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

Task 3: Modelling Options for a New Notional Building Specification

1.11 Modelling of national profile to improved standards

1.11.1 SAP modelling results

170. The seven building sub-types set out in Table 1.11i were modelled using the latest available version of SAP, SAP 10.1 (BRE, 2019), and the two levels of specifications set out in section 1.10: 'improved' (gas / ASHP) and 'advanced' (gas / ASHP). A summary of the cases modelled for each dwelling type is set out below:

• Gas + improved fabric + natural ventilation + PV ('Gas improved')
• Gas + advanced fabric + MVHR + PV ('Gas advanced')
• ASHP + improved fabric + natural ventilation ('ASHP improved')
• ASHP + advanced fabric + MVHR ('ASHP advanced').

171. The results for the 2021 options were compared to baseline results for 2015 compliant cases obtained using the specifications set out in section 0 and the same version of SAP (as described in section 1.5.9).

172. Key results from the modelling at the individual dwelling level are set out in Table 1.11a to Table 1.11d. The columns show the DPER and DER calculated using the SAP 10.1 methodology (including SAP 10.1 carbon emission factors and primary energy factors). These are compared to the equivalent results for the gas 2015 compliant ('BS2015') base cases previously presented in section 1.5.9. DPER is shown first here as this is proposed to be the primary target metric.

Table 1.11a: SAP 10.1 key modelling results for detached dwelling type – carbon emissions and primary energy

	DPER	% reduction in DPER vs 2015 gas case	DER	% reduction in DER vs 2015 gas case
Gas BS2015 case	69.0	0%	13.0	0%
Gas improved case	45.0	35%	8.6	34%
Gas advanced case	28.0	60%	5.0	61%
ASHP improved case	34.3	50%	3.3	75%
ASHP advanced case	27.4	60%	2.6	80%

Source AECOM modelling using BRE's SAP 10.1 software SAP.exe, Build 7, 20/01/20.

Results recorded in AECOM, '210507 Scotland Building Standards 2021 - SAP10.1 Results – v10.xls'

Table 1.11b: SAP 10.1 key modelling results for semi-detached dwelling type – carbon emissions and primary energy

Semi-detached house	DPER	% reduction in DPER vs 2015 gas case	DER	% reduction in DER vs 2015 gas case
Gas BS2015 case	80.2	0%	15.0	0%
Gas improved case	51.6	36%	9.9	34%
Gas advanced case	32.7	59%	6.1	60%
ASHP improved case	38.8	52%	3.7	75%
ASHP advanced case	31.4	61%	3.0	80%
ASHP BS2015 case	63.7	21%	6.1	59%

Notes The ASHP BS2015 case is included for information and forms part of the base case. Note that, as explained in section 1.5.8, this case over-complies with 2015 standards.

Source AECOM modelling using BRE's SAP 10.1 software SAP.exe, Build 7, 20/01/20.

Results recorded in AECOM, '210507 Scotland Building Standards 2021 - SAP10.1 Results – v10.xls'

Table 1.11c: SAP 10.1 key modelling results for mid-terrace dwelling type – carbon emissions and primary energy

Mid-terrace house	DPER	% reduction in DPER vs 2015 gas case	DER	% reduction in DER vs 2015 gas case
Gas BS2015 case	73.2	0%	13.7	0%
Gas improved case	45.6	38%	8.8	36%
Gas advanced case	28.3	61%	5.2	62%
ASHP improved case	37.2	49%	3.6	74%
ASHP advanced case	28.6	61%	2.7	80%

Source AECOM modelling using BRE's SAP 10.1 software SAP.exe, Build 7, 20/01/20.

Results recorded in AECOM, '210507 Scotland Building Standards 2021 - SAP10.1 Results – v10.xls'

Table 1.11d: SAP 10.1 key modelling results for flat dwelling type – carbon emissions and primary energy

Average flat / Block of flats	DPER	% reduction in DPER vs 2015 gas case	DER	% reduction in DER vs 2015 gas case
Gas BS2015 case	77.6	0%	14.5	0%
Gas improved case	58.0	25%	11.0	24%
Gas advanced case	40.5	48%	7.6	48%
ASHP improved case	39.6	49%	3.8	74%
ASHP advanced case	30.2	61%	2.9	80%

Source AECOM modelling using BRE's SAP 10.1 software SAP.exe, Build 7, 20/01/20.

Results recorded in AECOM, '210507 Scotland Building Standards 2021 - SAP10.1 Results – v10.xls'

173. Further results from the modelling of potential 2021 standards are set out in Table 1.11e to Table 1.11h. The tables show energy consumption by end-use, and energy generation from onsite PV (where applicable) as calculated using the SAP 10.1 methodology. Again these results are compared to the equivalent results for the 2015 compliant base cases.

174. The energy generated by PV is assumed to be split into electricity used on site, and electricity exported to the grid, with the split based upon the SAP 10.1 methodology. The proportions used on site or exported vary by dwelling type and compliance case, but generally around 50-55% is assumed to be used on site in the 2015 gas compliant cases for all dwelling types, around 50% for the flat block gas improved/advanced cases, and around 35-40% in the gas houses improved/advanced cases (where the overall amount of electricity generated is significantly higher).

Table 1.11e: SAP 10.1 key modelling results for detached dwelling type – annual energy consumption by end-use, and annual onsite energy generation ( kWh/yr)

Detached house	Space heating	Water heating	Pumps and fans	Lighting	PV generation
Gas BS2015 case	6,036	3,257	86	264	1,326
Gas improved case	5,062	2,642	86	264	3,565
Gas advanced case	2,378	2,688	469	268	3,565
ASHP improved case	1,937	903	0	264	0
ASHP advanced case	953	888	383	268	0

Source AECOM modelling using BRE's SAP 10.1 software SAP.exe, Build 7, 20/01/20.

Results recorded in AECOM, '210507 Scotland Building Standards 2021 - SAP10.1 Results – v10.xls'

Table 1.11f: SAP 10.1 key modelling results for semi-detached dwelling type – annual energy consumption by end-use, and annual onsite energy generation ( kWh/yr)

Semi-detached house	Space heating	Water heating	Pumps and fans	Lighting	PV generation
Gas BS2015 case	3,315	3,007	86	198	782
Gas improved case	2,647	2,441	86	198	2,141
Gas advanced case	935	2,498	266	204	2,141
ASHP improved case	1,063	852	0	198	0
ASHP advanced case	435	904	180	204	0
ASHP BS2015 case	1,748	1,524	0	198	0

Source AECOM modelling using BRE's SAP 10.1 software SAP.exe, Build 7, 20/01/20.

Results recorded in AECOM, '210507 Scotland Building Standards 2021 - SAP10.1 Results – v10.xls'

Table 1.11g: SAP 10.1 key modelling results for mid-terrace dwelling type – annual energy consumption by end-use, and annual onsite energy generation ( kWh/yr)

Mid-terrace house	Space heating	Water heating	Pumps and fans	Lighting	PV generation
Gas BS2015 case	2,774	3,018	86	204	782
Gas improved case	2,186	2,451	86	204	2,141
Gas advanced case	572	2,520	266	210	2,141
ASHP improved case	910	913	0	204	0
ASHP advanced case	294	885	180	210	0

Source AECOM modelling using BRE's SAP 10.1 software SAP.exe, Build 7, 20/01/20.

