Cundall's David Clark on green facade design

How many facades are designed for energy efficiency and thermal comfort as a basic requirement? Not many, if you look at the number of new buildings with full-height glazing. The only other buildings with this much glass are greenhouses and they’re for plants, not people.

We spend most of our lives inside buildings, so we should be designing facades for the benefit of the occupants – both their comfort and their energy bills.

Facades are an integral component of the complex systems that are modern buildings, but too often the architectural vision is set first and the engineering shoehorned in later. We need an integrated design approach to facades from the very start of projects because they influence so many aspects of building performance including thermal comfort, daylight, glare, views out, energy efficiency, ability to naturally ventilate (now and in an energy-constrained future) and noise levels.

The facade also directly influences the type of HVAC system that can be used in the perimeter zone. Many systems will not work if the facade’s performance is poor, so less energy efficient systems must be used to compensate for perimeter heat losses and gains – the ubiquitous four-pipe fan coil unit springs to mind.

We really need to start designing facades from the inside out. The first step should be to define the conditions to be achieved inside the building before deciding what it should look like from the outside. After agreeing the performance requirements, the whole project team can then begin developing the facade solution to achieve these – while also making it affordable and attractive, of course.

Optimising the facade design requires a more sophisticated approach than simply looking up the minimum U-values in Part L. With significant capital and recurrent costs at stake, it makes sense to use a bit of building science at the beginning.

Many engineers have the tools to evaluate the performance of different facade design options, such as shading analysis, energy modelling, thermal comfort assessments and daylight simulation. These allow informed decisions to be taken early rather than guessing and then fixing the problems later (usually by beefing up the mechanical systems to cope). This approach applies equally to upgrading existing building performance.

Full-height glass can be used to imaginatively frame views. But do you really need scenic vistas when you are under your desk plugging in the phone charger?

Glazing technology has advanced in leaps and bounds, but a typical high performance glass (U-value of 1.7) still has five times the heat loss of an insulated spandrel (U-value of 0.35).

So, can full-height glass ever be green? Well, for a particular building in a particular climate it might actually be the most carbon efficient solution. Letting heat out may result in less greenhouse emissions than trapping heat in, by reducing the demand for cooling – but this is determined by analysis, not guesswork.

An efficient facade design will take into consideration where the sun shines during the day and respond accordingly. East, west, north and south are subject to different external conditions so they should have different performance characteristics – buildings do not have to look the same on every face. Where is the logic in designing full-height glass for the east or west facades only to have to cover it with shading to keep out the sun, restricting views and reducing daylight?

Full-height glass can be used imaginatively to frame views rather than provide blanket vistas from every position. Do you really need scenic views when you are under your desk plugging in the phone charger? Is all that daylight entering below desk level really useful?

In deep-plan buildings the facade will have less influence on the total energy consumption, allowing full-height glass to be justified from an energy efficiency perspective. However, it is important not to forget about the comfort of those poor souls who have to sit next to that expansive wall of unshaded west-facing glass.

Greener facade design is limited only by the imagination, knowledge and collaboration of the project team. These are easy things to address. Why are we not we doing it more often?