The value of EPC ratings

With Minimum Energy Efficiency Standards imminent, EPCs will take on added value. Hywel Davies looks at how to avoid inaccurate ratings

Readers should know that, from 1 April 2018, it will be unlawful to lease – or renew a lease on – a building, or a part of a building, with an F- or G-rated Energy Performance Certificate (EPC). From 1 April 2023, existing leases on such buildings will no longer be lawful. As a result, many landlords and some tenants are actively assessing the energy ratings of their buildings ahead of the deadline.

Many EPCs are being dusted down, their accuracy reviewed and questions asked about what needs doing to ‘sub-standard’ properties to get them an E rating. For some F-rated buildings, the question is, ‘Is it really an F?’ In other cases, owners are seeking advice on how to improve the building, and what impact relevant energy efficiency measures might have on the rating.

Some readers may also be aware that a recent paper in Building Services Engineering Research and Technology (BSERT) looked at the advice given by a range of energy modellers on energy-saving measures – and generated some controversial press coverage. It asked 108 building modellers to comment on the importance of obtaining and using accurate values for 21 common modelling input variables – from U values to occupancy schedules.

The research team created an energy-simulation model of a house, then changed each of 21 variables to assess the impact on the model, and to create a ranking of the most significant effects of changes to that variable. This revealed the U value, ventilation rate and internal temperature set point as the most significant variables for the building being modelled.

There are ways to ensure advice on EPCs is given by the most competent advisers

Participants were initially asked to name the three most significant variables for determining energy demand, and to name three that they did not think were significant. Having done that, they were given a list of variables and asked to rank them in order of the effect on energy performance. This revealed a significant level of variation between the modellers: 68 identified U values as the most significant variable, with 64 choosing air-tightness and 48 internal heat gains. Only 39 opted for ventilation rate, and three identified the internal set point.

When asked to order 21 possible variables in order of impact, U values were ranked after glazing ratio, while the set point and ventilation rate were 8th and 12th on the list. The study claims that there is a lack of ‘modelling literacy’ among energy modellers.

While the BSERT study does reference a number of other papers, it does not refer to the Royal Academy of Engineering report on building engineering physics, and its importance in delivering a low carbon building stock in the UK1. Nor does it cite the academy’s follow-up study, which made the case for the creation of Centres of Excellence, to teach the various aspects of low-energy design across built-environment disciplines2.

The coalition government declined to support the centres, but four of them were established with the active support of CIBSE in 2013, and they have been delivering interdisciplinary training ever since. It is too early for their graduates to have reached the employment market yet, so the BSERT study serves to underline the importance of the centres, and demonstrates that the original call by the academy and CIBSE was well-founded.

But what of the building owners and occupiers who want accurate EPCs now? The BSERT paper is a warning to all that there is a range of expertise in energy modelling. However, there are ways to ensure that advice on EPCs – and how to improve them – is given by the most competent advisers.

CIBSE Certification operates a scheme for Level 5 Energy Assessors, who are approved to use simulation software – rather than the Simplified Building Energy Model – to produce EPCs. To be approved, they not only have to pass the CIBSE tests for Low Carbon Energy Assessors, but also be trained – and successfully examined in – the simulation package they use for modelling. We do not know whether any of the 108 modellers are accredited energy assessors – they were not asked.

So, by using a CIBSE Level 5 assessor, it is possible to ensure you are using an energy professional who does understand the impact of thermal insulation, ventilation rate and internal set points – and who can offer reliable advice to landlords on how to achieve an E rating on their building. They can even help a tenant to understand why a claimed E rating might not be that good.

CIBSE energy modellers are literate and ready to assist clients who want to understand the energy performance of their buildings.


  1. Engineering a low carbon built environment: The discipline of Building Engineering Physics, January 2010, RAE.
  2. The case for Centres of Excellence in sustainable building design, May 2012, RAE.
  • Dr Hywel Davies is technical director at CIBSE