The ideal rim seal should be 100 % gas and moisture tight and at the same time have a thermal conductivity equal to that of the evacuated aerogel to avoid thermal bridge effects along the glazing perimeter.

However, such solution does not exist and the main task has been to develop a solution as close to the ideal solution as possible.

Several ways exist for minimizing the thermal bridge effect: 1) use of materials with a low thermal conductivity, 2) minimizing the material thickness, 3) increasing the heat flow path length or a combination of the three [3].

The rim seal solutions used in sealed glazing units should both act as gas and vapour barrier as well as a structural element for keeping the desired glass distance.

In aerogel glazing the glass distance is kept by the aerogel layer, which has the sufficient strength to serve as spacer when evacuated. Therefore rim seal solutions for aerogel glazing do not need any structural strength, which makes foil solutions possible.

Metal foils with a thickness larger than 0.1 mm and glass are the only materials that are 100% tight against gas and moisture diffusion. Metal foils with a thickness < 0.1 mm are not airtight due to pinholes. Different laminated plastic foil solutions developed for vacuum insulation panels have a very low permeability that may be sufficient if a limited lifetime of the glazing is allowed.

Glass is considered too fragile leaving metal and laminated plastic foils as the most suitable solutions. The thermal bridge effect has been calculated for different metal foils and for a laminated plastic foil developed for vacuum insulation — the Mylar® 250 RSBL300 from DuPont [6]. The foil is made of several different plastic layers and a 13 nm thick aluminium layer. The total foil thickness is less than 0.1 mm. The thermal advantage of laminated plastic foil relative to stainless steel foils is shown in Figure 2.

The barrier properties of the Mylar® foil are according to specifications given by the manu­facturer (ASTM tests F1249 and D3985) sufficient to keep the required vacuum for at least 30 years if protected against water and UV-radiation.

Solar energy Total glazing U-value as function of window size

transmittance and foil rim seal solution

The advantage of aerogel glazing compared to other highly insulating glazing units are the high solar energy transmittance, which in cold climates has a large influence on the annual energy consumption for space heating.

The basic aerogel made as part of the European projects has a solar energy transmittance of approximately 70% for an aerogel thickness of 15 mm. A subsequent heat treatment of the aerogel to a temperature of 425 °C has shown to improve the optical quality

considerably and increasing the solar energy transmittance with approximately 6 %-points. Placing the aerogel between two layers of glass would reduce the solar energy transmittance due to absorption and reflection in the glass panes. A common 4 mm float glass absorbs approximately 10% of the solar energy and the iron content in the glass furthermore changes the colour of the transmitted daylight. Therefore float glass with a very low iron content makes progress, which reduces the solar energy absorption to less than 1% almost independent of glass thickness.

The reflection losses of the glass panes amount to approximately 8% for a single layer of glass. This value can be changed by surface treatment of the glass panes. A commercial durable treatment has been developed by the Danish company SUNARC A/S [7] and is mainly applied for solar collector covers. The surface treatment reduces the loss due to reflection to approximately 3%.

Table 1 shows the estimated benefit of using anti reflective treated low iron glass for aerogel glazing and common low energy glazing units. An improvement of the solar energy transmittance of approximately 13 %-point for both type of glazing is found, but even if the triple glazing is fully optimised the solar energy transmittance will still be lower than for the non-optimised aerogel glazing.