In much of the world, buildings are responsible for at least 40% of a country’s carbon emissions. This makes buildings more important than any other sector when trying to address the climate emergency. We clearly need to be building low energy, preferably zero energy, buildings. Unfortunately, if we look at the data for the energy use of most new buildings we know that we haven’t in general been delivering such buildings. In fact it is common for new buildings to use roughly the same amount of energy as much older ones.

At the University of Bath we believe modelling the energy use and carbon footprint of buildings can help us out of this position, but it has to be a certain type of modelling, done at a certain moment during the design cycle. In essence we are talking about modelling for design, not compliance with building regulations or codes.

When to Model: Early for Maximum Impact

Given that orientation, glazing ratios, wall construction, shape and many of the materials get decided very early on, unless we model almost from day 1 much of the potential to improve the energy and carbon performance of the building will have been lost.

The energy engineering team are unlikely to be appointed until much of the architecture has been fixed, so it is clear the architect might need to be doing the modelling, not the engineer. And whoever does it, the tools need to be quick to use and designed to be used at an early stage of the design cycle – when the values of many variables are unknown or only approximately known.

What to Model: Operational energy use

We clearly need to include the energy used to heat and cool the building plus that used to provide hot water and electricity. We call this the operational energy use of the building. We may also need to account for integrated renewables.

So what needs to be modelled to get a good idea of the operational energy use of a building?

1. The heat losses or gains through the opaque fabric (walls, roof and ground floor).

2. The energy balance of the windows. Windows lose heat but they also gain heat by letting sunlight into the building.

3. Building Systems. Such as the hot water distribution system.

4. Thermal Bridges. These are conducting elements such as metal wall ties or steel lintels, or window frames.

5. The movement of air into and out of the building. We normally break this down into infiltration (through cracks etc.) and ventilation (opening windows, etc.)

6. Heat from incidental gains, for example those from artificial lighting or body heat.

What to Model: Better to Guess than Ignore

Given this list of items to include in the operational energy model, the next question is how much detail do we need to provide about each item? The bad news is that you need to include quite a lot of information and ignoring even minor bits of the physics leads to large errors. The good news is that many of the variables can be represented with default values. The important thing is to include them, not ignore them. Ignoring them is really just including them but setting them to zero. Just about any guess you can make will be better than this.

What to Model: Embodied carbon

Not only is energy used to run a building, a lot of energy will have been used to make the construction materials, then assemble them on site, and this will have resulted in carbon being emitted to the atmosphere – we term these the upfront embodied carbon emissions. For a typical building these carbon emissions with be similar in size to the carbon emissions resultant from many years of operational energy use. However, as a low energy building will hopefully have a very low level of operational energy use, the embodied emissions can start to dominate the whole-life carbon foot print of the building.

Getting an accurate value for the embodied carbon emissions is difficult, especially during the early design stages as the detailed structural engineering will not have been completed. However benchmark figures do exist and these need to be included in the modelling so you get some idea how important embodied emissions might be, and whether any building integrated renewables might be able to offset them. Without this knowledge promising a client that you can deliver a zero-carbon building might be setting an unrealistic expectation.

Conclusion

So what can we conclude?

1. Firstly, given the critical role buildings will play in the climate emergency, we need to be able to deliver low energy buildings. We know some people can deliver such projects, we now need to help everyone to deliver them. We believe modelling can really help with this.

2. Being that most of the important energy and carbon decisions are made early in the design cycle, modelling needs to be done right at the start, and hence by those present at the start, who are unlikely to be modelling specialists.

3. This means that any tools need to be quick and easy to use and hence possibly based on an interface most people already use, such as EXCEL.

4. Such tools need to cover operational energy use, embodied carbon and renewables.

5. They also need to be free, to encourage their use.

6. Things like systems and their losses need to be included right from the start. After all you don’t what to find out late that the main focus should have been in reducing hot water distribution losses rather than heat loss from the roof.

7. Complex factors such as shading and distribution losses need to be built-in and set to sensible default values, rather than set to zero.

With is in mind, we created ZEBRA (or the zero energy building reduced algorithm) to allow almost anyone to model the energy use and carbon footprint of a building on day 1.

We hope you will find ZEBRA useful.