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

Research to identify potential improvements in energy and emissions performance for new domestic buildings. Produced in support of proposed improvements to energy standards for new buildings within Scottish building regulations in 2021.

Task 2: Develop Improved Notional Building Specifications

87. Task 2 of the analysis was to develop potential improved notional building specifications for new buildings in Scotland. The aim was to identify three proposed specification levels representing 'low', 'medium' and 'high' uplifts over Building Standards 2015. To achieve this involved the following steps:

1. Reviewing current notional buildings for design optimisation.

2. Identifying potential improved specifications based on a review of key existing data sources.

3. Analysing opportunities, constraints and risks.

4. Proposing specification options for review.

88. This section of the report sets out the evidence and rationale for the approach taken. It also summarises key findings and proposals.

1.6 Review of current notional buildings for design optimisation

89. A high-level review was undertaken looking at how the notional building could be defined at a strategic level to encourage design optimization – i.e. encouraging designs with lower energy consumption and carbon emissions.

90. One significant specification item which it has been suggested would benefit review in terms of impact on energy performance is the assumption that the size and shape of the notional building are the same as the actual building.[9] This will be considered in later analysis on potential modifications to performance targets for energy efficient design, focusing on built form (see section 1.18).

91. Another significant item is the differentiation of the notional building by fuel type. The Scottish Government wished to explore moving to a single notional building in 2021 instead. If the 2021 notional is based on gas, for example, this would allow heat pumps to receive a benefit in comparison, and the implications of this (in terms of potential relaxation of fabric specifications, for example) are considered at a later stage of analysis, in sections 1.10.6 and 0. It may also help to disincentivise higher carbon fossil fuels such as oil and LPG, though this may depend on whether carbon targets are set (as primary energy factors for these fuel types are similar to mains gas).

92. Specification items in the current notional building relating to opening assumptions have also been considered, but it was not thought that changing these assumptions would have a significant impact on design optimisation.

93. It should be recognised that the notional specification should be viewed in the light of overall targets, and that there are other approaches to encouraging design optimisation for energy efficiency beyond adjusting the notional building definition. For example, additional targets relating to limits on maximum values (as in Section 6 Table 6.3 of the Technical Handbook (Scottish Government, 2019a)), or setting limits on space heating requirements can do this, particularly if they are defined in absolute terms (as in the current optional targets in Section 7).[10] The space heating targets in particular reward more energy efficient exposed surface to floor area ratios, glazing designs which make use of solar gains (with potential for increasing availability of natural light as a side-effect), and other measures to make use of passive heating.

1.7 Identification of potential improved specifications

94. Several key data sources were reviewed to inform proposals for potential improved notional building specification options for 2021 Building Standards. These included:

  • The EPC database extract for new dwellings for years 2016-2018, processed as described in section 0 above.
  • Responses to the Scottish Government's 2018 call for evidence on energy standards for new buildings.
  • Proposals for Part L 2020 new dwelling notional buildings in England and Wales.
  • Research informing the development of 2015 standards.
  • The 2019 report on the assessment of cost-optimal energy performance requirements for the UK.

The findings from this review are summarised in the sub-sections below.

1.7.1 Review of EPC database extract

95. As the EPC database extract covered registrations over three years only, it was difficult to robustly identify trends in much of the data. In any case, fabric U-values and airtightness values did not appear to vary significantly by year. The distribution of average fabric U-values by main building element (external wall, floor and roof – no useful detailed data was available on windows) were more usefully analysed, with results shown in Figure 1.7a, Figure 1.7b and Figure 1.7c. Table 1.7a provides a summary of values at the 10th, 25th and 50th percentiles for the fabric elements, compared to the current 2015 notional building specifications. U-values over the maximum limits in Table 6.3 of the Technical Handbook (Scottish Government, 2019a) were excluded from this analysis. The findings suggest that a significant proportion of buildings improve upon the current notional building values and this has been used to inform U-value proposals in section 1.10.2.

Figure 1.7a: External wall U-value distribution in EPC database extract

Graph showing the range of wall U-values for new dwellings from energy performance certificate data and the proportions for each range of values. The most common U-value range is 0.21 to 0.22. 75% of walls were better than 0.21, 50% of walls were better than 0.19 and 25% of walls were better than 0.17.

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

Figure 1.7b: Floor U-value distribution in EPC database extract

Graph showing the range of floor U-values for new dwellings from energy performance certificate data and the proportions for each range of values. The most common U-value range is 0.15 to 0.16. 75% of floors were better than 0.17, 50% of floors were better than 0.15 and 25% of floors were better than 0.12.

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

Figure 1.7c: Roof U-value distribution in EPC database extract

Graph showing the range of roof U-values for new dwellings from energy performance certificate data and the proportions for each range of values. The most common U-value range is 0.1 to 0.11. 75% of roofs were better than 0.13, 50% of roofs were better than 0.11 and 25% of roofs were better than 0.1.

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

Table 1.7a: EPC database extract 10 th, 25 th and 50 th percentile U-values for main fabric elements, compared to 2015 notional building specification
Element 2015 Notional 10th percentile 25th percentile 50th percentile
Wall (W/m2K) 0.17 0.15 0.17 0.19
Floor (W/m2K) 0.15 0.11 0.12 0.15
Roof (W/m2K) 0.11 0.09 0.10 0.11

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

96. Heating trends show little variation by heating type by registration year; the exception being the proportion of dwellings with ASHP which increased from 4% of dwellings in 2016 to 9% in 2018 (this was offset by slight decreases in the proportions of mains gas-heated, direct electric-heated, oil-heated homes and homes connected to district heat networks). This is shown in Figure 1.7d.

Figure 1.7d: Heating type trends by registration year in EPC database extract

Graph showing the proportion of heating types in new homes for 2016. 2017 and 2018. Key fact – mains gas is the largest heating type, in more than 80% of new homes in all three years. Gas and electric heating account for over 90% of heating. Air source heat pump is the second most common heating type, increasing from less than 5% in 2016 to around 10% in 2018.

