Net-zero emissions is the globally agreed goal to avert catastrophic climate change – but what does it mean for buildings? 

How do you define a “net-zero building”? 

Net-zero is really a planet-scale concept. Net-zero emissions is the state that scientists convened by the UN have agreed the world needs to reach by 2050 at the latest, to limit global warming to 1.5°C and avert catastrophic climate change. This is what countries have agreed to work towards under the Paris Agreement. At this point, anthropogenic greenhouse gas emissions would be reduced to close to zero, and any residual emissions entering the atmosphere would be balanced by the removal of an equivalent amount, either by natural carbon sinks or technologies such as carbon capture and storage. The way we’ll achieve that state is by decarbonizing all human systems, including buildings. 

As for what level of decarbonization we need to achieve in buildings, there are many different regulations, standards and certifications that attempt to provide a framework for this process. The one thing they all have in common is that they are becoming progressively tougher, as the 2050 deadline, and the interim target of a 45% emissions reduction by 2030, races closer. According to the Science Based Targets initiative (SBTi), which translates climate science into corporate guidance, organizations must reduce their emissions by more than 90% to be in line with a 1.5°C trajectory.

So where does this leave buildings? “The UN’s definition of net-zero is ultimately the goal, so buildings need to follow the same terminology,” says David Leversha, net-zero carbon lead for WSP’s property & buildings division in the UK. “I think we need to be really concise and follow the UN and say that to call a building “net-zero”, we need to have reduced its energy use intensity by at least 70% and carbon emissions by 90% or more.”

“If our global goal is not to release any emissions to the atmosphere, we need to design buildings to be as energy efficient as possible, and to use clean, renewable sources of energy, and stop using fossil fuels,” says Martin Sing, energy and sustainability lead for WSP in Canada. 

But, he adds, this only addresses operational carbon emissions. It doesn’t take into account the carbon that’s emitted during the process of constructing a building: extracting the raw materials, processing them and manufacturing building components, transporting all of that to site and assembling it together. This “embodied” carbon has received less attention to date, but it can account for half of the total carbon footprint, and the proportion is rising as buildings become more energy efficient in operation and electricity grids are decarbonized. 

“It is technically feasible and realistic to deliver a building that is net-zero in operational terms today, and it’s been done multiple times,” says Sing. “But when it comes to delivering a net-zero embodied carbon building, we're not there right now because the manufacturers of materials aren't there. So that’s where we need to focus our efforts.” 

Some of this can be achieved by more material-efficient design, and some by embracing nature-based materials that sequester carbon as they grow, rather than materials derived from or produced using fossil fuels. “On smaller, low-rise building typologies – housing projects up to three or four storeys high, community hubs, visitor centres – you can use timber, bamboo, hempcrete,” says Leversha. “Then you can design them to Passivhaus standards to reduce energy consumption, and use renewables, and they will be net-zero in their build and probably net-zero in operation too.” 

For other buildings, there will still be some residual emissions, until we succeed in completely decarbonizing industry, energy networks and transport. 

Only up to 10%: that’s the maximum that the latest update to the SBTi says that organizations can offset to be compliant with the Paris Agreement and in line with a 1.5°C trajectory. Offsets have a role to play in the transition, but they’re not going to solve the problem. 

A higher proportion of offsets could result in a building that’s theoretically “carbon neutral”, but the benefit of this is diminishing as both regulators and public opinion take an increasingly sceptical line. In May, the EU parliament voted through draft legislation that would ban environmental claims based solely on offsetting schemes, while the UK’s Advertising Standards Authority also announced a crackdown on greenwash. 

“It’s interesting how quickly regulations are erasing any perceived marketing benefit from offsets,” says Antoni Paleshi, technical lead in energy and carbon analysis at WSP in Canada. “The SBTi update basically throws all of these voluntary ESG actions up in the air. Offsets can be beneficial if they’re genuinely new and promoting change in a place that needs it, but they’re effectively asking someone else to reduce their emissions. So it’s not really a fair way to say you’re net-zero because, really, you should be doing your own work.” 

More controversially, a draft standard just released by SBTi for consultation will no longer accept offsite renewable energy purchases in lieu of efficiency improvements, either via renewable energy credits or power purchase agreements. 

