November 23, 2020

At a time of national 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 envelope of the building. According to Louis Khan “A room is not a room without natural light.”

Historic windows and glazing at One Bishopsgate Plaza

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. Along came glass  – 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
  • 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 unit 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 remember that the data provided by many manufacturers refers to the glass alone which is not a true reflection of the overall performance of the window. Specifiers should always ask for the whole window performance.

It’s not what you do, it’s the way that you do it!

In order to achieve optimal performance, it is important to consider the way a window is manufactured at every stage of the process needs to be considered. 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 also 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 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 unit since it is a good conductor of heat and reduces thermal performance. Alternative products like the Swiss spacer ultimately delivers 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 use their knowledge, experience and most importantly test data to come up with the optimal solution in terms of performance and cost.