Since the introduction of Pilkington’s Spacia system a decade or so ago, vacuum glazing systems have begun to appear as an offered solution in the historic marketplace.
As the preferred supplier of insulating glazing for historic windows, Histoglass feel a duty to always assess new markets developments, available products, and best practice. The drive for the very best thermal performance always needs to be approached in a holistic way.
We are not convinced that any of the Vacuum systems currently offered in the market offers the right solution for historic window upgrades; we are working on developing system which addresses some of the major concerns we have about the current offering in the market but will only launch them when we are sure that they meet the specific needs of the UK historic built environment.
Our points of current concern include:
- The average insulation values. While vacuum glazing has a very low centre-pane value, this drops considerably towards the edge of the unit. This is particularly important when installing into Georgian frames, with a lot of edge areas per overall window. This also means that the average unit U-value is lower than the promoted figure.
- The insulation values. At U-values of around 0.7, this type of glazing can be lower than that of the main fabric of the building, in particular in historic buildings. This causes a real and present danger of moving the dew-point to the (stone and timber) walls, leading to interstitial condensation, potentially trapping moisture in the walls, causing it to degrade, in particular when non-breathable plaster and paint has been used.
- Rebate dimensions, sightlines and nozzles. No current system is fit for installation in Georgian glazing bars. The sightline of the vacuum glazing systems
- The spacers. A vacuum system requires spacers to keep the two panes apart. Currently, they come in all kinds of colours and are spaced out at varying distances, some as close as 10mm. In particular grey spacers are very noticeable and we have seen them delaminate and drop to the bottom.
- Historic glazing. Last but not least, no system currently on the market has managed to incorporate genuine historic glass types to maintain the character of historic windows. Clear float glass is simply not acceptable in any pre-1950s windows.
- Carbon footprint. Most vacuum glazing systems are manufactured in the Far East.
Vacuum glazing is a very interesting development and will have its uses. However, we cannot recommend or support any of the current systems in the market as there are still far too many unaddressed issues.
We will keep working on getting a system in place to address all of the above concerns. Once we have a system which we believe corresponds with our values and drive for perfection of offering solutions which stand the test of time, and are in line with the historic aesthetics, we will launch our vacuum range.