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

97. Trends in solar PV installations have already been discussed under section 1.5.6 and do suggest a clear change, likely linked to the increase in 2015 Building Standards-compliant dwellings over the period. As shown in Figure 1.7e, it was found that the proportions of dwellings with PV installed in each registration year increased significantly between 2016 and 2018 – from around 20% to 50% for flats, and from around 5% to 40% for houses.

98. In terms of array size, Table 1.5i showed the median and interquartile ranges for different dwelling types (not taking into account floor area variation). Typically the median values were close to those in the 2015 gas-heated notional building, with the median values for detached houses being higher and median values for flats being lower, though this is likely to relate to variation in the sizes of dwellings with PV installed within the dwelling type categories. It is not possible to draw strong conclusions from the data in terms of the feasibility or cost-effectiveness of larger array sizes, as sizes are likely to be significantly influenced (i.e. limited) by the targets set in the notional building.

Figure 1.7e: PV trends by registration year in EPC database extract

Graph showing proportion of new house and new flats with photovoltaic installations between 2016 and 2018.  Photovoltaic installations have increased from around 5% to almost 40% in houses and from 20% to over 50% for new flats.

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

1.7.2 Review of consultation responses

99. In June 2018 the Scottish Government conducted a consultation calling for evidence on 2015 energy standards for new buildings, with the aim of informing future standards (Scottish Government, 2018). It should be noted that specific questions were not posed in the consultation, though some examples of possible topic areas were provided. There were 41 published responses to the consultation, and these have been reviewed for particularly relevant information which might inform improved specifications for 2021 Building Standards. As might be expected, responses tended to provide broad indications of support for – or commentary on the advantages and/or disadvantages of – different strategies (e.g. pushing fabric, installing renewables, approaches to target-setting), rather than detailed input on specific suggested performance values for different elements of future notional buildings. It should be noted that the review did not aim to provide an exhaustive summary of responses. Key points are summarised below, covering fabric, ventilation, low and zero carbon technologies, other heating technologies, and overall standard setting.

100. In terms of fabric specifications, several responses gave support for pushing fabric further than current. Advantages noted included their longevity compared to services and low and zero carbon technologies. Several other responses indicated support for fabric improvements but also raised potential issues or limitations including approaching cost-benefit thresholds; potential health impacts relating to indoor air quality, overheating and condensation/damp; the need to consider and provide guidance on the impact of reduced U-values on thermal bridging/construction details; and similarly to address their interrelation with airtightness standards. Other relevant comments included a suggestion that fabric standards should be comparable with those in Europe , though detail was not provided on what this might mean. One response provided an indication of where developers may most commonly be incurring costs from pushing specifications beyond current notional building standards: walls, roofs and floors, and triple glazing (and also MVHR on the ventilation side).

101. Comments relating to ventilation were less common, and mostly related to concerns with ventilation/compliance strategies or the details of how standards are implemented. These included concerns that: developers are currently sometimes using inappropriately low airtightness rates to avoid installing low and zero carbon technologies,[11] that airtightness tests should be required for all properties (i.e. the use of a SAP default should be removed) to avoid inappropriate ventilation strategies in practice, that airtightness rates/ventilation strategies are currently leading to damp/mould issues. One respondent commented on disadvantages of mechanical ventilation systems, raising issues of embodied carbon impact, increased electricity use, maintenance, and use in practice.

102. There were many comments on low and zero carbon technologies. These included particular support for their inclusion in future standards from several. Respondents sometimes indicated broad support for a range of technologies whereas others were especially supportive of particular ones. Advantages noted included the role of new building installations in developing the wider supply chain (with implications for improved quality and reduced costs), increasing occupant familiarity with technologies; and in the case of solar PV its cost-effectiveness, the potential to adjust the notional building to take advantage of the economies of scale of larger array sizes by maximising use of available roof space, and benefits to occupants in terms of energy bill savings[12].

103. Potential disadvantages of or issues with low and zero carbon technologies were also raised in many responses. Costs to the consumer were highlighted including capital costs (passed on to occupants), operational and maintenance costs with some noting particular concerns for maintenance costs of some technologies in rural locations. Many of the responses also raised concerns about constraints on the electricity grid; particularly in relation to PV but also in relation to heat pumps. Such concerns also apply to some other technologies, especially direct electric heating.

104. Specifics of these concerns included: particular constraints on the grid in rural and island locations, and affecting large areas overall – though one respondent noted that the impact was greatest on commercial projects, with some respondents suggesting that PV systems are regularly having to earth generated electricity to protect the grid. One respondent suggested that PV requirements should not be increased until the electricity network had been upgraded.

105. Several other respondents however proposed mitigating measures. The potential benefits of PV in electricity system balancing and stabilisation were also commented on, particularly where combined with battery storage and the increased uptake of electric vehicles.[13] Support for electrical battery storage and better alignment of generation with use to reduce impacts on the grid was also expressed by other respondents. Suggestions included incentives or requirements for storage within SAP/Building Standards, and introducing the ability to reflect time of use of electricity within SAP. The need to improve alignment between Building Standards with Planning was suggested by one respondent. Another noted a need to build upon existing work with Distribution Network Operators to accurately identify and calculate grid constraints, review network upgrades and allow flexibility for embedded generation; and the potential to use export limiters where needed as a temporary measure until longer term solutions were found . For electric heating, one respondent suggested that minimum efficiencies should be specified (>100%) to mitigate impacts on the grid and avoid wasting electricity now that electricity carbon emission factors are so low, as is currently done in Norway.

106. Other concerns specifically about PV were raised by some respondents including lack of roofspace with optimum orientation – particularly for flats, inverter replacement frequency, and its embodied carbon impact. Other concerns specifically about heat pumps were raised by some respondents including the need for training, operational complexity, maintenance issues, performance in very cold weather, and longevity and performance particularly in island locations exposed to sea water.

107. Support for direct electric heating was specifically expressed by one of these respondents, who noted lower capital costs and complexity and increased longevity compared to heat pumps, and the low carbon emission factors for electricity in Scotland and particularly on some islands. Support for electric heating was given by another respondent too. Some respondents suggested that heat pumps were being/might be replaced at a later date with lower efficiency alternatives and it was noted that there is no provision to stop this happening. Other responses raised concerns with direct electric heating; including disbenefits for consumers in terms of energy costs/inefficiency and impacts on the grid.

