In conjunction with the mass or shape of a building the building façade is a key component in the realisation of the building’s architecture and therefore is considered early in the design process from an aesthetic perspective. What is less considered is the impact that the façade has on the building’s performance.
In addition to the less quantifiable benefits that contribute to occupant comfort, such as that connection with the outside (perhaps with nature if you’re lucky), the façade enables measurable benefits relating to energy use, such as heat gain and heat loss and natural daylight for example. As the primary moderator between the internal and external environments the façade influences more than half of the building’s energy use.
One of the earliest decisions that is made in the façade design process is what is the optimal percentage glazing, commonly referred to as the glass to wall ratio. But how do we know how much glass is too much glazing is too much glazing, and how much is enough?
Many designers work off a long-standing rule of thumb, the 40:60 ratio, being 40% glazing and 60% being opaque. There must be some logic to this, why in some jurisdictions there is a limit of 40% glazing can be applied to the building? We can only surmise that such a rule of thumb and / or limitation is there to limit heat loss through the lesser thermally performing elements of glazing while limiting excess heat gains through the façade.
What does 40% glazing actually look like, and what is the impact of increased or decreased glazing percentages. The table below shows what the various glazing ratios look like as “notional” windows in Forma, and punched windows, extending to Curtain Walls, in Revit, and the impact that it has on the baseline thermal performance for 40% glazing.
As you can see although the decline in the overall thermal performance dissipates as you increase the amount of glazing, this decline can be easily offset by improving the thermal performance (U-values or R-values) of the envelop elements including the roof, the floor, the walls, and of course the glazing itself.
In this example, the glazing selected for calculating the above baseline thermal performance for 40% glazing is a standard double glazed unit, therefore there is significant room for improving the glazing unit thermal with a low-e double glazing, and in extreme cases there is the possibility to specify a triple glazing, or indeed vacuum glazing, which although more expensive can significantly improve the overall thermal performance far beyond the baseline for 40% glazing.
Then it becomes about understanding the impact of such a highly glazed façade on excessive passive solar heat gains – which can be managed using a solar control glass or introducing some form of shading – but that is a whole other blog in itself!
You can use a series of formulas to manually calculate the overall thermal performance of the building based on the U-value and the ratio of the façades components, or you can use FenestraPro’s Forma extension and/or Revit application to give you real time feedback on the implications of the changes as outlined above as well as passive solar heat gain and natural daylight taking into consideration the location, orientation, the impact of shading from surrounding buildings and shading devices and of course changes on the component specification – giving you the freedom to design beyond limitations.
