic material in which the solid part is quartz.

Internationally, Europe is leading in the field of developing the aerogel process. The two nearest competitors are probably Japan and USA, but regarding sample size and optical and thermal properties the research those countries seems to be well behind Europe. Monolithic silica aerogel is not yet produced for other than scientific purposes however major improvements have been made in the last years. Until last year, Airglass  was producing the best material with respect to transparency and heat conductivity. Furthermore, nowhere else can sample sizes of about 60 x 60 cm^2 be made. In brief, Airglass standard method is a high temperature process with supercritical alcohol drying.

Among other things, the potential risk of this process have stimulated the interest for a more safe process and the so-called CO2 process is an alternative method. To use this, the alcohol must be replaced with liquid CO2 in the gel between the sol-gel step and the supercritical drying step. Besides the explosion are avoided in case of leak during the drying step, Maximum temperature of the process is lowered from about 280 degrees (C) to 40.

During two JOULE II projects the French partners of this proposal developed and filed a patent on a aerogel process which gives aerogel samples with the best thermal and optical properties ever report, a heat conductivity of 17 mW/m K (at atmospheric pressure) and a solar transmittance of 90%. The corresponding figures for Airglass aerogel material are 20 mW/m K and 85 %. Due to the small sample size, the heat conductivity in vacuum (gas pressure below 50 -100 hPa) of the new aerogel cannot be determined but it will certainly be lower than the 8 mW/m K which is the heat conductivity of Airglass aerogel in vacuum.

The major problem of aerogel, with respect to a general application in windows, is the optical quality of the vision through the material, the so-called image blur. This effect has been reduced during the last years, although not to the same level as the one of ordinary glass panes. The aerogel glazing is a sandwich construction consisting of an aerogel tile between two glass panes and the unit is sealed at the rim. In order to take advantage of the insulation capacity of the aerogel material, the gas pressure within the glazing is below 50 - 100 hPa (which is a rough vacuum). At this gas pressure, the gaseous heat transfer is suppressed due the small diameters of the pores.

For a glazing based on 20 mm evacuated aerogel, the centre U-value is below 0,4 W/m^2 K which is comparable with the rest of the thermal envelope of a building. But at such highly insulating glazings, the overall U-value depends strongly on both the cold bridge effect of the rim seal, size of the glazing and the frame in which the glazing is mounted. Using standard rim seal techniques, the overall U-value will be about the double of the centre U-value and if the rim seal problems is solved, the frame must be thermally improved also. Otherwise the overall U-value is almost unchanged. In the above mentioned JOULE II project both of these problems were solved satisfactory.

The techniques applied to a conventional low-energy glazings are extra layer of glass, low-e coationg and insulating gas filling. Except for the last one, these measures reduce the transmittance of the glazing, eg a triple pane glazing with two low-e coatings and gas filled has a solar transmittance of about 30 % and a daylight transmittance of about 50 %. Depending on the gas, its centre U-value is in the range of 0.5 to 0.8 W/m^2 K. An alternative concept which might enter market in the future is the vacuum glazing, which is made of two glass panes with a distance of about 0.2 mm. The gap is evacuated and the panes are kept apart by means of small support pillars. Using two low-e coatings, the centre U-value should be about 0.8 W/m^2 K and a solar transmittance will be around 35 %. The solar transmittance of the above mentioned aerogel glazing will be about 65 %, so in both fields the aerogel glazing has a better performance.