Results recorded in AECOM, '210507 Scotland Building Standards 2021 - SAP10.1 Results – v10.xls'

Table 1.11h: SAP 10.1 key modelling results for flat dwelling type – annual energy consumption by end-use, and annual onsite energy generation ( kWh/yr)

Average flat	Space heating	Water heating	Pumps and fans	Lighting	PV generation
Gas BS2015 case	2,259	2,849	86	169	700
Gas improved case	1,867	2,370	86	169	1,186
Gas advanced case	564	2,426	204	172	1,186
ASHP improved case	766	858	0	169	0
ASHP advanced case	279	808	118	172	0

Source AECOM modelling using BRE's SAP 10.1 software SAP.exe, Build 7, 20/01/20.

Results recorded in AECOM, '210507 Scotland Building Standards 2021 - SAP10.1 Results – v10.xls'

175. The primary energy and carbon emission results by individual dwelling and fuel type (Table 1.11a to Table 1.11d) were compared to help assess the suitability of the three potential options for the notional building set out in section 1.10.6 (i.e. Option 1, a single fuel notional building based on gas; Option 2, a single fuel notional building based on ASHP; or Option 3, a notional building differentiated by fuel type).

176. In terms of primary energy, as was expected, percentage level of improvement differed across all dwelling types for the equivalent cases for gas / ASHP (i.e. improved / advanced, which respectively have the same fabric and ventilation specifications for gas / ASHP, but which both omit PV in the case of ASHP). For the improved cases, the ASHP results were significantly better than the equivalent gas model results for all dwelling types. However for the advanced cases, the ASHP results were close to the equivalent gas model results for the houses only. This latter observation can be explained by observing that improving fabric specifications and switching to MVHR have a greater impact where a lower efficiency space heating system is being used. The results are further apart for the flats where space heating demands are lower.

177. However it can be seen that the gas advanced case comes out similar to/better than the ASHP improved case in terms of primary energy across the dwelling types modelled. In terms of carbon emissions, both ASHP cases (improved / advanced) have much lower results than either of the gas cases.

178. Some of the implications of these findings for the notional building options set out in section 1.10.6 are as follows (with the main focus being on primary energy targets, as this is the main target metric):

• Option 1 – a single notional gas-heated building:
  • A single notional gas building could be set but this would give ASHP a significant benefit in many situations (particularly at the 'improved' specification level), allowing specification relaxation which may not be sufficiently limited by improved backstops.
  • In terms of carbon, having this as an additional metric should not impact on ASHP – benefits/impacts relating to some other fuel types e.g. oil/LPG are discussed in section 1.17.
• Option 2 – a single notional ASHP-heated building:
  • A single notional ASHP building could be set at the improved specification level, but this would make it challenging for gas to comply, e.g. requiring similar specification to the gas advanced case.
  • A single notional ASHP building set at the advanced specification level would be even more challenging and would make gas heating challenging for at least some dwelling types.
  • The notional building ASHP efficiency could be reduced to provide more parity between gas and ASHP cases; however this potentially gives a confusing message particularly in the context of increased use of heat pumps and concerns about energy bills, and could allow poor fabric efficiency where better performing ASHP is used in practice.
  • In terms of carbon, having this as an additional metric where a single notional was based on ASHP would mean that gas-heated dwellings would be expected to be unable to realistically comply (in either the improved or advanced case).
  • In terms of cost-benefit, for the advanced specification this would be affected by the advanced measures having a smaller impact on the ASHP models compared to gas.
• Option 3 – a notional differentiated by fuel type (gas/ASHP):
  • This could have the benefit of helping to prevent some of the issues identified above with Options 1 and 2, and also perhaps the benefit of providing more clarity for the consultation.
  • As a downside, it potentially gives less benefit to/incentive for ASHP vs gas in terms of capex, but there would be some savings from excluding PV (and a small saving from excluding WWHR).
  • In terms of carbon, having this as an additional metric would not then rule out gas.

179. It should be noted that comparisons between gas and ASHP cases may vary when looking at a wider range of building types. This would be expected to have particular relevance if setting a single-fuelled notional building. Comparisons would of course also be affected by changes to underlying assumptions – for example if they are reviewed at a later date when primary energy and carbon emission factors for electricity would be expected to have reduced.

180. The benefits/impacts of potential targets (including carbon targets) relating to other sensitivity fuel/heating types (oil, district heating) are considered separately under sections 1.16 and 1.17.

181. Other relevant key findings from the SAP modelling include that the advanced cases for all dwelling types show a high risk of overheating, based on a SAP Appendix P assessment. It had already been noted that the ground floor flat showed a high risk in all cases (including 2015 compliant). Overheating risks and mitigation measures would need to be considered separately as they are outside of the scope of the current work – and it is understood that separate work is taking place on this topic.

182. It should also be noted that the mid-terrace and flat dwelling types required an extension to be made in SAP to the plant size ratios modelled for heat pumps in the advanced cases, as the design heat losses in these dwellings are so low, even though the heat pump product modelled was chosen as being particularly suited to low energy dwellings. This also meant that space heating efficiencies in these cases were somewhat lower in the modelling than the 250% figure proposed in section 1.10.4. Depending in part on where the notional building target is set, this is likely to require further exploration with BRE as the SAP contractors, and with the heat pump industry to investigate the suitability of a range of heat pumps for providing heating in very low energy dwellings and the efficiencies which can be achieved in practice and as modelled in SAP.

183. Consideration of the above findings relating to the notional building options led Scottish Government to decide to proceed with Option 3 for the modelling – a notional building differentiated by fuel type (gas/ASHP). As noted previously, this is similar to the approach adopted in 2015 standards and helps ensure that high standards are achieved for different fuel types. Two levels of standards were therefore assessed at the national level in the following analysis:

• Scenario 1: 'Improved' standards in 2021:
  • Gas + improved fabric + natural ventilation + PV ('Gas improved'), where dwelling sub-type is gas-heated, and
  • ASHP + improved fabric + natural ventilation ('ASHP improved'), where dwelling sub-type is ASHP-heated.
• Scenario 2: 'Advanced' standards in 2021:
  • Gas + advanced fabric + MVHR + PV ('Gas advanced'), where dwelling sub-type is gas-heated, and
  • ASHP + advanced fabric + MVHR ('ASHP advanced'), where dwelling sub-type is ASHP-heated.

1.11.2 Fuel mix

184. The SAP modelling results were used to assess the benefits at a national level. The seven building sub-types and annual build numbers used for the baseline were assumed in the counterfactual scenarios, and these were assumed to be unchanged over the analysis period. These assumptions were defined and explained in section 1.4 (Table 1.4b). The assumptions made are replicated in Table 1.11i.

