Double glazing isn’t always a suitable prescription for thermally inefficient older buildings, particularly in conservation areas where the original style has to be maintained. Could a slimline Japanese system be a better remedy?

The Japanese have an insatiable appetite for innovation and new technology. Ten years ago they had something new to exploit: vacuum glazing. It offered the same thermal properties as double glazing, but had the thickness of a single pane of glass. It quickly found a niche in high-tech office developments where cost was not the most important factor.

Now, more than a decade later, vacuum glazing is going to be available in the UK, only this time the company behind it has a different type of customer in mind. Nippon Sheet Glass, which pioneered the technology in partnership with the University of Sydney, acquired Pilkington in 2006. According to John Phillips, the European product manager for insulating glass at Pilkington, it was this purchase that opened up the opportunity to bring vacuum glazing to the UK. However, his immediate thought was not to market it to modern commercial projects but rather to tackle the long-standing problem of how to make older, historic buildings more energy efficient while keeping with the original design. “It was the perfect solution for replacement windows in heritage projects where architects have for decades been looking for a way to keep the slim look of the original windows while still being able to comply with the Building Regulations.”

Launched under the firm’s EnergiKare brand, the Legacy vacuum glazing looks an attractive proposition. Each unit is made up of one pane of clear float glass and another of low-emissivity glass, which are separated by a 0.2mm gap to give an overall unit thickness of about 6mm. Its size and its light weight mean it can be used as a like-for-like replacement for single panes in windows such as box sashes without modifying the frames, and normal putties and sealants can be used. Early talks with English Heritage have proved positive, and although it would not be acceptable on grades I and II-listed buildings, it would be an option in conservation areas or properties where architects simply want to preserve the original aesthetics.

The big benefit, of course, is the glazing’s U-value of 1.4W/m2K. This is comparable to a 24mm-thick double-glazed unit. Depending on the frame the glass is fitted in, it can outperform the minimum requirements of future Building Regulations.

A protection cap plugs the hole through which the air is drawn out from between the two panes.

A protection cap plugs the hole through which the air is drawn out from between the two panes.

So far, so good. But the technical performance comes at a price. The obvious visual one is the protection cap that appears in the corner of every pane of glass, regardless of size. This plugs the hole through which the air is drawn out from between the two panes, and is therefore required by the production process. According to Phillips, it has been possible to create a vacuum by drawing air out of the edge of the unit in the labs but so far it has not been possible to do the same in a factory, so the cap will be a feature of the glazing for the foreseeable future. For the UK market it will be changed from a silver disk to a black plastic cap, 6mm in diameter to try and make it less obtrusive. One way of reducing the number of caps appearing in a multi-pane window is to use surface-mounted glazing bars, which is possible when new frames are being made.

The 20mm grid of micro-spacers is less of a problem; these sit between the two panes to prevent them imploding. With a diameter of 0.5mm, they are obvious only when close up and, argues Phillips, they blend into the view.

Another factor is shape. The vacuum units come as small as 200 x 350mm and as large as 1,350 x 2,400mm but the manufacturing process means that there must be two sides with straight edges at 90 degrees. That rules out a lot of shapes.

Also, the glass cannot be toughened – the production process requires the unit to be heated to up to 400°C to create the vacuum, which would destroy any heat treatment previously applied. There are two options where toughened glass is needed, one is to switch back to conventional 6mm single panes, the other is to apply a film coating.

The big question is cost. Vacuum glazing could be about four times that of double glazing. Phillips is coy on giving exact figures but makes it clear that the product will not be competing on price. The big influence is transportation. So far the only production facility for the vacuum glazing is in Japan, which means it must be shipped to the UK – delivery times are about eight weeks. Shipping costs are based on volume. Phillips says: “To ship 50m2 of glass if it’s made up of 3m x 4m panes would require a large container and a lot of packaging and supports. However, if someone wants 50m2 made up of small panes, the shipping cost would drop dramatically.” The idea is to batch orders to make it cheaper.

Ultimately the cost will drop dramatically if manufacturing begins in the UK. This will depend on demand but Phillips says the set-up costs would not be huge.

The market share for double glazing in the UK is about 20 million m2 a year. The market that Pilkington is targeting with its vacuum glazing is good for about 300,000m2 a year. “If we get just 1% of that market, I’ll be happy,” says Phillips.

But failing this there could be another niche for the technology – super glazing for new builds. In theory, it could be possible to produce vacuum glazing with a U-value as low as 0.6W/m2K by using a wider spacing of the panes, a higher vacuum or better performing glass. This would require significant R&D costs, but with ever tightening Building Regulations, there could be a market for it in the fairly near future.