108. Other comments relating to heating systems included concerns about biomass such as lack of skilled installers and maintenance teams, and costs of maintenance particularly in rural locations – noting that 2015 Building Standards encouraged biomass in such locations. One respondent also questioned the performance of waste water heat recovery systems (WWHR), stating that it needs very low flow rates to work. Another noted support for futureproofing if/where low carbon heating is not included in developments.

109. Overall standard setting was commented on in many of the responses. It should be noted that the introduction of primary energy as a target metric was not mentioned in the Scottish Government's consultation document, and so was not mentioned in the responses. Additionally, whilst a range of information gathered from responses relating to standard setting has been included here, consideration of some of the approaches proposed (for example consideration of allowable solutions; potential targets other than primary energy, carbon emissions or performance target modifiers for energy efficient design relating to built form; other changes outside of defining the Section 6 notional building) is outside of the scope of AECOM's current work and so would need to be considered separately by Scottish Government.

110. Several of the responses included support for setting targets based on energy consumption – with suggestions of basing this on net consumption recognizing the contribution of renewables or on energy efficiency.[14] Consideration of the impact of built form on energy efficiency within Section 6 was also recommended by a couple of respondents. Another specifically noted that targeting Section 7 Gold Aspect 2 instead of Silver Aspect 2 (maximum annual space heating demand for houses/flats of 30/20 kWh/m²/yr instead of 40/30 kWh/m²/yr) had been reported as being challenging based on current construction methodologies and requiring a change in mainstream techniques.

111. Some of the responses called for the use of Passive House standards, either introduced in part/in a staged approach (e.g. QA, airtightness requirements, space heating requirements) or as exemplar standards or alternative compliance routes – one respondent noted that extra-over costs of building to Passive House standards are estimated at between 0%-10% as a proportion of build costs.

112. In terms of the maximum values for different elements specified in Section 6 Table 6.3, limited comments were made. One respondent noted that the values were generally strong but suggested that reducing the U-value for cavity party walls to zero (instead of 0.20 W/m²K) would be an effective and low-cost change. Various responses also indicated that the airtightness value (of 15 m³/m².h@50Pa) was no longer appropriate.

113. Support for retaining carbon as a metric was expressed by several respondents, with some of these expressing support for a zero/net zero carbon target in the near term but with the need for clarity on timescales and careful implementation noted; and one suggesting that a target equivalent to Section 7 Gold Standard (i.e. a 27% improvement over 2015 standards[15]) should be considered. There was support for review and use of the Silver/Gold system from other respondents too – though this extends beyond carbon targets alone. Some respondents noted concerns with the changes to the carbon emission factor for electricity and its impact on certain technologies (reduced PV attractiveness, increased direct electric attractiveness), with the need for mitigating measures/supplementary targets suggested.

114. In relation to zero carbon targets, support for allowable solutions was specifically noted by one respondent, opposition noted by another, and qualified support from another (i.e. if their overall contribution was limited and depending on the use of funds).

115. Various comments were also made relating to the performance gap, including: the difficulty of checking/verifying SAP assessments, the need for encouragement for quality assurance, and for measuring and reporting real energy and carbon performance.

116. In terms of more general comments on the development and implementation of standards, one respondent in particular noted concerns with increased costs of future standards (including differences from other UK administrations and impacts on smaller developers) and suggested that wider measures would be needed to help mitigate these (e.g. consideration of the other recommendations in the Sullivan Report; clarity on and alignment with related policies and strategies e.g. for heat decarbonisation, training, business support; changes to planning authorities' abilities to set targets beyond Building Standards). This respondent also called for clarity on the timing and definition of future standards – a point echoed by others. Other comments were made relating to the need for holistic review of standards (e.g. Section 7, consideration of thermal comfort, flood resilience etc.) and to the need to carefully assess deliverability.

1.7.3 Review of England and Wales Part L 2020 proposals

England and Wales have both recently held consultations on proposed changes to Part L1A (their equivalent of Section 6 standards for new dwellings) which are due to come into effect in 2020. Details of these will be considered when shortlisting potential specifications for 2021 standards in Scotland, if and where appropriate. The key specifications proposed for their notional buildings (preferred options) are summarised inTable 1.7b. Table 1.7bTable 1.7b: Part L 2020 consultation preferred options for notional building specification – England and Wales, compared to Scotland 2015 gas notional building

Element Scotland 2015 notional building (gas) England Part L 2020 consultation preferred option Wales Part L 2020 consultation preferred option
External Wall U-value (W/m2K) 0.17 0.18 0.13
Corridor Wall U-value (W/m2K) 0.17 0.18 0.18
Party Wall U-value (W/m2K) 0.0 0.0 0.0
Roof U-value (W/m2K) 0.11 0.11 0.11
Floor U-value (W/m2K) 0.15 0.13 0.11
Window U-value (W/m2K) 1.4 1.2 1.3
Window g-value 0.63 0.63 0.63
Door U-value (W/m2K) 1.4 1.0 1.0
y-value (W/m2K) 0.08 Based on SAP 10.1 Appendix R (option 2 column; values unchanged from SAP 9.92) Based on SAP 10.1 Appendix R (option 2 column; values unchanged from SAP 9.92)
Ventilation type Intermittent extract fans with trickle vents Intermittent extract fans with trickle vents Intermittent extract fans with trickle vents
Air permeability rate (m³/m².h @50Pa) 7 5 5
Space heating source Condensing gas boiler Condensing gas boiler Condensing gas boiler
Domestic hot water source As for space heating As for space heating As for space heating
Boiler efficiency 89.0% (SEDBUK) 89.5% (SEDBUK) 89.5% (SEDBUK)
Heat emitters Standard radiators Large (low temp) radiators Large (low temp) radiators
Space heating controls Time and temp control, weather compensation, interlock, delayed start ErP Class V, time and temp control, interlock ErP Class V, time and temp control, interlock
Hot Water Controls / insulation (where applicable) Cylinder thermostat, separate timer, fully insulated primary pipework Cylinder thermostat, separate timer, fully insulated primary pipework Cylinder thermostat, separate timer, fully insulated primary pipework
Shower flow rate (l/min) n/a 8 8
WWHR Efficiency of 36% 2 showers if TFA>100m2, otherwise 1 shower Efficiency of 36% Utilisation of 0.98 Connected to all showers Efficiency of 55% Utilisation of 0.98 Connected to all showers
Fixed lighting capacity (lm) n/a 185 x TFA 185 x TFA
Lighting efficacy (lm/W) n/a 80 80
PV installation kWp equivalent to 1% of total floor area; 8.3 m²/kWp assumed for calculating roof area limit Area equivalent to 40% of building foundation area; 6.5 m²/kWpassumed Area equivalent to 40% of building foundation area; 6.5 m²/kWpassumed

Notes Further details are provided in SAP 10.1 Appendix R (BRE, 2019).

