Building regulations - new domestic buildings: identification and assessment of energy standard improvements

4. Definition of Business as Usual (BAU) Specification

In order to determine the proposed BAU specification case, the following data sources were analysed as key indicators of typical performance:

• Scotland EPC data from 2021 to 2023;
• Example archetypes supplied by a Scottish Government stakeholder; and
• Existing 2023 notional building standards.

The BAU specification seeks to present a cost optimal, minimum level of performance which could be delivered to achieve the notional TDER (as calculated in SAP 10.2 of the April 2024 Technical Handbook notional building specification) or demonstrate a marginal improvement.

4.1 Fabric

The U-values proposed for key fabric elements (floors, roofs and windows) are aligned to the 50^th percentile values determined from analysis of the EPC register, and with consideration of the maximum area-weighted average U-values set out in Table 6.2 of the April 2024 Technical Handbook. Whilst poorer performing than the notional, these are shown to be achievable and deliverable in implementation. Industry standard practices, such as standard thicknesses of supplied insulation, limit minor incremental improvements to U-values. Furthermore, element thicknesses typically reach tipping points for other parts of the design, such as maximum spans of timber/wall ties or strip or point foundation widths. Consulting the EPC register, the mean U-value performance was found to be 0.20, 0.12 and 0.15 W/m².K for walls, roofs and floors respectively. These were all poorer performance than the March 2021 and June 2023 notional specifications, indicating that the most cost-effective solutions may rely more on the carbon savings from building services than from fabric. It is noted that the dataset contains entries from 2021, which may be built out to standard pre-dating the March 2021 update to the notional specification. When filtering was applied, the mean performance value of the external wall improved to 0.19W/m².K for dwellings delivered between 2022-2023. Very few dwellings within the sample were required to comply with the June 2023 notional performance standard. However, as an indication of future performance, a wall U-value of 0.17 W/m²K has been presented in the BAU specification to align with the maximum area-weighted average U-value set out in Table 6.2 of the April 2024 Technical Handbook.

Scottish Government provided feedback from working group members, which indicated a typical approach to complying with the 2023 standards. These did not push U-values significantly beyond the mean values in the 2021-2023 EPC dataset. The wall U-value was marginally improved to 0.17W/m².K and the roof to 0.11W/m².K. Most of the improvement on the TDER was obtained through the use of mechanical ventilation with heat recovery (MVHR) and lower air tightness than the 2023 notional performance specification.

Analysis of the EPC dataset indicated that low air tightness is not a common approach for new build housing in Scotland. This is likely to be due to the requirement for supply and extract ventilation for dwellings with low air tightness. The EPC dataset indicated that approximately 25% of new build homes applied this strategy and the mean air tightness level was observed to be 4.6m³ /h.m²@50pa. An air permeability of 4.6 m³ /h/m² @ 50 Pa is suggested for the BAU. As this level of air tightness is below the level permitted for naturally ventilated dwellings in Section 3.14 of the technical handbook, it is proposed that the BAU will have continuous mechanical extract ventilation.

The EPC register did not provide specific window performance data. The most common new build solution was for low emissivity double glazing units to be installed, this was also supported by the stakeholder feedback. Considering the average performance of double-glazing units for the window types expected on new dwellings, a U-value of 1.4W/m².K has been assumed. This aligns with the maximum area-weighted average U-value set out in Table 6.2 of the April 2024 Technical Handbook.

The g-value of the glazing and thermal bridging performance are assumed to be aligned to the notional specification. It is noted that final g-values may now be driven by overheating mitigation requirements. Thermal bridging contribution to total dwelling heat loss rates are expected to significantly change depending on the archetype. Thermal bridging is also sensitive to the wider specification performance and the construction type of the dwelling. Given the variability across new build housing stock, this modelling does not consider changes to thermal bridging performance, rather the current Section 6.1 notional performance thermal bridging assumptions (i.e. SAP 10.2 Appendix R psi values) has been maintained.