185. Two alternative fuel mix scenarios, agreed with the Scottish Government, were considered which are also presented in Table 1.11i. In both cases, these were modelled separately for both the improved and advanced standards, and a full transition to the new standards is assumed to be achieved by 2025, as explained in section 1.11.3 below. The fuel mix scenarios are:

• A core 'with fossil fuels' case which assumes the same fuel mix as the base case, across the entire analysis period.
• A 'without fossil fuels' case which assumes a move to 100% off-fossil fuels.

186. In each case the specification for the building sub-type modelled is the relevant (gas / ASHP) specification set out at the end of section 1.11.1 above (see section 1.10 for detailed specifications). Note that in the core 'with fossil fuels' case, ASHP is not assumed other than for some semi-detached houses.

Table 1.11i: Seven building sub-types modelled in analysis – fuel mix assumptions

Building Sub-types - baseline	Proportion of build mix	Annual build numbers	Fuel to assume in 'with fossil fuels' case	Fuel to assume in 'without fossil fuels' case
Detached house, gas	30.32%	5,521	Gas	Electricity (ASHP)
Semi-detached house, gas	22.84%	4,158	Gas
Mid-terrace house, gas	8.06%	1,467	Gas
Ground-floor flat, gas	9.37%	1,706	Gas
Mid-floor flat, gas	9.37%	1,706	Gas
Top-floor flat, gas	9.37%	1,706	Gas
Semi-detached house, ASHP	10.67%	1,943	Electricity (ASHP)
Total	100%	18,207

NotesPercentages do not add up to totals due to rounding.

Source AECOM analysis of EPC database extract 2016-18 SAP new build (provided by Scottish Government, December 2019)

Scottish Government, Housing Statistics for Scotland – All sector new build (completions), September 2019 (data for calendar years 2016-2018 used to derive an average annual total build rate)

Fuel assumptions for 'with / without fossil fuels' cases agreed with Scottish Government.

1.11.3 Transitional period

187. The national profile modelling assumes a transitional period as new standards are introduced (i.e. not all buildings built in 2021 will be to 2021 standards). The assumptions made were agreed with Scottish Government and are set out in Table 1.11j.

Table 1.11j: Transitional period assumptions for 2021 standards

Proportions of new dwellings built to relevant standard in each year	2021	2022	2023	2024	2025 onwards
2015 standard	80%	60%	40%	20%	0%
2021 standard	20%	40%	60%	80%	100%

Source Agreed with Scottish Government.

1.12 National impacts (carbon benefits) with fossil fuels

188. To form an initial estimate of the carbon benefit of the different potential future standards, prior to undertaking a full CBA, the energy results summarised in Table 1.11eTable 1.5m to Table 1.11h were applied to the national build profile, taking into account the assumptions on build/fuel mix and build rates set out in section 1.11. Initially the core 'with fossil fuels' scenario was assessed.

189. A 25 year analysis period was used. The two counterfactual cases ('improved' and 'advanced') were compared to the 2015 compliant base case. The carbon emission factors applied are those published by BEIS to support the HM Treasury Green Book supplementary appraisal guidance on valuing energy use and greenhouse gas (GHG) emissions (BEIS, 2019). The factors for electricity are projected to decrease over time and are summarised in Table 1.12a. The factor for gas is 0.184kgCO₂e/kWh.

Table 1.12a: Carbon emission factors used in benefit analysis – electricity ( kgCO ₂e/ kWh)

Year	2021	2022	2023	2024	2025	2026	2027	2028	2029	2030	2031	2032	2033
Consumption	0.283	0.269	0.255	0.240	0.224	0.207	0.189	0.171	0.151	0.130	0.116	0.103	0.092
Generation	0.258	0.246	0.233	0.219	0.205	0.189	0.173	0.156	0.138	0.118	0.105	0.094	0.084

Year	2034	2035	2036	2037	2038	2039	2040	2041	2042	2043	2044	2045
Consumption	0.082	0.073	0.065	0.058	0.052	0.046	0.041	0.040	0.038	0.037	0.036	0.034
Generation	0.075	0.066	0.059	0.053	0.047	0.042	0.037	0.036	0.035	0.034	0.032	0.031

Notes Generation-based emission factors are used for electricity generated by PV and exported. PV generated electricity used on site is assumed to offset consumption and therefore the consumption-based emission factors are applied.

Source BEIS, Green Book supplementary guidance: valuation of energy use and greenhouse gas emissions for appraisal – data tables 1 (electricity – long-run marginal domestic consumption-based figures and generation-based figures) and 2a (natural gas) (BEIS, 2019).

190. It should be noted that the carbon emission factors are different from those used in SAP 10.1. They are lower for gas, and for electricity they are initially significantly higher but are projected to decrease over time becoming lower from 2030 onwards and continuing to decrease until 2050. A different calculation of carbon savings from the counterfactual cases across the build mix using SAP 10.1 carbon emission factors (for a single year) is given in section 1.14.

191. The results by year are presented in Table 1.12b. Total emissions increase over time as the number of homes included in the analysis increases, though emission factors for electricity decrease. The estimated total carbon savings for the counterfactual cases across the analysis period are summarised in Table 1.12c. This shows that the 'improved' case is estimated to achieve a 21% reduction in carbon emissions compared to the base case; and the 'advanced' case to achieve a 42% reduction.

Table 1.12b: Annual carbon emissions for base case and counterfactual cases – 'with fossil fuels' scenario ( ktCO ₂e/yr)

Year	2021	2022	2023	2024	2025	2026	2027	2028	2029	2030	2031	2032	2033
Base	20	40	60	80	100	120	140	161	182	202	223	244	265
Improved	18	34	48	61	73	86	100	115	131	148	165	183	200
Advanced	17	31	42	51	58	66	75	84	95	107	119	130	142

Year (cont.)	2034	2035	2036	2037	2038	2039	2040	2041	2042	2043	2044	2045	All
Base	285	306	327	348	369	390	410	431	452	472	493	514	6,632
Improved	218	236	254	272	290	308	327	343	360	376	393	410	5,147
Advanced	155	167	179	192	204	217	229	240	251	262	274	285	3,673

Source AECOM modelling using assumptions on build mix, fuel mix and emission factors as set out above, and using SAP 10.1 energy modelling results as set out above.

Results recorded in AECOM, '210507 Scotland Building Standards 2021 - SAP10.1 Results – v10.xls'

Table 1.12c: Total carbon emissions for improved and advanced cases – 'with fossil fuels' scenario

Scenario	Total carbon saving (ktCO2e/yr)	% reduction compared to base case
Improved	1,485	22%
Advanced	2,959	45%

Source AECOM modelling using assumptions on build mix, fuel mix and emission factors as set out above, and using SAP 10.1 energy modelling results as set out above.

