Will the UK’s new home energy-performance standard offer certainty or a lottery?

By Steven Heath, Technical Director
August 09, 2023

Our first new-build case study published last week highlighted a significant performance gap in an estate of 20 new homes in the North West. Measured space heat demand peaked at 6 times that predicted in the EPC while modelled heat pump running cost peaked at a greater differential.

Today we report a real performance study on an estate of 12 new build homes built to Part L 2013 Building Regulations. The results offer a more nuanced picture with a less dramatic performance gap & fresh insights into how getting build quality right can bring homes back in line with design targets. The outliers in the study however further highlight the importance of real performance measurement in Government’s 2025 Future Homes Standard if a shift to low-energy electrically-heated homes is to be a success.

The EPC space heating demand estimates for the 12 homes ranged from c1,750 kWh pa to c2,850 kWh. This low expected demand is driven as much by the small size of the homes - between 74m2 & 85m2 – as low-energy design ambitions.

Before the results, a short description on what is being measured to compare to the EPC estimates. Raw data collected via small sensor boxes is worked on by an algorithm to create multiple real performance metrics: one being kWh pa home heat requirement in ‘averaged’ conditions. Knauf Energy Solutions (KES) can measure the efficiency of the home (the ability of the home to slow heat loss to the outside) and then plug a ‘basket of average weather, thermostat & heating system timer settings’ through that model to give a kWh heat demand over a year. Very similar to an Energy Certificate’s annual space heating estimate – but accurate, reliable, and trustworthy.

The graph shows EPC estimated heat demand vs KES’ measured demand. A performance gap is evident on the right-hand side of the graph. Homes 1-4 in the study are smaller, mid-terrace homes, 5-7 are end- terrace properties (so more exposed facades but still small) while homes 8-12 are larger semi-detached homes.

The small mid-terrace properties perform mostly in line with design expectations, however as homes got bigger, and with more surface area exposed to the weather, a consistent performance gap opens. In short, where more can go wrong, more did. The expected heating bills - if heating with a gas boiler at current prices - sees the gap between prediction and measurement as high as 65%. Significant, but not nearly as significant as our previous case study, and a relatively benign cost impact when considering a traditional gas boiler and lower gas unit cost to electricity.

As with our previous case study we saw common build errors including poorly fitted rigid board insulation in the walls, unsealed service penetrations (wires, pipes, etc.) into the loft and poorly fitted loft insulation. Unlike the previous study we weren’t limited to just ‘fixing’ the loft insulation by removing the existing material, sealing all service penetrations, and fitting lower lambda insulation ensuring the crucial connection with wall insulation at the soffits and gable ends.

We were also able to blow-in mineral fibre insulation to the 50mm residual cavity in the exposed walls of all homes. To note, these two retrofit measures would not have changed the dial on the EPC space heating estimates much – a small reduction for the additional insulation in the external walls while the loft insulation would be deemed a like-for-like replacement.  

These two retrofit steps saw measured improvements in all 12 homes. Most notably, it brought the measured space heating figures in line with, or better than, the original EPC design expectations in all but 3 homes. 

The apparent link between greater property size / level of detachment and presence of a performance gap is broken.  5 of the 8 end-terrace and semi-detached homes came back in line with EPC / SAP software expectations. We were not able to do further diagnostics on the 3 remaining properties to understand what was driving the continued underperformance although, at least for property 12 the ‘underperformance’ may be within a modelling performance gap rather than indicating a flaw with construction.  

A glass-half full interpretation of these results suggests that homes can be built to perform against design expectations. And in some instances, the design model (SAP software) can act as a reasonable design proxy for completed home target energy performance. A positive spin on the criticism that ‘as these homes grew in size and exposed surface area, a performance gap became evident’, is that this study shows a performance gap can be closed when installing the right insulation with the correct care and attention. The fact there were still ‘performance gap’ outliers post-retrofit, indicates the need to show that care and attention from the outset, as attempting to go back and address issues once homes are built will not always be as straightforward as our retrofit solution.

Given the Future Homes Standard transition to electric heating, it is worth modelling the heating these home archetypes may have installed. Unlike the larger homes in our other new-build case study, these could be considered suitable for direct electric heating rather than heat pumps.  

At an electricity kWh unit cost 4 times the cost of gas, bills would take a significant hike relative to the gas boiler cost modelling. The FHS will offset the higher electricity unit cost with a more stringent target fabric efficiency requirement leading to a lower expected heat demand. So fewer kWh needed to achieve indoor comfort even if the kWh unit price is higher. But how will we know this efficient fabric has been achieved in reality?

Without real performance measurement, the EPC estimates would be unverified. Occupants would have no understanding whether the EPC was an accurate, reliable and trustworthy reflection of running costs under average occupancy. It is they who will bear the risk of any performance gap which, in an electric heating world is likely to have more significant implications than now.

The Future Homes Standard can’t leave unsuspecting homebuyers to carry the risk these outcomes aren’t delivered. The technology is here, and is being assessed by Government’s SMETER programme[1], to measure home fabric efficiency. Affordable, scalable technology that can tell occupants if they received the efficient home promised in the design. The FHS should require at least 5% of homes to be real performance measured by an approved technology.

The role of measurement is also there to tell the housebuilder what is working. And in this case study we show closing much of the performance gap is not likely to be too challenging where care and attention is taken. When published, search the Future Home Standard consultation for reference to ‘real performance’ measurement to know whether the policy has any chance of giving new home buyers the warm, low-energy, low-carbon, low-cost-to-run homes they are paying for.

As a corollary, we re-checked the EPC space heating figures for this write up. 7 of our 12 homes had received brand new updated EPCs as part of a wider stock condition survey unrelated to our project. The new EPCs estimated kWh pa space heating double the original EPC / SAP software calculation. 5 of the new home predictions were now 2 to 3 times higher than the Knauf Energy measured value, while the 2 remaining home EPC estimates (outliers in the original analysis) are now much closer to their measured value.  It’s time to stop guessing and start measuring!


[1] Smart meter enabled thermal efficiency ratings (SMETER) technologies project: technical evaluation - GOV.UK (www.gov.uk)