Low temperature radiators (and associated low boiler flow temperatures) can be seen as a future-proofing measure.

For PV, there are some minor differences in other assumptions compared to the Scotland 2015 notional building e.g. roof pitch.

Source MHCLG, The Future Homes Standard, 2019 Consultation on changes to Part L and Part F (MHCLG, 2019f)

BRE, SAP 10.1 Appendix R (BRE, 2019)

Welsh Government, Consultation Document, Building Regulations Part L and F Review (Annex A) (Welsh Government, 2019b)

117. It can be seen that in some areas the proposed specifications go beyond the equivalent notional building 2015 standards in Scotland to varying degrees – including external wall U-values (Wales only), floor U-values, window U-values, thermal bridging, air permeability rates, boiler efficiency, boiler flow temperatures, WWHR efficiency (Wales only), and PV array sizes.

118. Both consultations note that heat pumps would provide alternative compliance options in 2020, and that the next iterations of standards in 2025 would be expected to be based upon low carbon heating; which is anticipated to be typically delivered using heat pumps and/or heat networks (as well as possibly direct electric heating in some circumstances). They also note that 2025 standards would be expected to include higher fabric standards (in particular noting that triple glazing is likely to be part of specifications).

119. Whilst reviewing the backstop values is outside of AECOM's scope, it is recommended that that the Scottish Government carefully considers the minimum energy efficiency standards for the individual building fabric and building services elements. This is likely to be particularly important if the 2021 notional building is based on gas, as in this case where low carbon/primary energy heating is specified by developers, it can be that the design solution significantly improves upon the notional building target. This potentially allows significantly poorer fabric and service efficiencies to be adopted.

1.7.4 Review of research informing 2015 standards

120. Work was undertaken on behalf of the Scottish Government in 2011 and 2012 to inform proposals for 2015 domestic Building Standards. Whilst a summary of all of this research is not considered necessary, there are some points which may provide useful context for future standards. As the 2012 report built upon the 2011 research, only the 2012 report has been focused on here.

121. The 2012 report was based on modelling for 14 dwelling types, primarily testing both gas and electric (ASHP) heating specifications to meet 2010 standards and how these might change to meet potential future carbon targets (45% reduction over 2007 standards, or 60%). Overall in their central cost benefit scenario the improvements resulted in net costs (with only the improvements to electrically heated dwellings showing a significant net benefit).[16]

122. The report also considered the impact of including a 'useful energy for space heating' target of 40 kWh/m2/yr for houses and 30kWh/m2/yr for flats, finding that applying this would lead to increases in capital costs but to more efficient fabric, as would be expected. The report noted that it would serve a similar function to backstops but would also account for the energy efficiency of the built form (e.g. ratio of floor area to exposed surface area; window orientation). This target is included as an optional standard under Section 7 (Aspect Silver level 2) but was not taken forwards as part of 2015 consultation proposals.

123. The Scottish Government's consultation report on the 2015 proposed changes (Scottish Government, 2014) provides more useful context. It shows that there was support from a majority of the consultation respondents for the following key domestic proposals (percentages agreeing with shown, with a total of around 80 respondents per question): inclusion of low carbon equipment for electricity and biomass packages (58%); addition of PV for gas, LPG and oil packages (56%); introduction of waste water heat recovery for all packages (58%); improvement to U-values (69.7%). Whilst it should be recognised that these responses were provided prior to implementation of the standards, they provide some background for future changes.

1.7.5 Review of UK cost-optimal report

124. In 2019 a report by AECOM and Currie & Brown was published by MHCLG providing the second cost optimal assessment of energy performance requirements for the United Kingdom, based on analysis undertaken in 2016 (MHCLG, 2019c). The report compares to relative lifecycle-cost-effectiveness of packages of measures to reduce the primary energy in new domestic buildings, based on a semi-detached house and a block of flats – using similar dwelling typologies to those in the current analysis. Elemental values were not separately assessed for new buildings. Macroeconomic costs were assessed with central energy prices, at a 3.5% discount rate and over a 30-year period in the central analysis scenario. An example graph is shown in Figure 1.7f. The cost-optimal point was chosen as being at the base of the curve where lifecycle costs and primary energy were plotted. It can be seen however that there are solutions available with lower primary energy and relatively small increases in lifecycle costs.

Figure 1.7f: UK cost-optimal analysis results for a semi-detached house

Graph showing distribution of cost-optimal measures to reduce the primary energy demand in a semi-detached house. This compares that energy demand with the total lifecycle cost of measures. The example, taken from the cost optimal report published in 2019, shows that there are a range of high, medium and low cost solutions which can reduce primary energy demand to a total below the reported UK average for new homes of that type.

Source MHCLG, UK cost-optimal report, Figure 6.1a (MHCLG, 2019c).

125. The headline finding from the report was that dwelling in Scotland at 2015 standards (based on the notional building specification for gas-heated dwellings) were assessed as being beyond cost-optimal levels. Semi-detached houses were estimated to have primary energy levels of 74 kWh/m2/yr, and flats 71 kWh/m2/yr, compared to estimated cost-optimal levels of 96 kWh/m2/yr for houses, 77 kWh/m2/yr for flats.

126. The report included tables summarising various solutions which appeared along the bottom of the cost-optimal curves, i.e. had relatively low lifecycle costs for the level of primary energy modelled (see the example cost-optimal curve shown in Figure 1.7f). This included several solutions found to have a lower primary energy than 2015 standards – these have been shown in Figure 1.7g, for the semi-detached house.