Results recorded in AECOM, '210507 Scotland Building Standards 2021 - SAP10.1 Results – v10.xls'

192. Sensitivity analysis was undertaken to estimate the impact of excluding the benefit of carbon emission savings associated with electricity generated by PV and exported to the grid. This indicated that carbon emission reductions compared to the base case would decrease to 18% for the 'improved' case and 39% for the 'advanced' case, where other assumptions remain unchanged from the core case. It should be noted that for consistency, in this analysis the carbon benefit of exported generation was removed for the base case as well as the improved case.

1.13 National impacts (carbon benefits) without fossil fuels

193. The 'without fossil fuels' scenario was also assessed, using the same process and assumptions as for the 'with fossil fuels' scenario, set out in sections 1.11 and 1.12. The results by year are presented in Table 1.13a. The estimated total carbon savings for the counterfactual cases across the analysis period are summarised in Table 1.13b.

194. This shows that under the 'without fossil fuels' scenario, the 'improved' case is estimated to achieve a 73% reduction in carbon emissions compared to the base case; and the 'advanced' case to achieve a 75% reduction. As would be expected, this is significantly higher than the estimated 22% / 45% reductions for the improved / advanced cases under the core 'with fossil fuels' scenario, where the majority of new dwellings are assumed to be gas-heated over the analysis period.

Table 1.13a: Annual carbon emissions for base case and counterfactual cases – 'without fossil fuels' scenario ( ktCO ₂e/yr)

Year	2021	2022	2023	2024	2025	2026	2027	2028	2029	2030	2031	2032	2033
Base	20	40	60	80	100	120	140	161	182	202	223	244	265
Improved	18	35	49	60	68	75	80	83	85	84	84	84	83
Advanced	18	33	46	56	62	68	72	75	76	75	76	75	75

Year (cont.)	2034	2035	2036	2037	2038	2039	2040	2041	2042	2043	2044	2045	ALL
Base	285	306	327	348	369	390	410	431	452	472	493	514	6,632
Improved	82	81	79	78	76	74	72	73	73	74	74	74	1,799
Advanced	74	73	71	70	69	67	66	66	67	67	67	68	1,631

Source AECOM modelling using assumptions on build mix, fuel mix and emission factors as set out above, and using SAP 10.1 energy modelling results as set out above.

Results recorded in AECOM, '210507 Scotland Building Standards 2021 - SAP10.1 Results – v10.xls'

Table 1.13b: Total carbon emissions for improved and advanced cases – 'without fossil fuels' scenario

	Total carbon saving (ktCO2e)	% reduction compared to base case
Improved	4,834	73%
Advanced	5,001	75%

Source AECOM modelling using assumptions on build mix, fuel mix and emission factors as set out above, and using SAP 10.1 energy modelling results as set out above.

Results recorded in AECOM, '210507 Scotland Building Standards 2021 - SAP10.1 Results – v10.xls'

1.14 National impacts (Costs)

195. The capital costs (in 2020 prices) of each home type for the 2015, improved and advanced cases, and for both gas heating and ASHP, are shown in Table 1.14a to Table 1.14e.

Table 1.14a: Capital costs by cost case and fuel type – detached house

Detached house	Fabric	Ventilation	Heating and hot water	Photovoltaics	Balance of construction cost	Total	Percentage uplift on 2015
Gas BS2015 case	£58,154	£600	£9,030	£2,388	£98,788	£168,960	0%
Gas improved case	£62,135	£600	£9,930	£4,564	£98,788	£176,017	4%
Gas advanced case	£65,205	£4,216	£7,284	£4,564	£98,788	£180,057	6%
ASHP improved case	£62,135	£600	£11,942	£0	£98,788	£173,465	3%
ASHP advanced case	£65,205	£4,216	£9,296	£0	£98,788	£177,505	5%

Table 1.14b: Capital costs by case and fuel type – semi-detached house

Semi-detached house	Fabric	Ventilation	Heating and hot water	Photovoltaics	Balance of construction cost	Total	Percentage uplift on 2015
Gas BS2015 case	£32,765	£600	£7,102	£1,860	£54,778	£97,106	0%
Gas improved case	£34,967	£600	£7,852	£3,180	£54,778	£101,378	4%
Gas advanced case	£36,565	£3,511	£6,220	£3,180	£54,778	£104,254	7%
ASHP improved case	£34,967	£600	£9,914	£0	£54,778	£100,260	3%
ASHP advanced case	£36,565	£3,511	£8,282	£0	£54,778	£103,136	6%

Table 1.14c: Capital costs by case and fuel type – mid-terraced house

Mid-terraced house	Fabric	Ventilation	Heating and hot water	Photovoltaics	Balance of construction cost	Total	Percentage uplift on 2015
Gas BS2015 case	£24,754	£600	£7,102	£1,860	£54,346	£88,662	0%
Gas improved case	£26,680	£600	£7,852	£3,180	£54,346	£92,658	4%
Gas advanced case	£27,986	£3,511	£6,220	£3,180	£54,346	£95,243	7%
ASHP improved case	£26,680	£600	£9,914	£0	£54,346	£91,540	3%
ASHP advanced case	£27,986	£3,511	£8,282	£0	£54,346	£94,125	6%

Table 1.14d: Capital costs by case and fuel type – average flat (in block of 12 over 3 floors)

Average flat in block of 12	Fabric	Ventilation	Heating and hot water	Photovoltaics	Balance of construction cost	Total	Percentage uplift on 2015
Gas BS2015 case	£19,885	£450	£6,086	£935	£56,758	£84,114	0%
Gas improved case	£21,388	£450	£6,296	£1,584	£56,758	£86,477	3%
Gas advanced case	£22,650	£3,012	£4,999	£1,584	£56,758	£89,004	5%
ASHP improved case	£21,388	£450	£9,243	£0	£56,758	£87,840	4%
ASHP advanced case	£22,650	£3,012	£7,946	£0	£56,758	£90,367	7%

Table 1.14e: Capital costs by case and fuel type – semi-detached house – ASHP

Semi-detached house (ASHP)	Fabric	Ventilation	Heating and hot water	Photovoltaics	Balance of construction cost	Total	Percentage uplift on 2015
ASHP BS2015 case	£32,765	£600	£7,794	£0	£54,778	£95,938	0%
ASHP improved case	£34,967	£600	£9,914	£0	£54,778	£100,260	4%
ASHP advanced case	£36,565	£3,511	£8,282	£0	£54,778	£103,136	7%

196. The total costs of building homes with the advanced case fabric standards are lower than might be expected because some of the additional cost is offset by a reduction in the cost of providing radiators and associated pipework. This is drawn from evidence developed for the Committee on Climate Change which shows that ultra-low energy homes can be kept warm with reduced numbers of heat emitters (Committee on Climate Change, 2019c). In this study a 50% reduction in the costs of radiators and distribution pipework was allowed for advanced practice fabric standards compared to a 75% cost reduction assumed in the CCC study for construction to Passivhaus standards. This recognises that the advanced case fabric standards are significantly higher in performance than a typical home compliant with Section 6 2015 but that they are not at a level consistent with the Passivhaus specification, largely as a result of the lower level of targeted airtightness. A 50% saving is applied to both the house and flat types which is slightly different from the assumptions used by the CCC but reflects that the same fabric specifications are used in all dwelling types and that as a consequence the reduction in space heating demand is greater in flats than in houses^[28].