4.2 Services

Three subsets of specifications are proposed for the building services to represent the three permissible heating technologies options under the Scottish New Build Heat Standard. This considers that individual gas boilers are no longer permitted to be installed under new build heat standards, although SAP 10.2 methodology assumes a gas boiler within the notional dwelling specification for dwellings which are not served by a ASHP or DHN connection. The specifications associated with each of the technology options are defined through the subsequent sections.

4.2.1 Air Source Heat Pump (ASHP)

An ASHP is expected to be the most common heating technology (anticipated to be installed in 65-70% of new build dwellings under the New Build Heat Standard). For dwellings with greater number of exposed sides (i.e. detached and semi-detached houses) it is expected to be significantly more challenging to meet the TDER without a high efficiency heating system. This is due to the higher space heating demand associated with these archetypes and relatively low level of cost effectiveness to reduce delivered energy consumption through fabric and renewables. The heating system efficiency is sensitive to dwelling design demand which in turn is sensitive to dwelling size. The efficiencies of each system are presented in Table 4 below.

Hot water heating is assumed to be delivered by the main heating system, with the hot water storage sized relative to the system capacity. Hot water cylinder standing losses have been based on an Ecodesign for Energy-Related Products (ErP) rating of B, which is assumed to be representative of current new build dwelling performance.

As discussed in the fabric section, analysis of EPC data illustrates that approximately three-quarters of dwellings propose natural ventilation rather than a mechanical system. As an air permeability of 4.6 m³ /h/m² @ 50 Pa is proposed for the BAU, continuous mechanical extract ventilation is required. This will take the form of decentralised continuous extract fans in wet rooms with trickle ventilators throughout other rooms.

Lighting performance is assumed to be aligned to the notional specification. The performance of installed luminaries in new dwellings is expected to align closely with the wider market requirements for Ecodesign.

Additional Low or Zero Carbon Technologies (LZCs) such as WWHR and PV are not proposed with respect to the proposed BAU specification. Based on the EPC data and example archetypes these are more commonly shown to be combined with fossil fuel or heat network heating systems to lower the (DDER) and Dwelling Emission Rate (DER).

4.2.2 Direct Electric (DE)

Electric panel heaters are proposed as an alternative BAU space heating source (anticipated to be installed in 10-15% of new build dwellings under the New Build Heat Standard). This includes both convective and radiant emitters as well as purely radiant emitters, as SAP 10.2 does not differentiate. AECOM is aware that there is a rising popularity of radiant electric panel heating, and the behaviour of this technology could be better understood through the detailed modelling of the HEM. For direct electric generally, hot water heating can be delivered by either immersion or a ducted air source heat pump supplying DHW only. Based on previous cost data provided by Currie and Brown (relevant to England but assumed to be valid for Scotland), an electric heat pump for domestic hot water has been found to be more cost effective than an immersion tank due to the reduction in PV generation requirement. However, dialogue with Scottish Government has concluded that very few developers are implementing this approach. The BAU case therefore proposes the use of a dual immersion heater for hot water. A slightly smaller cylinder size has been assumed when compared to the ASHP strategy. Table 5 sets out this sizing for each archetype. The standing loss from the cylinder has been based on the insulation requirement to achieve an ErP rating of B.

The ventilation and lighting are proposed as per the ASHP specification.

Use of LZCs is required to achieve compliance with building regulations. PV has been proposed to the DE BAU specification has been sized relative to the ground floor area of the dwelling to define the BAU specification.

4.2.3 District Heat Network / Communal Heating (DHN)

This scenario proposes DHN connection/communal heating system to serve space heating and domestic hot water demands of each individual dwelling in the block (anticipated to be installed in 15-20% of new build dwellings under the New Build Heat Standard). The DHN was assumed to be served by ASHP and peaking plant (assumed to be gas boilers) in a 90:10 split based on typical efficiencies for the central plant. An indirect HIU was assumed with the SAP 10.2 default standing loss rate. Distribution losses have been set at 1 as to not cause any issues with the notional dwelling calculation.

The ventilation and lighting are proposed as per the ASHP specification.

Use of LZCs is required to achieve compliance. PV was included within the BAU specification and sized relative to the ground floor area of the dwelling in order to define the BAU specification.