Figure 1.7g: Results of the UK cost calculations for the most cost-optimal packages, semi-detached house

Table taken from the 2019 cost optimal report which shows examples of elements of building specification used in the development of cost-optimal packages of measures modelled in the report.

Source MHCLG, UK cost-optimal report, Table 5.12a (MHCLG, 2019c).

Notes Costs are shown on a per square metre of floor area basis.

The green box shows cost-optimal solutions with primary energy below current standards, as modelled in the report.

The orange box shows the cost-optimal solution chosen as being at the lowest point of the curve.

127. Various comparisons can be made using the data in Figure 1.7g (and similar findings could be seen looking at the flat dwelling type). For example:

  • Solutions which were modelled as having lower primary energy than current standards typically include fabric packages with wall U-values around current notional building levels.
  • Going down to the 0.12 wall U-value package – which also includes a floor U-value of 0.10 compared to 0.15 in most other packages – involves a leap in lifecycle cost and capital cost (this can be seen by comparing the top two rows: the difference in macroeconomic cost per square metre is around £24; the difference in initial investment cost per square metre is around £32; the difference in primary energy is around 5kWh/m2/yr).
  • Triple glazing also appears to involve a leap in lifecycle cost and capital cost (this can be seen by comparing the fourth and fifth rows: the difference in macroeconomic cost per square metre is around £13; the difference in initial investment cost per square metre is around £14; the difference in primary energy is around 3kWh/m2/yr. These cases also have a change in fabric package, but this can be seen elsewhere to have a limited impact – comparing the row highlighted in orange with the one above it – and these costs/primary energy savings have been subtracted off).
  • WWHR is included in several solutions, and this involves a low increase in lifecycle cost and a relatively low increase in capital cost (this can be seen by comparing the row in the orange box with the row two above it; the difference in macroeconomic cost per square metre is around £3; the difference in initial investment cost per square metre is around £8; the difference in primary energy is around 7kWh/m2/yr).
  • MVHR is included at the lowest levels of primary energy; here there is a leap in lifecycle cost and capital cost (this can be seen by comparing the third and fifth rows; the difference in macroeconomic cost per square metre is around £57; the difference in initial investment cost per square metre is around £38; the difference in primary energy is around 13kWh/m2/yr).
  • Solar thermal is included at the lowest levels of primary energy; this appears to involve a leap in lifecycle cost and capital cost for a relatively small saving in primary energy when compared to gas + waste water heat recovery (a direct comparison is not possible, but the increase can be approximated by comparing the second and fourth rows and subtracting out the difference for MVHR shown above; the resulting difference in macroeconomic cost per square metre is around £51; the difference in initial investment cost per square metre is around £63; the difference in primary energy is around 5kWh/m2/yr).
  • All solutions below current standards include PV at 40% of the building foundation area (PV areas are reduced where solar thermal is also installed), and this involves a low increase in lifecycle cost and a leap in capital cost for a significant reduction in primary energy (this can be seen by comparing the bottom row in the green box with the row in the orange box; the difference in macroeconomic cost per square metre is around £3; the difference in initial investment cost per square metre is around £37; the difference in primary energy is around 48kWh/m2/yr but electricity factors have changed significantly since).
  • Thermal bridging values are improved in all solutions shown above current standards.
  • In the most improved case of all the solutions shown, the primary energy was reduced to 12kWh/m2/yr. Higher figures were shown for the flat dwelling type, likely due to the relatively limited roof area available per flat.

128. It is however important to note that results would change with updated input assumptions. In particular, the primary energy factors for electricity in SAP 10.1 are significantly lower than those used in the cost-optimal analysis.[17] In addition, the costs of some measures may also have changed,[18] and the performance of some technologies may have improved. However the analysis provides some useful findings for potential future standards, particularly in terms of the cost impacts of different measures.

1.8 Review of risks relating to overheating and indoor air quality

129. A targeted review was undertaken to consider the potential increased risks associated with fabric improvements relating to poor indoor air quality/ventilation and summer overheating in new homes. AECOM recently undertook work on these topics to inform the consultation on Part L 2020 in England, and reports summarising this research were published as part of the consultation package (MHCLG, 2019g; MHCLG, 2019d; MHCLG, 2019e).

1.8.1 Indoor air quality risks

130. The 2019 report on ventilation and indoor air quality was based on various levels of inspection/monitoring of 80 new build homes in England with design airtightness values of under 7m3/m2.h@50Pa, designed to England Part F 2010 and Part L 2010/2013 standards (25 with dMEV and 55 naturally ventilated).[19] The research found that only three of these homes met the minimum recommendations in England's Approved Document F relating to extract fan flow rates and trickle ventilator provision, despite testing and commissioning requirements. Poor indoor air quality was found in a number of the homes. In these cases the minimum ventilation provisions recommended in Approved Document F were not being met in practice, and it was suggested that this would explain some of the issues found. Concerns were also raised about whether the recommendations themselves provide sufficient fresh air in naturally ventilated bedrooms e.g. trickle ventilators being hidden when curtains were closed[20]. Some concerns were also raised in the report about noise impacting on the use of ventilation systems and subsequently reducing ventilation rates – including some residents reporting that they turned off extract fans due to their noise, and some that they closed trickle vents due to external noise.

131. The research has informed changes proposed to Approved Documents F and L in England in MHCLG's consultation (MHCLG, 2019f). In particular, in relation to Part L, the consultation proposes not accruing energy savings in SAP for improving the airtightness in naturally ventilated dwellings beyond 3m3/m2.h (defined as 'highly airtight' homes) to reduce the risk of insufficient natural ventilation in airtight properties. It is also noted that MHCLG has not included mechanical ventilation in the consulted notional building options. In relation to Part F, in the case of natural ventilation the consultation proposes setting guidance for the size of background ventilators on a per room (rather than per house) basis, and only providing guidance for less airtight homes.[21] In the case of continuous mechanical extract ventilation, the proposals also include recommending background ventilators in more airtight dwellings. In terms of airtightness testing, it is proposed to report results in SAP to the nearest 0.5m3/m2.h to account for uncertainty, to require all properties to be tested and to revise the testing methodology.