197. These capital cost estimates are based on a 'central belt' price level. In other areas of Scotland prices may be different reflecting the availability and costs of materials and labour. Drawing on Currie & Brown's experience in delivering projects across Scotland^[29] the following adjustments on the base (central belt) costs are considered reasonable to reflect the additional costs of working in more remote parts of the country. The impact on the build cost of the semi-detached house for the different cases is shown in

198. Table 1.14f for the highest cost location the Western Isles.

• Central Belt (Glasgow, Edinburgh etc) – 100
• Borders / Dumfries & Galloway - 103
• Grampian (Aberdeen) - 103
• Highland - 110
• Orkney & Shetland - 125
• Western Isles – 130

Table 1.14f: Potential variation in build costs for homes built in the Western Isles – semi-detached house

Semi-detached house	Central cost	Cost for projects in Western Isles	Variation in overall cost from 2015 base
Gas BS2015 case	£97,106	£126,238	£0
Gas improved case	£104,074	£131,791	£5,553
Gas advanced case	£103,719	£135,531	£9,293
ASHP improved case	£102,906	£130,338	£4,100
ASHP advanced case	£102,551	£134,077	£7,839

199. The capital, maintenance and renewal, energy (variable cost) and lifetime costs of each case and fuel type are shown in Table 1.14g. These costs are the net present value costs over a 60 year period for a home built in 2021(in 2021 prices). Information is presented for the detached, semi-detached, mid-terrace and average flat in a three-storey block against a gas heated 2015 compliant reference case and also for the semi-detached house against a 2015 reference building with an ASHP. Lifetime energy costs are derived from energy price projections published by BEIS, renewal and maintenance costs are derived on an elemental basis in line with the assumptions in Appendix A. Renewal and maintenance costs reflect only those elements that are linked to the variations in specification and exclude other common elements such as decoration and other services (e.g. lighting).

Table 1.14g: Lifetime costs by home, case and heating type (£ present value per home)

Lifetime cost category	Detached 1. Improved with Gas	Detached 2. Advanced with Gas	Detached 3. Improved with ASHP	Detached 4. Advanced with ASHP	Semi 1. Improved with Gas	Semi 2. Advanced with Gas
Reference heating system	Gas	Gas	Gas	Gas	Gas	Gas
Change in capital cost	£7,057	£11,097	£4,505	£8,545	£4,272	£7,148
Change in 60 year energy cost	-£5,185	-£5,667	£5,544	£3,710	-£3,401	-£3,874
Change in renewals cost	£1,467	£1,455	£3,268	£3,256	£873	£898
Change in maintenance cost	£0	£826	-£2,002	-£1,177	£0	£826
Change in lifetime cost	£3,339	£7,710	£11,316	£14,334	£1,744	£4,998

Lifetime cost category	Semi 3. Improved with ASHP	Semi 4. Advanced with ASHP	Mid-terrace 1. Improved with Gas	Mid-terrace 2. Advanced with Gas	Mid-terrace 3. Improved with ASHP	Mid-terrace 4. Advanced with ASHP
Reference heating system	Gas	Gas	Gas	Gas	Gas	Gas
Change in capital cost	£3,154	£6,030	£3,996	£6,581	£2,878	£5,463
Change in energy cost	£3,412	£2,236	-£3,352	-£3,755	£3,463	£2,089
Change in renewals cost	£3,203	£3,228	£860	£850	£3,190	£3,180
Change in maintenance cost	-£2,002	-£1,177	£0	£826	-£2,002	-£1,177
Change in lifetime cost	£7,767	£10,318	£1,504	£4,501	£7,529	£9,555

Lifetime cost category	Flat (average) 1. Improved with Gas	Flat (average) 2. Advanced with Gas	Flat (average) 3. Improved with ASHP	Flat (average) 4. Advanced with ASHP	Semi 1. Improved with ASHP	Semi 2. Advanced with ASHP
Reference heating system	Gas	Gas	Gas	Gas	ASHP	ASHP
Change in capital cost	£2,363	£4,890	£3,726	£6,253	£4,322	£7,198
Change in energy cost	-£1,521	-£1,932	£3,092	£1,842	-£4,076	-£5,252
Change in renewals cost	£561	£932	£3,655	£4,025	£2,577	£2,602
Change in maintenance cost	£190	£1,016	-£962	-£137	£0	£826
Change in lifetime cost	£1,593	£4,906	£9,510	£11,983	£2,823	£5,374

1.15 Comparison with Sullivan Report recommendations

200. Carbon emission savings were also estimated using a single analysis year and SAP 10.1 carbon emission factors, to provide a comparison with Sullivan Report recommendations. The same build rates and fuel mix were used as for the core 'with fossil fuels' analysis above (see Table 1.11i), and it was assumed that 100% of homes were built to 2021 standards in the year assessed. The SAP 10.1 carbon emission factors (as shown in Table 1.5p) were applied to the energy results summarised in Table 1.11e to Table 1.11h.

201. The two counterfactual cases ('improved' and 'advanced') were again compared to the 2015 compliant base case. The estimated total carbon emissions and savings for the year are summarised in Table 1.15a. The percentage reductions are higher than in the benefit analysis for the core scenario presented in section 1.12 above mainly due to differences in carbon emission factors. The figures show that the 'improved' case is estimated to achieve a reduction of around 32% in carbon emissions compared to the base case across the build mix; and the 'advanced' case to achieve a reduction of around 57%. This compares to a commitment in the Sullivan Report to achieve an aggregate emissions reduction of at least 27.5% on 2015 standards across the build mix; indicating that either case would exceed this target.

Table 1.15a: Total carbon emissions and savings for base, improved and advanced cases – Sullivan Report comparison (based on a single year and SAP 10.1 carbon emission factors)

Scenario	Total carbon emissions for the year (ktCO2e/yr)	Total carbon saving for the year (ktCO2e/yr)	% reduction compared to base case
Base	23	0	-
Improved	16	7	32%
Advanced	10	13	57%

Source AECOM modelling using assumptions on build and fuel mix as set out above, and using SAP 10.1 energy modelling results and carbon emission factors as set out above.

Results recorded in AECOM, '210507 Scotland Building Standards 2021 - SAP10.1 Results – v10.xls'

1.16 Sensitivity analysis

202. Analysis of the EPC database extract provided by Scottish Government showed that gas-heated and ASHP-heated dwellings account for the majority of recent new build homes within Scotland. However, sensitivity analysis was undertaken to investigate the ability of dwellings to comply with potential 2021 standards where fuel/heating types other than gas and ASHP were specified. The heating types chosen for assessment were in part based upon analysis of the EPC database extract, as well as upon investigating cases where in discussion with the client it was thought that particular questions might arise which require further consideration from a policy perspective.

