How to design naturally ventilated low energy buildings with real fresh air

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The simple design moves that give you naturally ventilated low embodied carbon systems with genuinely fresh air, writes Andy Lerpiniere

What is a low-embodied carbon ventilation system? It’s a simple question, with a simple answer: natural ventilation. There are no fans with bearings or electric motors, or the carbon used in making them. All you need to do is open the windows, it is about as simple as ventilation can be.

At one time all buildings were naturally ventilated, and we understood how to make those buildings work, to make them comfortable, pleasant places to inhabit. With the invention of cooling systems, we no longer needed to rely on outdoor air to provide comfort as well as to remove odours. Buildings could be designed with great glass facades that could never have been made comfortable with natural ventilation.

The days of throwing cooling at the problem are largely behind us. The drive towards reduced energy use and sustainability has taken us in a different direction: highly insulated, extremely airtight buildings that require much less heating and cooling energy, but demand to be mechanically ventilated.

There is much that is good in this approach. The controlled delivery of fresh air in winter, with heat recovered from the exhaust air stream, is undoubtedly an energy efficient solution, particularly when compared to the simplest ‘open the window’ natural ventilation schemes. The argument of course is that the uncontrolled supply of cold winter air directly affects heating bills and the comfort of the occupants.

Benefit balance

But natural ventilation can also deliver low energy in use. The key is in controlling the supply of fresh air. This adds an element of complexity, a requirement for equipment that can control how air enters the building, such as automatically operated vents that are linked to carbon dioxide sensors and temperature sensors. Such sensors would be carefully positioned to monitor conditions in the occupied zone, typically at head height, and connected to a central building management system so that operation can be monitored. Hardly ground-breaking, in controls terms, but it does move the system away from the beautifully simple ‘open the window’ solution.

The key is in controlling the supply of fresh air, which adds an element of complexity

The potential energy saving, from reduced heating requirements, and the improved levels of comfort, can justify the increased complexity and embodied energy.

There are other reasons why we’ve chosen to move towards hermetically sealed buildings, especially within our cities – principally noise and pollution. It’s interesting then to think about the future of our cities. Looking beyond current congestion charging and low-emission zones, vehicles of the future will be predominantly, if not fully, electric. Gas-fired boilers, and combined heat and power engines, will be phased out. The main sources of noise and pollution will be much reduced and one day will no longer exist. Outdoor can then truly be considered ‘fresh’.

On the other hand, global warming means that we will have increased temperatures. Part of the solution may be that we move out of the city to cooler climes during the summer. Lockdown has taught us that there are other ways of working. I hope, more importantly, that it has also instilled a sense that there are other approaches to how we live and work. Not working during the hottest two weeks of the year may be part of a pragmatic solution to dealing with increasing temperatures in low-energy buildings.

Summer fun

It is still possible to design naturally ventilated buildings that work in the summer. Heat can be kept out by limiting glazing; just enough for good levels of daylighting, positioned to avoid too much direct solar gain. Shading can keep out the sun. Creating buffer zones can ‘protect’ the most important, most used spaces. Thermal mass, provided with low-carbon stone, will cool overnight and deliver that cooling back into the building by day. And the use of controlled natural ventilation, with carefully designed cross ventilation enhanced by natural stack effect.

Designing and delivering naturally ventilated buildings can deliver low carbon on all fronts, and be beautiful places to be.  

This article was originally published on The RIBA Journal.