132. A study commissioned by Scottish Government in 2014 looked at the impact of occupant behaviour in naturally ventilated homes, based on a survey of 200 homes and monitoring of a sample of 40 of these homes, alongside analysis of other monitoring undertaken as part of the TSB's Building Performance Evaluation programme (Sharpe et al., 2014). This followed previous research which looked at the impact of increased airtightness on indoor air quality and concluded that the guidance in Scotland's Domestic Technical Handbook on Standard 3.14 relating to natural ventilation was fit for purpose at airtightness levels of 5m3/m2.h@50Pa or above; but based on the assumption that trickle ventilators (and internal doors) would all be open (BRE, 2011).

133. The 2014 study found that in practice trickle vents are infrequently adjusted by occupants with the majority being left closed, and suggested that the main driver was generally thermal comfort (i.e. avoiding heat loss, rather than providing fresh air and enabling control of moisture and pollutants). The impacts of closed trickle vents included low ventilation rates (evidenced here by monitored CO2 levels), but issues were also found in dwellings were trickle vents were left open.

134. The 2014 study helped to inform changes to ventilation requirements within the Domestic Technical Handbook in 2015, which included the introduction of a requirement for carbon dioxide monitors to main bedrooms in new homes with an airtightness level below 15m3/m2.h@50Pa, and changes to the calculation of required trickle ventilation areas which increased their provision in practice.

135. A further recent report commissioned by Scottish Government considered the effectiveness of dMEV in providing whole-house ventilation in the context of increasing airtightness, based on a study of 223 new homes with airtightness levels between 3 and 5m3/m2.h@50Pa (Sharpe, et al., 2018). It found poor overnight ventilation in over 50% of homes (citing a variety of contributing factors), and that around 40-50% of installed systems were sub-optimal or non-compliant. It concluded that the evidence suggested that "whilst there are some situations where a dMEV system can assist with the ventilation provision of modern airtight homes, the ability to act as a whole house system is limited, particularly in larger more complex layouts, and where ventilation loads are high" (p.5).

136. Whilst the reports were written in the context of upcoming improvements to energy standards for dwellings, neither specifically looked in detail at potential future changes to fabric design and the potential impacts on indoor air quality. However as problems were found with homes built to iterations of energy standards below 2015 levels it is reasonable to expect many of these issues may become more problematic in the future unless addressed – the 2019 report in particular noted an expected increased reliance on background ventilators as general infiltration is reduced. The reports focused on improvements relating to ventilation guidance and testing rather than recommending any limits on fabric performance.[22] As such it is suggested that this information is considered under any separate review of ventilation requirements which may be undertaken.

137. Points most relevant to the current work include proposals on limiting energy savings airtightness in naturally ventilated homes in SAP; it is suggested that the Scottish Government consider these. Other proposals in the English consultation relevant to Part F could also be considered as these may help to mitigate unintended consequences of more airtight dwellings.

138. In terms of informing the notional building specification in 2021, the research suggests that there are potential risks associated with any ventilation system type, in terms of design, construction, installation, commissioning and operation. Wider work would be needed to address these. AECOM's proposals for the airtightness specification in the 2021 notional building will take into account the 'highly airtight' definition in the consultation version of England's Approved Document F (design air permeability rates lower than 5m3/m2.h@50Pa; or as-built air permeability rates lower than 3m3/m2.h@50Pa) (MHCLG, 2019a), with a value of 5m3/m2.h@50Pa proposed if and where a natural ventilation strategy is assumed and a value of 3m3/m2.h@50Pa if and where MVHR is assumed.

1.8.2 Summer overheating risks

139. The 2019 research on overheating in new homes included a phase 1 report focusing on better understanding the dwellings most at risk, based on dynamic modelling and different dwelling types and locations in England and on the CIBSE TM59 definition of overheating (MHCLG, 2019d; CIBSE, 2017).

140. The dwelling types assessed included different built forms (detached/semi-detached/terrace/flat), sizes (large/small flat, mid-rise/high-rise), aspects (dual/single for flats), ventilation strategies (natural ventilation/MEVMVHR was not analysed), heating systems (individual gas boiler/communal) and construction types (masonry for houses, concrete-/steel-frame for mid-/high-rise flats – so not including timber frame, but with low thermal mass assumed), all compliant with England Part L 2013 (with fabric specifications generally similar to the England Part L 2013 notional building). The core analysis made various assumptions, including: all living rooms facing south, continuous occupancy, unrestricted window opening, and no external shading or internal blinds/curtains (the latter were looked at in phase 2 of the work).

141. The study found that all dwellings modelled failed to comply with the CIBSE TM59 criteria, with higher risks for flats, and greatest risks in London locations. Results were found to be sensitive to orientation (with West-facing living rooms performing worse), flat location in block (with ground-floor flats performing worse due to lower wind speeds and ventilation rates; in practice window-opening restrictions would also have an impact),[23] window opening behaviours (including restrictions on opening), and weather data – with the last two variables being particularly significant. Fabric infiltration rates were not found to affect the results significantly.

142. The phase 2 report assessed the costs and benefits of different overheating risk mitigation strategies for different building types and locations, including modifications to building design (but also occupant behaviour). Further dynamic modelling was undertaken to assess the most cost-effective package for each scenario, from five graded options which all prioritised passive measures, to reduce overheating risks to comply with CIBSE TM59. Costs included capital, replacement, and energy costs and the cost of carbon. Quantified benefits included reduced mortality and improved mortality.

143. MHCLG has announced their intention to hold a consultation on overheating in new dwellings in early 2020, following which new overheating regulations and guidance will be produced and are expected to come into force in mid-late 2020 alongside revised Part L and Part F regulations (MHCLG, 2019f). It is noted that SAP is not set-up to assess compliance with the CIBSE TM59 criteria which requires dynamic thermal modelling.