203. The analysis of the EPC database extract showed that the most common other heating types were oil and district heating, with each respectively accounting for around 3% of homes in the database extract. These were followed by direct electric (around 2.5% of homes), then LPG (under 1%) and biomass (under 0.5%). The EPC database extract is discussed in more detail in section 1.4, with key findings on heating type presented in Table 1.4a.

204. Based on this analysis, oil, direct electric, and district heating for gas, heat pumps, and energy from waste were evaluated. Sensitivity analysis was undertaken to assess the ability to comply with potential 2021 standards.

205. For oil, the ratio of the gas carbon emission factor to the oil carbon emission factor in SAP 10.1 is 1:1.42, whereas the ratio of the gas primary energy factor to the oil primary energy factor is around 1:1.04.^[30] This means that a carbon target would be significantly more challenging than the primary energy target.

206. For direct electric storage heating, the ratio of gas carbon emission factor to electricity carbon emission factor in SAP 10.1 is 1:1.54, whereas the ratio of the gas primary energy factor to electricity primary energy factor is around 1:33. This means that a carbon target would be more challenging than the primary energy target.

207. A key benefit of gas CHP arises from offsetting grid electricity with that generated from the CHP engine. With electricity carbon emission and primary energy factors reducing over time, the benefits associated with gas CHP electricity generation are significantly reduced making gas CHP less attractive than previously and making it harder for gas CHP heat networks to comply. A carbon target would also be more challenging than the primary energy target due to reductions in carbon emission factors for electricity. Whilst the primary energy factors for electricity have reduced, there is a significantly greater percentage reduction for carbon emission factors. The ratio of the gas carbon emission factor to the electricity carbon emission factor in SAP 10.1 is 1:0.65, and the ratio of the gas primary energy factor to the electricity primary energy factor is 1:1.33. This compares to comparable ratios of 1:2.4 and 1:2.52 in SAP 9.92. In terms of both primary energy and carbon emissions, district heating also requires additional measures to comply compared to an individually-heated gas case because of the distribution losses associated with the heat network.

208. For heat pump fueled district heating, the ratio of carbon emissions and primary energy factors compared to gas are the same as for direct electric storage heating which would mean a carbon target would be more challenging than the primary energy target.

209. For a district heating scheme which uses waste combustion for heat, the ratio of gas carbon emission factor to waste combustion carbon emission factor in SAP 10.1 is 2.84:1, whereas the ratio of gas primary energy factor to waste combustion primary energy factor is 1:1.03. This means that a carbon target would be much easier to comply with than a primary energy target.

• While the primary energy ratio is nearly equal it does not mean that a waste combustion heat network would be similar to a gas based heat network in terms of compliance; as the waste fuel has a more varied calorific value than gas due to the nature and variety of waste. This means that a waste fueled boiler or CHP engine would likely have a much lower heating efficiency, and therefore be required to burn much more waste to produce the same heat output.
• This situation is the same for waste heat from a power station which has a similar ratio of gas primary energy factor to waste heat from a power station primary energy factor.

210. The following sensitivity cases were modelled. In each case the same individual built forms were used as in the core modelling, and compliance was tested against the relevant 2021 'improved' case (gas/ASHP) in terms of primary energy. Carbon emissions were also compared to aid in the assessment of a potential secondary carbon metric. For all fuel/system types, the sensitivity cases included PV.

• Oil:
  • A detached house with an oil boiler was modelled as the majority of oil-heated dwellings in the database extract were detached houses.
  • The primary energy and carbon emission results were compared to those of the 2021 'gas improved' case.
• Gas CHP district heating:^[31]
  • A mid-floor flat was modelled as the majority of dwellings on district/communal heating in the database extract were flats.
  • It was assumed that gas CHP provided 75% of the annual heat supplied, with gas boilers providing the remainder.
  • A distribution loss factor of 1.5 was assumed, which corresponds to the value assumed in SAP for heat networks complying with the Heat Network Code of Practice for the UK (CIBSE; ADE, 2015).
  • The primary energy and carbon emission results were compared to those of the 2021 'gas improved' case.
• Heat Pump (+ gas CHP) district heating:
  • A mid-floor flat was modelled as the majority of dwellings on district/communal heating in the database extract were flats.
  • It was assumed that the district heating network was supplied by electric heat pump for 40% of the annual heat supplied, with gas CHP providing 50% and gas boilers providing the remainder.
  • A distribution loss factor of 1.5 was assumed, which corresponds to the value assumed in SAP for heat networks complying with the Heat Network Code of Practice for the UK (CIBSE; ADE, 2015).
  • The primary energy and carbon emission results were compared to those of the 2021 'gas improved' case and the 2021 'ASHP improved' case.
• Energy from waste district heating:
  • A mid-floor flat was modelled as the majority of dwellings on district/communal heating in the database extract were flats.
  • It was assumed that the district heating network was supplied by a waste combustion fueled CHP that is operational for 90% of the year. When operating it is assumed 100% of the heat consumption is met by the heat generated by the plant. When the plant is offline for maintenance it is assumed gas boilers provided 100% of the heat consumed.
    • i. The CHP is assumed to be part of an Energy from Waste plant which is electrically led with a power efficiency of approximately 25%, and heating efficiency of approximately 14%.^[32]
  • A distribution loss factor of 1.5 was assumed, which corresponds to the value assumed in SAP for heat networks complying with the Heat Network Code of Practice for the UK (CIBSE; ADE, 2015).
  • The primary energy and carbon emission results were compared to those of the 2021 'gas improved' case.
• Direct electric:
  • A detached house and a mid-floor flat were modelled.
  • An electric storage heater system was modelled for both buildings, with secondary heating provided by room heaters. Water heating was provided by immersion heaters.
  • The primary energy and carbon emission results were compared to those of the 2021 'gas improved' case and the 2021 'ASHP improved' case.

211. For each sensitivity case, the 2021 'improved' case specifications (including WWHR and PV) were initially modelled (with heating systems changed). This provided an initial comparison, but as expected the primary energy and carbon emissions were higher than in the 'gas improved' case (so the specifications would not achieve 'compliance' should these be the 2021 targets). The 'advanced' case specifications (including WWHR and PV) were then modelled to see whether these would comply. If not, or if they over-complied, then adjustments were made to see what would bring them closer to compliance.

212. In terms of adjustments made, lower capital cost measures were applied first where possible, with cost-effectiveness also being considered (fabric was generally prioritised before MVHR, and wall/floor improvements before windows). The modelling did not look beyond the 'advanced' case specifications in terms of fabric or ventilation, but did look at a higher efficiency PV panel where the advanced case did not achieve compliance alone. This assumed an efficiency of 4.5 m²/kWp (as opposed to 6.5 m²/kWp in the core cases). The analysis is indicative of the scale of additional measures necessary for compliance.

213. For the oil sensitivity case, the following findings were observed:

• to achieve compliance in terms of primary energy alone, the 'improved case' specification could be adopted but with wall, floor and thermal bridging values adjusted to the 'advanced case' specification (i.e. wall U-value 0.13, floor U-value 0.10, and y-value 0.04).
• to achieve compliance in terms of carbon emissions, if a carbon metric were to be set, a specification close to the 'advanced case' specification would be expected to be required (the advanced case itself would slightly over-comply with a difference in dwelling emission rate of around 0.3kgCO₂e/m²/yr).

214. For the gas CHP district heat network sensitivity case, the following findings were observed:

• to achieve compliance in terms of primary energy alone, the 'advanced case' specification would not be sufficient. If a higher efficiency PV panel was also to be adopted, this would comply and there could be some relaxation elsewhere of the advanced specification (e.g. if the window specification was relaxed to double glazing, this would still be very close to compliance).
• in terms of carbon emissions, even if the 'advanced case' specification were adopted and a higher efficiency PV panel to be specified, the case would still be far off compliance if a carbon metric was to be set (with a dwelling emission rate over 30% higher than that for the comparison individually-heated gas 'improved' specification case).
• It should be noted that the mid-floor flat would be expected to form the most challenging example in terms of compliance of all the core dwelling types modelled. Block averaging across flats may also help to some degree in this case.

215. For the heat pump (+ gas CHP) district heat network sensitivity case, the following findings were observed:

• to achieve compliance in terms of primary energy alone the 'advanced case' specification would not be sufficient when compared to the 2021 'ASHP improved' case. If a higher efficiency PV panel was also to be adopted, this would comply and there could be some relaxation elsewhere of the advanced specification (e.g. relaxing the window specification to double glazing). When compared to the 2021 'gas improved' case the 'advanced case' specification would be sufficient to comply. The 'improved case' specification would not be sufficient to comply in either case.
• in terms of carbon emissions, the 'advanced case' specification would be sufficient to comply against the 2021 'gas improved' case but not against the 2021 'ASHP improved' case. Even if the 'advanced case' specification were adopted and a higher efficiency PV panel to be specified, the carbon emissions would still be approximately two and a half times higher than the 'ASHP improved' target. The 'improved case' specification fails to comply against both the 2021 'gas improved' case and the 2021 'ASHP improved' case.
• It should be noted that the mid-floor flat would be expected to form the most challenging example in terms of compliance of all the core dwelling types modelled. Block averaging across flats may also help to some degree in this case.

216. For the energy from waste district heat network sensitivity case, the following findings were observed:

• to achieve compliance in terms of primary energy alone, neither the 'improved case' or 'advanced case' specification achieved compliance against the 2021 'gas improved' case. The primary energy for the 'advanced case' specification was approximately five times that of the 'gas improved' case target. The cause of this difference is the low efficiency of a waste to energy plant (14% heat and 25% power) compared to a gas boiler (approximately 90%) which is not matched by a comparable reduction in the primary energy factor of waste as a fuel. The combustion process of waste is inherently less efficient than a fuel such as gas due to the mixed composition and high moisture content. The fact that the waste would otherwise go to landfill is not accounted for in the primary energy factor given to waste in SAP leading to high primary energy consumption associate with waste combustion, even as a CHP process
• in terms of carbon emissions, neither the 'improved case' or 'advanced case' specification achieved compliance against the 2021 'gas improved' case. The 'advanced case' specification failed to comply by approximately 25%.
• It should be noted that the mid-floor flat would be expected to form the most challenging example in terms of compliance of all the core dwelling types modelled. Block averaging across flats may also help to some degree in this case.

217. For the direct electric sensitivity case, the following findings were observed:

• to achieve compliance in terms of primary energy alone, the 'advanced case' specification would be sufficient for both the detached house and mid-floor flat for both the 2021 'gas improved' case and the 2021 'ASHP improved' case (achieving around 30% improvement on the 2021 'gas improved' case and a 5% improvement on the 2021 'ASHP improved' case). The 'improved case' specification would not be sufficient for compliance against either the 2021 'gas improved' case or the 2021 'ASHP improved' case.
• in terms of carbon emissions, the 'advanced case' specification would be sufficient for both the detached house and mid-floor flat for both the 2021 'gas improved' case and the 2021 'ASHP improved' case (achieving around 80% improvement on the 2021 'gas improved' case and a 40-50% improvement on the 2021 'ASHP improved' case). The 'improved case' specification would not be sufficient for compliance against the 2021 'ASHP improved' case but would comply with the 2021 'gas improved' case (producing around 50% of the carbon emissions compared to the 2021 'gas improved' case for both the detached house and mid-floor flat).

There is a policy decision as to whether a gas or ASHP notional building should be used for direct electric heating. The analysis shows that a direct electric compliant solution is possible with a specification between 'improved' and 'advanced' for both heating system types with an ASHP notional building resulting in a more stringent target. Direct electric heating is a well-established technology that has lower carbon emissions compared to fossil fuels. However, direct electric heating is much less efficient than a heat pump and as a result fuel bills are more expensive than gas heating and, if deployed at scale, will have greater impact on the national grid.

1.17 Review of the need for a carbon target

218. The Scottish Government has indicated the intention to retain the carbon dioxide equivalent emissions target as an additional secondary metric. The Scottish Government is looking to drive down greenhouse gas emissions and will need to continue to record progress against national carbon targets. There was also support for a carbon target from several respondents to the Scottish Government's 2018 call for evidence on energy standards, as discussed in section 1.7.2 above.

219. Primary energy and carbon dioxide emissions targets can have different impacts depending on the fuel type and on how the notional dwelling is set. As a result of grid decarbonisation, carbon emission factors for electricity are now significantly lower than gas whilst primary energy factors for electricity remain higher than gas. This would make carbon targets more challenging for gas-heated homes than primary energy targets if these were set based on a single electric-heated notional. However, given that the proposed preferred approach is to set a notional building differentiated by fuel type (gas / ASHP), the addition of a carbon metric would not be expected to have the impact of making it more difficult for gas-heated dwellings to comply.

220. Adopting a secondary carbon target would not be expected to impact on the compliance specification for direct electric-heated dwellings (whether based on a gas-heated or ASHP-heated notional), as in these cases the primary energy target is more challenging. Similarly, a carbon target would not be expected to impact on biomass-heated homes as the primary energy target would be much more onerous than a carbon target (as carbon emission factors for biomass are very low). The impact of a secondary carbon target would be on some of the other less commonly used (but higher carbon) fuel types / heating systems.

221. A potential benefit of setting a carbon target would be to introduce more differentiation between gas and higher carbon fossil fuels such as oil and LPG. Under the 2015 standards, the notional is differentiated into five cases – gas, LPG, oil, electricity (ASHP-based), and biomass. The same specifications are applied for gas, LPG and oil except for fuel/boiler type and assumptions on secondary heating. This means that there is no disincentive for higher carbon fossil fuels than gas. As shown in the sensitivity analysis described in section 1.16, comparing all higher carbon fuel types to a 2021 gas notional building with a primary energy target will make it more challenging for higher carbon fuels to comply compared to gas, albeit the impact is limited, particularly for LPG, as the primary energy factors are similar to gas. If an additional carbon target is adopted, it is much more onerous for higher carbon fuels to comply as their carbon emission factors are significantly higher than that of gas. Given the policy intention of phasing out fossil fuels in 2024, it could make sense to introduce a greater disincentive for higher carbon fossil fuels in 2021 which reflects their carbon impact.

222. Similarly, a secondary carbon target would introduce more differentiation between individually gas-heated homes and homes on a gas CHP-based heat network. However, as shown in section 1.16, it would be very challenging for such homes to comply with a carbon target. This may make sense in terms of reflecting carbon impact, but there may also be other strategic reasons for wanting to allow new homes to continue to connect to existing gas CHP-based heat networks. Proposals for Part L 2020 in England and Wales have reflected this through the proposed introduction of technology factors for heat networks but there could be other approaches taken. These may not necessarily be primarily within the remit of Building Standards and the notional building targets – though one possible option could be to explore a district heating based notional building sub-type (or sub-types). Differentiation between new and existing heat networks may also be useful - new heat networks could potentially be treated differently as they may be encouraged to adopt lower carbon fuels, whereas gas CHP-based networks could be more likely to be existing networks being expanded for new homes to connect. The approach for non-domestic buildings should be considered at the same time, as similar findings would be expected there, and it may be appropriate to have a common approach.

223. Whilst the implications of a secondary carbon metric have not been considered in detail for all possible fuel types or heating systems, the ones considered particularly relevant for new homes in Scotland have been discussed above. Further sensitivity analysis covering a wider range of fuel/dwelling types or specifications could also be undertaken, and other views could be sought.

224. The analysis suggests that there would be benefits of applying a secondary metric based on carbon, in terms of encouraging a move away from higher carbon fossil fuels and heating systems, and provides an indication of what specifications might be needed should developers still continue to specify such systems.

225. It also finds that a carbon metric would raise significant challenges for gas CHP district heating. It may be that separate policy changes or mechanisms are needed to address this challenge if the Scottish Government takes the view that it wishes to continue to encourage gas CHP district heating.

1.18 Accounting for energy efficient design in target setting

226. As touched upon in section 1.6, there are aspects of the notional building target setting methodology which may not always reward all aspects of energy efficient design. One of the issues which has been raised previously within industry is the lack of incentive for design changes which improve thermal efficiency through adjustments to built form and shape. Because the notional building dimensions used for target setting are defined as being the same as the actual building dimensions, this means that built forms which have lower external wall areas (such as semi-detached compared to detached houses) and designs which adopt more simple rectangular shapes in terms of layouts are not directly rewarded for reduced heat loss area since the target emission/primary energy rate changes as well as the design emission/primary energy rate when they are altered. This issue was the subject of a report by NHBCF in 2016 (NHBC Foundation, 2016) and was also raised by some respondents to the Scottish Government's 2018 call for evidence on energy standards – as discussed in section 1.7.2 above.

227. The NHBCF report presented the idea of a 'form factor', which can be used to compare the relative efficiency of different built forms and shapes. In the report, it was calculated as follows:

• Form Factor = Total heat loss area of exposed walls^[33], roofs, floors and openings (m²)/ Total Floor Area (m²)

228. Some limited analysis was undertaken as part of the current project to compare form factors and key results for the core dwelling types which reflect different built forms. The aim was to assess whether differences in form factor could potentially be used to modify the performance targets set by the notional building (i.e. primary energy, or carbon if used as a secondary target metric).

229. It should be noted that the basis for the calculations undertaken was different from that used in the NHBCF report which compared five dwelling types with the same total floor area but different forms (i.e. flat, mid-terrace, semi-detached, detached and bungalow, all with simple rectangular layouts). The comparison within the current project did not compare like-for-like total floor areas but was undertaken to give an idea of trends within form factor and energy results for the different core dwelling types, to inform a discussion of how energy efficient design could potentially be rewarded within Building Standards. Similar to the NHBCF dwelling types, the core dwelling types in the current analysis all have simple layouts – so comparison focuses on built form rather than built shape.

230. The analysis compared form factors and primary energy results across the dwelling types modelled. It found that there was not a simple relationship between form factor and primary energy. Comparisons of primary energy results across dwelling types would also be impacted by assumed heating type which could be problematic depending on how any modification was implemented. Similar observations would be expected where carbon emissions were being compared. Previous analysis for Part L in England showed similar findings – even where different dwelling forms and shapes with the same total floor area and fabric specification were compared – and also showed that there were a number of variables relating to the differences in built form which impacted on primary energy results (for example, relative U-values and areas of different exposed elements; thermal bridging lengths and resultant y-values). The NHBCF report noted other variables impacting on results when looking at built shape as well.

231. The analysis also compared form factors and space heating demands. These are less affected by heating system design (though water heating gains have some impact). The relationship here appears somewhat clearer – with dwellings with higher form factors having higher space heating demands, as would be expected and as also shown in the NHBCF report.^[34] However particularly given the small sample of dwelling types modelled there is not enough evidence to define this relationship more precisely or to base a potential policy change upon.

232. Further work would be needed to establish if there was a means of using form factors to inform the adjustment of notional building targets which would reflect differences in space heating demands in a robust and fair way, avoiding loopholes and unintended consequences. This would need to consider a wider range of dwelling types. Challenges would include determining on what basis a good practice comparison form factor case would be set, and how this would be applied – for example within SAP or without. As an example, a notional building within SAP could be used where a standardised built form was considered, but this would potentially be very complex to define, and reflecting good practice in terms of shape would be potentially still more challenging. Alternatively, form factors could perhaps be used to implement a change outside of the notional building by applying an adjustment factor or penalty to the main proposed target metric of primary energy, or to a secondary carbon target metric, but the above analysis suggests that this would also be challenging to implement well.

233. These observations arguably reflect those implied in the NHBCF report, which noted in its conclusions section that "it is not possible to quote a single 'best practice' value of Form Factor at which designers might aim" (p.21), and focused on the idea of promoting greater understanding of form factor – suggesting that efficient building shape should be included as a key design consideration for new homes (as is stated in the Passive House standard).

234. As already discussed in section 1.6, there are various alternative approaches to encouraging design optimisation for energy efficiency beyond adjusting the notional building definition or targets. For example, setting limits directly onto space heating requirements can do this, particularly if they are defined in absolute terms (as in the current optional targets in Section 7 of the Domestic Technical Handbook). Given the findings of the above analysis this might be a more pragmatic approach to consider further, and it has the benefit of being a metric which already has currency within industry given its use in Section 7 and in the Passive House standard for example. As already noted in section 1.6 of the current report, the Section 7 targets would potentially need reviewing/checking to align with improvements made to Section 6 standards, to identify how changes in the methodology in SAP 10.1 may affect the figures, and perhaps also to generally review their evidence base including consideration of a range of dwelling types.