Building regulations 2025 - new non-domestic buildings: energy standard improvements - modelling report

6. Conclusions

The aim of this project was to assess and identify potential improvements in energy and emissions performance for new dwellings and non-domestic buildings constructed in Scotland set via Section 6 of the Scottish Building Standards (energy). This was to inform the setting of targets within the next set of energy standards, programmed for implementation in 2028. This report focuses on the project findings for new non-domestic buildings.

Impact of potential improvements to the notional building

Potential improvements to the current notional (reference) building were identified based on a review of current practice in Scotland and other relevant literature. Three new alternative standards (“Low”, “Medium” and “High”) were developed. The EPC database was analysed for indications of what building specifications are currently typical practice (“low” based on the 50^th percentile), good practice (“medium” based on the 75^th percentile) and best practice (“high” based on the 90^th percentile). Their benefits and costs were assessed at an individual building and national level. This analysis indicated that the current typical practice (“low”) is generally closely aligned to the current notional building specification and so broadly matches the current Section 6 standards, i.e. current typical practice does not go beyond the current standard. In comparison, the “medium” and “high” scenarios do model significant improvement beyond current typical practice (“low”).

Following a ‘fabric first’ approach, the performance of the building envelope was first considered for improvement. It was found that there was little evidence to support significant improvement in this area, and so all three options adopt the same fabric standard as the current standard in most areas. The principal exception to this is the inclusion of triple glazing for the “high” option. The “low” scenario differs from the current notional building to align with typical practice in terms of lower performance in air tightness, whilst the “medium” and “high” scenarios include improvements in air tightness.

After adopting building fabric improvements to reduce building energy demand, other measures were included to reduce the building’s energy consumption. Notably, the “medium” and “high” scenarios include improvements in heating, cooling and lighting efficiencies as well as an increase in the amount of solar PV compared to the “low” scenario. To reflect current practice as demonstrated in the EPC database, the “low” scenario uses a lower cooling efficiency and higher performance in heat generator efficiencies compared to the current Section 6 notional building.

For all three options, it is proposed that the notional building is based on zero direct emission heating systems plus PV with an increased array size compared to the current notional building. This would simplify the current approach where the extent of the PV depends on the heat source(s) in the actual building.

The “Low”, “Medium” and “High” options are estimated to reduce carbon emissions by -0.7%, 10.1% and 11.4% respectively across the build mix. This was evaluated using SBEM v6.1e and the proposed new carbon emission factors, across 11 building archetypes.

It is estimated that the capital costs of solutions range from -1% (lower) for the “Low option to +11% (higher) for the “High” option than the current standard across the different building types and the three alternative standards.

The national cost benefit analysis shows that the “Low” option results in a benefit of £2m whilst the “Medium” and “High” options result in a net cost of £33m and £330m respectively.

Differences between modelling in SBEM and PHPP

The Scottish Government aims to understand how the business-as-usual performance might differ across two different compliance engines: SBEM and Passivhaus Planning Package (PHPP). The way in which PHPP could be applied for building regulations compliance calculations needs to be understood.

In addition to differences in the building physics algorithms, PHPP requires significantly more detailed data inputs than are required for SBEM calculations. Therefore, a decision would be needed on the level of data entry that would be required for an alternative compliance calculation using PHPP rather than SBEM.

The results indicate that the building physics of the PHPP model does align closely enough with SBEM to predict a similar level of impact for individual and groups of specification changes, when the PHPP model is set up to align with the SBEM inputs. Therefore, it is likely that this would also be the case for a PHPP model that has been populated in line with PHPP conventions. This means that a potentially more accurate model, which is used to assess compliance with PH targets for energy consumption and space heating/cooling demand, could offer a robust comparison against a notional performance standard.

Further work may be able to better identify why PHPP and SBEM differ more significantly for specific energy uses. If methods to reduce this difference were developed, then this could further strengthen the case for using PHPP as a compliance model as well as a model for assessing performance against absolute targets. This work may be challenging due to the wide range of building typologies that Section 6 is required to regulate. It is also important to consider PHPP’s validity and accuracy for building typologies which are less commonly assessed (i.e. distribution warehouses and hotels).

In summary, it is evident that PHPP and SBEM generally agree on the impact of specification changes for non-domestic buildings, when an aligned modelling approach is applied (that is, using inputs in PHPP to as closely match those used in SBEM as possible). Further evidence would be required to confirm that the use of full PHPP conventions would agree to the same extent across a broad range of building types.

Scottish Government would need to consider carefully the approach to using PHPP as a compliance model for Section 6, including further testing using PHPP conventions as described above, before it could be adopted as an alternative approach to demonstrating compliance. It would be key to confirm that if both PHPP and SBEM were allowable compliance routes, the choice of compliance engine does not provide developers with the ability to game the system by selecting which compliance engine provides an easier route for Section 6 compliance for different building types.

Investigations into additional targets

The analysis has considered the potential for introducing a space heating demand metric as a new primary compliance measure within Section 6. The analysis has identified two key challenges with doing so.

1. The analysis has shown that, for certain typologies, the BAU specification can comply with an absolute target of 15kWh/m²/yr for space heating and cooling (as per LETI and Passivhaus guidance), although it should be noted that this would only be the case if the NCM gains assumptions were applied. However, for many of the building typologies assessed, using 15kWh/m²/yr as an absolute target for space heating demand would significantly improve standards beyond those modelled as best practice in this study.

It will be for the Scottish Government to consider whether a single space heating demand target applicable to all building types would be appropriate. An alternative is that the target could be varied between building types such that they are more similarly challenged. Further work would be required to develop these targets to an extent that drives the intended fabric performance improvements across all building types.

2. Should a space heating target be adopted as the primary compliance metric, this may not drive improvements in other energy end uses that are not impacted by fabric performance, such as lighting or domestic hot water, which can dominate the overall building energy demands for some building types. Hence, there would still be the need for an additional metric to drive broader improvements.

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. Our view is that to introduce an incentive for built form would be complicated to implement successfully and would be likely to have unintended consequences. Significant further research would be necessary to assess and develop such an approach in a robust and fair way across a wide range of non-domestic building types and avoiding unintended consequences or loop-holes.