144. Earlier research into low energy homes in Scotland (Morgan et al., 2015; Foster et al., 2016) has suggested that there is growing evidence of overheating already occurring in Scotland despite more severe climate projections being delayed compared to more southerly parts of the UK. The research focused on Building Performance Evaluation studies for 26 low energy new build homes in Scotland – including 5 Passive Houses – built under 2007 or 2010 standards, which already showed incidences of overheating. The homes were analysed based on Passive House criteria for overheating,[24] and under a third were found to overheat for less than 10% of the year, with over half of homes exceeding the threshold temperature for more than half the year. Findings suggested that design and occupancy factors had more of an impact than location or climate, but the authors noted the difficulty of identifying primary contributing factors or trends. However it was identified for example that none of the homes had external shading, that occupant understanding and control use was often poor, and a range of other factors such as uninsulated pipework were common. The prevalence and potential impact of lightweight timber construction was also noted; this has been associated with increased overheating risks in previous studies (Holmes & Hacker, 2007; Peacock, Jenkins, & Kane, 2010; Dengel & Swainson, 2012), and is particularly relevant for Scotland. The heat loss parameters (which relate to fabric and ventilation heat losses) were low (under 2.1W/m2K) for all the dwellings but there was not a clear correlation with overheating – in fact the two homes with the highest heat loss parameters showed some of the worst levels of overheating.

145. There were not clear findings to suggest the Passive House homes performed worse or better compared to the others in terms of overheating, and the sample was quite small. It was observed however that there were issues with imbalanced MVHR systems and insufficient ventilation rates (affected by occupancy density) in most of the Passive House homes. The research cited an earlier study suggesting that installing external solar shading and adjusting glazing ratios could significantly contribute to mitigating future overheating risks in Passive Houses (McLeod et al., 2013), and noted that other simple passive design measures would also be helpful (e.g. provision of high level openings for purge ventilation, higher ceilings on upper floors).

146. This research into overheating in low energy buildings in Scotland was cited in a later report for Scottish Government looking at options for climate change mitigation in the built environment (Aether, 2017). This report concluded that further research is necessary investigating the benefits of the Passive House approach and how to adapt it to Scotland's climate and culture.[25] A more recent report on climate change risks in UK housing by the Committee on Climate Change also noted that there are limited studies on overheating in Scotland more generally, implying that further research is needed (Committee on Climate Change, 2019b). It estimated that total heat-related deaths in Scotland may increase from around 40 per year to between 70 and 280 per year by the 2050s.[26]

147. In terms of implications for the notional building specification for 2021 in Scotland, whilst previous research has highlighted the increased overheating risks associated with higher levels of energy efficiency, it is unclear where thresholds are in terms of specific levels of fabric performance, nor whether these would have a significant impact if other factors were taken into account – for example if effective ventilation (especially at night-time) was possible. The MHCLG report also suggested that reduced fabric infiltration associated with more airtight homes would not necessarily have a significant impact on risk; ventilation from other sources is more significant. In addition, the level of risk found in the research, even in the north of England, should be different from equivalent modelling for dwellings in Scotland due to differences in climate.

148. The scope of the current research is focused on energy performance and is limited to SAP modelling, which only includes a relatively basic check on overheating risk. It is suggested that the Scottish Government may want to consider further the risks of overheating and how these may affect building specifications, and may wish to undertake more detailed modelling of proposed future standards to assess their overheating risk including in relation to current standards. The consultation on overheating in England should also be considered in terms of whether Scotland may wish to adopt similar changes.

1.9 Analysis of opportunities and constraints

149. A high-level analysis of the costs and savings of different improvement measures is provided in Table 1.9a. This analysis is based on SAP 10.1 modelling of the 2015 compliant specification for a semi-detached gas-heated home (as set out in section 0, adjusted for the various improvement measures) for primary energy and carbon emission savings, and on rough cost data and AECOM's experience for the capital cost analysis and commentary on energy cost savings, replacement and maintenance costs.[27]

150. Considerations of key opportunities and constraints drawn from this analysis and the review of evidence described in section 1.7 are summarised in Table 1.9b.

Table 1.9a: Costs and savings analysis table, comparisons to baseline 2015 compliant gas specification for semi-detached house
Specification element Capex PE saving CO saving Energy consumption Replacement and maintenance
Walls Low – 0.15 Med – 0.13 Low Low Savings in space heating -
Roof Low – 0.09 Low Low Savings in space heating -
Floor Low – 0.13/0.11 Med – 0.09 (higher end) Low Low Savings in space heating -
Window Med – 1.2 High – 0.8 Low – 1.2 Med – 0.8 Low – 1.2 Med – 0.8 Savings in space heating Replacement costs (30yrs)
Door Low – 1.0 Low Low Savings in space heating Replacement costs (30yrs)
Thermal bridging Low – 0.05 Low (but higher end) Low (but higher end) Savings in space heating -
MVHR + improved airtightness Med High High Savings in space heating Replacement costs (MVHR unit 20yrs) Maintenance costs (significant as annual)
PV Med High High Energy savings from generation – electricity Replacement costs (15yrs inverters, 25yrs panels)
PV + battery Very high (High if battery only) High (Med if battery only) High (None if battery only) Battery allows greater onsite savings from generation – electricity Replacement costs (as above plus 12yrs battery)
Low temp rads Low Low (but higher end) Low (but higher end) Savings in space heating Replacement costs (20yrs)
WWHR Low Med Med Savings in water heating Replacement costs (20yrs, but tray systems only)
Solar thermal Very high High High Savings in water heating Replacement costs (panels 15yrs, separate cylinders 20yrs) Maintenance costs (significant as additional)
ASHP High Very high Very high (higher percentage than PE) Significant savings in space and water heating compared to direct electric. Also compared to baseline, but switch to higher cost electricity vs gas. Replacement costs (ASHP 15yrs, separate cylinders 20yrs) Maintenance cost saving (as assumed lower than gas boiler)

NotesCapex is based on uplift from baseline gas compliant specification for a semi-detached house. Uplifts are classified as 'low' if less than around £500; 'medium' if around £500-£1500, 'high' if around £1500-£2500, 'very high' if around £2500-£4000. Values included here for fabric elements show the U-values/y-values modelled.

PE and CO2 savings are classified as 'low' if <5% compared to baseline of semi-detached house on gas, 'medium' if 5-10%, 'high' includes savings from 10-25%, 'very high' >50% (no options fall between these latter two categories).

