Understanding how buildings use energy is key to tackling the climate crisis, not only from an architect's point of view, but also from a building user's point of view. Sustainable architecture should fundamentally aim to be energy efficient to reduce our demand for natural resources, but when it comes to talking about building energy there is a lot of terminology and many buzzwords that get used.
Some of the terms that often get used in the context of sustainable architecture include ‘passive’, ‘net zero’ and ‘offsetting’. But what does each of these mean and what are the implications on building energy demand?
The context for this conversation is that we currently don’t have a realistic clean energy source that has no environmental ramifications. No matter how we produce and distribute energy it has some impact on the environment. In theory, the only way to produce a completely sustainable building would be to avoid the use of energy altogether. In practice however, this isn't realistic as human health and comfort is also a priority that needs to be considered in inhabited spaces. So we have to find ways to design that minimise our energy consumption, whilst maintaining internal comfort.
The first and most important design strategy for reducing energy consumption in a building is passive design. This refers to how the design uses the physical characteristics and physics of the building to move air and protect from the elements whilst creating a comfortable internal environment.
The shape of the building can be used to drive natural ventilation, shade from overheating and use physical mass to protect from outdoor temperatures. However, this has limitations when you bring into play weather extremes. High or low external temperatures, wind, rain etc will have an impact on what temperature the internal space reaches. This can also be affected by large numbers of people in a space, which can cause temperatures to rise and also contributes to higher levels of CO2, which makes a space feel stuffy and can cause drowsiness.
So passive design can work in certain circumstances, and should be used where possible, but it doesn’t work for all building types and uses. This is when mechanical systems are introduced to drive ventilation, provide heating and achieve good light levels.
Low energy design
The vast majority of buildings use energy for heating, cooling, lighting and ventilation, which consume enormous amounts of energy globally. By designing to passive principles we can reduce the energy demand of the building but it is also possible to design for low energy mechanical systems.
Passivhaus is a design standard that aims to minimise energy consumption of a building by highly insulating and providing good air tightness, but crucially, these systems are supplemented with energy efficient mechanical systems that maintain internal comfort. The ‘MVHR’ (mechanical ventilation with heat recovery) draws stale air out of the building, and extracts the heat from this to warm up fresh air coming in. This means that wasted heat from our bodies, computers and kitchens gets reused to heat the building.
By thinking resourcefully about space and energy we can reduce the energy intensiveness of our buildings and further reduce our demand for natural resources.
Net Zero Energy Building
Ideally, a sustainable project will have implemented as many energy efficient design considerations as possible from the outset, but there will likely still be a remaining demand for energy in the building.
Net Zero Energy describes the idea that the total amount of energy consumed in the building can be matched by energy generated on site. This could include energy generators like solar panels, wind turbines, or ground source heat pumps. You therefore end up with a ‘net zero’ expense on energy as all the energy you need is created by the building.
Net Zero Carbon design
‘Net Zero’ often gets mentioned in isolation, but it’s important to understand the difference between ‘net zero energy’ and ‘net zero Carbon’ design approaches. In principle, these both relate to the idea that the amount of energy or carbon required in a building is balanced out by applying additional measures that balance the scales.
In the construction of buildings, carbon is emitted as part of the construction process (embodied carbon) and in the operation of the building (operational carbon). To reduce this impact, designers can consider measures to reduce the amount of carbon emitted.
Operational carbon is directly related to the energy efficiency of the building as it is largely a result of the amount of energy used in heating, lighting, ventilating and powering the spaces. To reduce these, passive design strategies and on site energy can greatly reduce the operational carbon emissions.
Embodied carbon is more tricky and is connected to the types of materials and construction methods used. The best way to reduce embodied carbon is to source materials that are least energy intensive to extract and manufacture, or that sequester carbon. Sequestering refers to the process of absorbing and ‘locking in’ carbon, specifically in wood, which absorbs carbon during the time that it grows. This is then contained in the physical material of the timber, which can be stored in the fabric of a building. The overall effect is that more carbon is locked into the fabric of the building itself, than was emitted as part of the construction process.
Net zero Carbon buildings are achieved by balancing out the amount of carbon emitted in the construction process. This process of balancing is commonly known as ‘offsetting’. In addition to the example mentioned above, using timber to offset carbon, offsetting could be achieved by including on site energy strategies, or by contributing to planting trees which absorb more CO2 during their life cycle.
Offsetting a term that is often advertised by companies or projects as a sustainability credential, to advertise ‘greenness', and whilst it is a good measure to balance out an otherwise sustainable design, it is not best practice by itself. The best approach is not to offset at all, but aim to reduce emissions from the outset of a project. It’s important to highlight that large scale extraction of fossil fuels can’t be undone, regardless of what offsetting measures are used.
Offsetting measures such as planting trees, whilst great for biodiversity and CO2 absorption, will not have a meaningful effect on carbon emissions in the short term. If we are attempting to offset large amounts of energy spent as a result of inefficient building design, we are applying a band aid solution to a deeper problem.
Terms such as ‘offsetting’ are often presented as a complete approach to sustainability, but do not address the more fundamental work that needs to be done to meaningfully impact climate change. Reducing building energy use and carbon emissions are central to creating sustainable architecture and it is essential to make sure that terminology of this topic is well understood to help us evaluate sustainability claims when they are made.
- Maria Henshall