The Balancing Act of Facade Design: Glazing Ratios

A design’s brief and context sets up some essential limits within which the creative mind can roam. Our brief for this post is a new build five-storey office set within London’s typical urban grain, with party walls on two sides. The question is, how do you make the right architectural decisions but ensure that the finished design is high performing? In a few blogs, we’ll focus on facades and explore how architects can create high performing and aesthetically appealing fronts to their buildings.

North-South Facade
North and south view of the proposal showing a typical London urban block

As we touched on in a previous post, creating a successful facade is a balancing act that actually involves three goals: improving beneficial daylight, minimizing direct sun exposure, and reducing energy use. Keeping our building form as simple as possible to start with, we’ll focus on determining the ideal glazing ratio. Our defining metrics will be the Spatial Daylight Autonomy (sDA), the Annual Sunlight Exposure (ASE) and the Energy Use Intensity (EUI).

Because our building sits on an infill site in London with party walls to the east and west, our main concerns will be the north and south facades. All glazing ratio options were tested on the same building form – they all comply with Part L 2010. Using Sefaira’s SketchUp PlugIn, we can immediately tell that the building is cooling dominated. Getting the cooling load down will be the last defining factor in our search for the ideal glazing ratio(s).

Ratio Test Models
A few of the glazing ratio test models

We tested nine combinations for north and south glazing ratios. To start, we tried equal glazing ratios for both facades.

Initial results from glazing ratio tests
Initial results from glazing ratio tests

As expected, the largest glazing ratio, 80 & 80% on both facades, gave the highest sDA levels, but it also has the highest exposure (or glare) and EUI.

Next, we tested alternate values for north and south.

All results from glazing ratio tests
All results from glazing ratio tests

The general rule of thumb when designing in the UK is to harness natural light from the north and be wary of glare from the south. Interestingly, the south offered almost double the sDA the north glazing offered, with characteristically higher levels of unfavourable glare. We can also see that higher glazing ratios on the south reduced the cooling load by 7%. It could be that the slight tilt of the building off the cardinal points is exposing the north facade to bad gains from the east.

With this knowledge, I’d like to focus in on the the three options that deliver the highest daylighting levels- (N80%, S80%), (N50%, S80%) and (N80%, S50%).

The shortlist
The shortlist

In our next study, we’ll investigate how much we can get this ASE value down by introducing shading devices such as horizontal or vertical projections or brise soleils whilst still harnessing as much daylight as possible.