This means that deep emissions cuts of 90% in both energy use and the building fabric appear to be the only way to reach net-zero. 

Carbon reductions are traditionally measured as percentage improvements against a historic baseline – an assumption about the energy performance or embodied carbon of a generic building of a certain type. But this isn’t necessarily that useful, or a true representation of reality. “You're setting a baseline based on something that's fictitious,” says Sing. “Even if that’s the minimum standard, general practices often go above those standards to start with.”

There’s also the “performance gap” to consider: between the “regulated” loads that buildings are designed for, from heating, lighting and ventilation, and their real energy consumption, which can be several times higher due additional installations and occupant activity.   

Efforts are now underway around the world to move beyond percentage reductions, towards absolute values. For example, the UK Green Building Council is leading a massive industry-wide effort to develop a net-zero carbon building standard, which will set maximum allowable budgets for operational carbon in terms of energy use intensity in kWh/m2, and for embodied carbon in kgCO2/m2.

To do this, it’s taking both a top-down and a bottom-up approach, explains Kate Dougherty, head of net zero for building services at WSP in the UK, which is contributing to the standard. “There's only a certain amount of carbon we can emit over the coming years if we want to reach the net-zero goal. So the top-down approach is saying, OK, how much of that carbon budget should be attributed to buildings? Then, within buildings, how much do laboratories get? How much do offices get, how much do schools get? That gives you your kilograms of carbon and your kilowatt hours.” 

The bottom-up approach, meanwhile, is a massive industry-wide collaboration to gather together existing benchmarks, meter readings and embodied carbon calculations for the different building sectors, and then carry out modelling to identify current best practice, and what’s likely to be achievable in future based on technological improvements. “Then if the bottom-up and the top-down meet in the same place, that becomes the target. But it's highly likely that there'll be a gap, so then we have to work out what to do about that – do we have a stepped approach and how does that work?”

The challenge of reconciling the necessary with the possible will raise some difficult questions – particularly for new-build construction.  

In many cases, yes, particularly in developed countries. Given the high amounts of embodied carbon in new concrete and steel, retrofitting existing structures to reduce operational emissions may be the only way to create net-zero buildings in the short and medium term. 

“That's going to be a huge part of the UK net-zero definition,” says Dougherty. “One of its underlying core principles is that you can't knock anything down and rebuild it unless you've got a really good reason to do so, from a carbon perspective.” This is already planning policy in London, where for every proposed development there has to be a whole-life comparison of the carbon involvement in a new building over a 60-year lifespan and a deep retrofit. “You might not be able to make an existing building as operationally efficient, but given that the embodied carbon is about 50% of the total, you have to be very, very efficient in your new building to make it better in carbon terms.” 

Looking at whole-life carbon also informs the choice of materials, and makes the case much stronger for using components reclaimed from demolition, says Geoffrey Quintas Neves, head of sustainability and energy at BG Consulting Engineers in Switzerland, part of WSP. “You have to take into account not only the embodied carbon from the first construction, but all of the maintenance over the 60-year life cycle too. You might decide to invest more at the beginning in a material that only needs to be replaced twice, rather than three times. It’s also a great opportunity to take into account all of the reclaimed materials that could be used in a circular way.”

As targets and standards become progressively tougher, designing buildings is becoming a crystal-ball gazing exercise. 

“Building code compliance is no longer a one-day single-date thing,” says Matt Coulter, assistant vice president of WSP’s Built Ecology practice in the US. “You can build to code-minimum today, but in the next seven years, the requirements will change significantly. So we’re having conversations with owners about the financial risks of going above the code now or waiting until later to retrofit. The important thing is to understand this is coming, leave yourself pathways and push it as far as you can, so you don’t end up with a stranded asset or a large capital cost to deal with it.” 

“As a trusted advisor, we should be telling our clients the truth,” adds Leversha. “Everything is evolving so rapidly, we're not quite sure where these definitions will ultimately end up. We could advise them to just scrape through the next round, or we can explain the sensible logic of net-zero, help them follow those processes as far as practically possible, and create something that will stand the test of time.”