The Cost and Delivery of Net Zero Carbon Buildings

Adam Mactavish

Director

Currie & Brown

Since the concept of a zero-carbon building first gained widespread recognition in 2007/8, the costs of meeting the standard have been a topic of ongoing and sometimes heated discussion.

This debate is not surprising as, in my experience, trying to ascertain the costs of a building to a zero- carbon standard has been like trying to nail jelly to the wall. Not only has the definition of what constitutes zero carbon changed regularly, but the costs, performance and carbon impact of key technologies have changed dramatically.

For example, in the last decade the costs of photovoltaics have fallen by over 80%, while the efficiency of lighting has doubled. Meanwhile the carbon intensity of grid electricity is now less than a quarter of the level of 2010. Add to this the inherent variability in costs between different types of project, location, design, or client and the challenge in establishing a clear evidence base for costs becomes clear.

Our experience of working with a wide range of engineers and designers on ultra-low energy and carbon projects suggests a few key considerations are important in achieving the most cost-effective results:

  • Design, orientation, and form factor – an efficient design can achieve a substantial reduction in energy demand and carbon emissions. These benefits are not recognised by the Part L assessment method, which assesses performance against a notional building of the same design. When targeting absolute energy and carbon standards, however, an efficient form is a real advantage and can reduce the amount of work (and cost) required of the fabric.
  • Development of an airtightness and ventilation strategy – our modelling for the Committee on Climate Change illustrated that very high levels of airtightness (<1 m3m2hr) can make a major saving in heat demand and can provide a little leeway to increase U values of external walls without compromising performance. On some projects, for example high-rise developments, this can be really valuable in managing the depth of the wall construction. Establishing a supply chain that can reliably deliver airtight buildings will be a real aid to reducing the costs of ultralow energy and carbon buildings.
  • Electrification of heating – heat pumps are likely to become the typical form of providing space heating and hot water to new buildings. Gathering experience and supply chain partners who can help in transitioning to these technologies will be important in managing costs, while ensuring they operate effectively and meet occupiers’ needs. Where teams can deliver reliably low space heat demand, it may also be possible to consider direct electric heating (with heat pumps for hot water). This approach can result in lower cost than even a gas boiler alternative and can have lower total occupancy costs than a heat pump once plant replacement is considered. There is a risk, however, of higher bills for occupiers if the building does not perform as well as designed.
  • Wastewater heat recovery – often overlooked when looking for energy efficiencies, vertical heat recovery of shower wastewater can deliver significant reductions in running costs in buildings with high hot water demand, for example houses or hotels. For a relatively small capital cost these savings can help make heat electrification more affordable for occupiers.
  • Realisation of savings in the heating system – in housing in particular the investment in a highly energy-efficient building fabric should enable a significant reduction in the size of the heat distribution system, which will deliver cost savings that help offset the costs of the high-performance fabric.

For new homes to be able to achieve total or near total decarbonisation on site and save households several hundreds of pounds per year, an additional outlay, broadly equivalent to a couple of years’ tender price inflation, appears to be a pretty sound investment.

Adam Mactavish, Director at Currie & Brown