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The Magic of Glass and how to specify it

Following months of lockdown, there has never been a better time to reflect on the importance of windows and particularly glass as a way of giving us an opening to the outside world. So often taken for granted, glass is pretty magical stuff and essential to the building envelope of the building.

Before glass our ancestors found innovative ways to allow light into their buildings using animal hide, translucent animal horn, cloth or wood followed by alabaster – arguably one of the greatest inventions, made from sand its transformation is a kind of alchemy. The New Scientist says, “Glass is not a slow moving liquid, it is a solid, albeit an odd one. An amorphous solid because it lacks the ordered molecular structure of solids… It is too rigid for it to qualify as a liquid.”

When it comes to specifying glass for timber windows there is a lot to consider. Before you even begin to consider building regulations, it’s important to go back to basics. Consider:

  • What the building will be used for
  • The building’s orientation
  • Solar control (gain and reduction)
  • Acoustic requirements
  • Light transmittance
  • Safety
  • Security
  • Fire
  • Aesthetics
  • Planning
  • Budget
  • Size of the unit/wind loading

So how do you begin to navigate this minefield?
Tom Wright, MD at George Barnsdale, says, “The  most important thing to remember is that the optimal performance of glass in a window relies on the whole window system, not just the glazing unit. The way in which the glass unity is made and the way it is installed into the frame both have a huge influence on long term performance.”

For architects considering all performance criteria, it is important to consider the way a window is manufactured at every stage of the process. For example, the Glazing Unit itself is more likely to mist up and prematurely fail if the depth of the seal is too shallow. Many units are still being installed with this problem, particularly historic types which have never passed EN1279.

The way units are installed into the frame has a major effect. Many glazing units have sealant or putty surrounding them, which may look like a feasible way to provide an excellent barrier to moisture. However, over time sealants degrade, making units more prone to failure. Alternative methods are available that also incorporate drained and vented systems which are far more effective, key advice is to be sure to check the design of the glazing interface.

The choice of spacer bar can even make a difference to performance. Aluminium can cause condensation on the inside around the edge of the unity since it is a good conductor of heat and reduces thermal performance. Alternatively, products like Swiss Spacer ultimately deliver better results. Since there has been a history of false claims in the spacer industry, the German Glass Association has introduced a certification scheme to provide quality assurance.

Sound advice
In terms of acoustic performance, don’t just take the centre pane value, the frame thickness also plays an important role and can contribute to air leakage. The best manufacturers will have overall window test data rather than just glass performance and it is important that specifiers request this if they are serious about real performance.

In general, there is a lot more to specifying glass than simply looking at raw IGU U values and performance data and the optimal solution is to tell the manufacturer what you are trying to achieve and allow them to test their knowledge, experience and most importantly test data to come up with the optimal solution in terms of performance and cost.

Our full Guide to Glass can be found here at: