Building regulations - new domestic buildings - modelling of proposed energy improvements: research report

Conclusions

237. The aim of this project was to assess and identify potential improvements in energy and emissions performance for new domestic and non-domestic buildings constructed in Scotland set via Standard 6.1 (carbon dioxide emissions). This was to inform the setting of targets within the next set of energy standards, programmed for implementation in 2021. This report focuses on the project findings for new domestic buildings.

238. Improvements to the current notional (reference) building were identified based on an analysis of where construction in Scotland is already going beyond the current notional building specifications, and relevant literature including Part L consultation options in England and in Wales. These improved measures were assessed based on various criteria including their relative cost-effectiveness, feasibility and associated risks (e.g. poor indoor air quality and summer overheating). Based on this, two new alternative standards ("Option 1" and "Option 2") were proposed and their benefits and costs were assessed at an individual building and national level.

239. Option 1 comprises improvements to the fabric efficiency of the notional building and the inclusion of WWHR. Option 2 includes further improvements, including (but not limited to) triple glazing and the adoption of MHVR with improved air tightness. Developers can build to alternative specifications as long as they meet, or improve upon, the performance of the notional building.

240. The intention was, if practical, to base the notional building on a single fuel/heating system type. This would simplify the current approach where the fuel in the notional building depends on that included in the actual building. It could also help encourage the transition to lower carbon fuels. For both options, it is proposed that the notional building is based on gas heating + PV, with typically an increase in the array size compared to the current notional building. An exception is proposed if a heat pump is used in the actual building, where an air source heat pump (ASHP) is included and the PV element removed in the notional building. This is to help address the concern that using a heat pump in practice could significantly over-comply if compared to a gas-heated notional building with potentially an opportunity for a significant relaxation in fabric energy efficiency even with an improvement in the backstop values.

241. It is noted that the Scottish Government has indicated that for the next (2024) revision of Building Standards, the intention is to move to renewable or low carbon heating systems in new homes. One method to delivering this would be to build from the proposed approach for notional buildings to be based on an 'ASHP only' specification whatever the fuel in the actual building; this would result in a demanding target that would make it difficult for fossil fuel based heating to comply.

242. Options 1 and 2 are estimated to reduce carbon emissions by 32% and 57% respectively across the build mix. This was evaluated using the SAP 10.1 methodology, including SAP 10.1 carbon emission factors, across 7 buildings selected to represent common building and fuel types in Scotland. This compares to a recommendation in the 2007 Sullivan Report to achieve aggregate emission reductions equating to at least 27.5% on 2015 standards. Hence both options would meet this commitment.

243. It is estimated that Options 1 and 2 for a semi-detached home, as an example, will increase the capital costs by 4% and 7% respectively if gas heating + PV is used. This cost is reduced if ASHP is used, increasing the capital cost by 3% and 6% respectively, providing encouragement to lower carbon fuels. In general, it is estimated that the capital cost is lower for an ASHP for individual homes but higher for apartment buildings. A key reason for this difference for apartment buildings is that the amount of PV required is based on the foundation area of the building which reduces the cost of PV per apartment unit (and makes the gas heating + PV option more attractive) as the number of storeys increases.

244. The national cost benefit analysis shows that Option 1 results in a net benefit of £46m whilst Option 2 results in a net cost of £250m. This difference is driven by the incremental capital, renewal and maintenance costs for Option 2 being nearly double those for Option 1 and this negates the greater savings in energy use from Option 1.

245. In response to a 2018 amendment of the Energy Performance of Buildings Directive. the Scottish Government proposes that primary energy becomes the main target metric for building regulations compliance. Sensitivity analysis based on oil and gas CHP district heating, both higher carbon options than individual gas boilers, suggests that if the carbon emission target is retained as a secondary metric it will help encourage lower carbon fuels.

• A detached house with an oil boiler was modelled and the primary energy and carbon emission results were compared to the Option 1 'gas' notional building. To comply with both the primary energy and carbon targets, the 'advanced' fabric efficiency specifications would be required. The carbon target was the most stringent of the two targets and dictated the compliant solution.
• A mid-floor flat with a gas CHP was modelled and the primary energy and carbon emission results were compared to the Option 1 'gas' notional building. In this case it was not possible to comply with both the primary energy and carbon targets with the 'advanced case' specification and a higher efficiency PV panel. This was particularly dictated by the more stringent carbon target.

The Scottish Government does need to consider this within its broader strategic goal to encourage district heating. It also needs to consider that energy from waste district heating performs very poorly against the Option 1 'gas' notional building primary energy target which does not account for the fact that waste would otherwise go to landfill.

246. There is a policy decision as to whether direct electric heating should be compared to the 'gas' or 'ASHP' notional building. Sensitivity analysis showed that a direct electric solution could comply both with the Option 1 'gas' notional building and the Option 1 'ASHP' notional building using a specification somewhere between 'improved' and 'advanced' – the Option 1 'ASHP' notional building being more demanding. Direct electric heating is a well establishing technology and lower carbon compared to gas heating but less efficient than a heat pump resulting in higher fuel bills than gas heating and, if adopted at scale, a greater impact on the national grid.

247. The analysis also considered where the notional building target setting methodology may not reward energy efficient design. In particular the analysis focussed on the lack of incentive for improved efficiency through adjustments to built form and shape as the notional building dimensions used for target setting are defined as being the same as the actual building dimensions. One approach identified is to use a 'form factor' in the target setting methodology based on the total heat loss area per unit floor area. Analysis suggests a correlation between the form factor of the building and space heating demand. Further work would be needed to establish if there is a means of incorporating form factors in the target setting methodology which would reflect differences in space heating demands in a robust and fair way across a wide range of dwelling types and avoiding unintended consequences or loop-holes.