Energy consumption – looking at the 'PE saving' column will give an idea of the scale of energy savings, though in cases of ASHP and PV this is also affected by different fuel type (with different primary energy factors and costs).

Savings calculated from PV (both with and without battery) currently include some savings from electricity exported to the grid. It has been suggested that these could potentially be excluded from calculations to incentivise battery storage, but the analysis here follows the SAP 10.1 approach.

Replacement and maintenance column provides notes on applicable costs (replacements over 60 year period; figures are blank if no replacement/maintenance costs expected during this time. Figures in brackets show estimated life expectancy for replacements – see capex column for an idea of the scale of costs).

Source AECOM analysis, with cost input from Currie & Brown.

Table 1.9b: Opportunities and constraints analysis table

Element Opportunities Constraints
  • Fabric efficiency built into dwelling
  • No maintenance/replacement costs
  • Low-medium cost
  • Costs appear to go up a bit more significantly below U-value of around 0.15
  • Savings relatively low
  • Fabric efficiency built into dwelling
  • No maintenance/replacement costs
  • Low cost
  • Savings relatively low
  • Fabric efficiency built into dwelling
  • No maintenance/replacement costs
  • Low cost options
  • Costs appear to go up significantly below U-value of around 0.11
  • Lower values harder to achieve for dwelling types with larger floor areas/perimeters
  • Savings relatively low
  • Fabric efficiency built into dwelling
  • No additional maintenance costs
  • Medium savings for triple glazing
  • Potential capital cost reductions in future
  • Medium cost, high for triple glazing
  • Replacement costs
  • Savings relatively low for 1.2 U-value
  • Market readiness for move to triple glazing
  • Fabric efficiency built into dwelling
  • No additional maintenance costs
  • Low cost
  • Savings relatively low
Thermal bridging
  • Fabric efficiency built into dwelling
  • No maintenance/replacement costs
  • Low cost
  • Common performance gap issue
  • Skills gaps
MVHR + improved airtightness
  • High primary energy and carbon savings
  • Medium cost
  • Potential cost reductions in future
  • Medium cost
  • Annual maintenance costs
  • Replacement costs
  • Common performance gap issue
  • Skills gaps
  • Possibly limits use of alternative ventilation systems if basis of notional
  • High primary energy and carbon savings
  • Medium cost
  • Significant energy (electricity) cost savings for occupants (where direct connections to dwelling)
  • Potential cost reductions in future
  • Current approach does not maximise use of roofspace / cost-effectiveness
  • Medium cost
  • Constraints on electricity grid
  • Constraints on roofspace particularly for higher blocks of flats, constraints on orientation
  • Modelled carbon and primary energy savings will reduce as grid decarbonises over time
  • Carbon and primary energy savings would also be reduced if proportion exported is excluded from calculations
PV + battery
  • As above (except higher cost)
  • Alleviating impacts on electricity grid
  • Increased energy cost savings for occupants
  • Potential cost reductions in future
As above (except higher cost)
  • High cost of batteries (initial and replacement)
  • Relatively low additional primary energy savings compared to cost
  • No additional carbon savings
  • Total cost savings not fully recognised (e.g. savings in terms of grid connection/reinforcement, potential for time of use tariffs etc.)
  • Market readiness would need consideration
Low temp rads
  • Low cost
  • Future-proofing (for ASHP if gas-based notional)
  • Change in common practice, potential skills gaps
  • Low cost
  • Medium primary energy and carbon savings
  • Potential cost reductions in future
  • Apparently low uptake to date (based on EPC database)
  • Lower savings where tray systems used
Solar thermal
  • High primary energy and carbon savings
  • Potential cost reductions in future
  • High cost
  • Savings are in gas use so lower value for residents (compared to electricity)
  • Replacement costs
  • Additional maintenance costs
  • Limits on ability to connect flats particularly for higher blocks
  • Availability of roofspace – impacts on /relates to space for solar PV
  • Very high primary energy and carbon savings
  • Future-proofing as grid decarbonises further
  • Potential cost reductions in future
  • High cost
  • Impacts on electricity grid
  • Availability of smaller heat pumps for very well-insulated dwellings
  • Market readiness for move to heat pumps
  • Skills gaps
  • Exposure issues for island locations
  • Limits on alternative compliance routes if basis of notional

NotesComments in table are based on analysis in section 1.7 above.

1.10 Selection of 2021 notional buildings

1.10.1 2021 modelling specifications – introduction

151. The specifications used in the modelling of potential 2021 standards have been shown in sections 1.10.2 to 0, where fabric, ventilation, heating and PV are considered separately. The specifications take into account key findings in sections 1.7 to 1.9 above, considerations on heating options discussed in section 1.10.6, and feedback from Scottish Government on earlier draft proposals. Overall, the proposed specifications form four cases:

  • 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').

152. The intention was that these four modelled cases could be grouped into a smaller set of two or three options for the 2021 targets; either by defining notional buildings differentiated by fuel type or by selecting a single fuel notional building option which also allows compliance using other fuels – but without introducing overly onerous requirements or risking unintended consequences by allowing too much relaxation of specification elements such as fabric. This is discussed further in sections 1.10.6 and 1.11.1.

153. Where modelled 2021 specification details are not shown they are assumed to be as per 2015 compliant specifications (e.g. lighting specification, detailed assumptions for PV, additional window design assumptions).

154. It should be noted that some of the specification details may also differ if and when they are entered into a notional building specification (for example, items such as opening areas or boiler types/hot water cylinder sizes may be standardised across building types); the tables below show the modelled specifications used for the purposes of the current analysis.

155. When reviewing notional building specifications it is important to consider that they are not prescriptive; they are used to set standards but developers can deviate from them in practice as long as sufficient flexibility is allowed for – for example relaxing some elements of the specification and compensating elsewhere if required.

156. For some specification elements there would be an expectation that work would be required outside the Building Standards remit (e.g. to build supply chains, and upskill designers, installers and commissioners) to achieve them at scale in practice and to avoid performance gaps. It is recommended that the Scottish Government consider these points further.

1.10.2 2021 modelling specifications – fabric

157. The modelled fabric specifications are set out in Table 1.10a, where they are compared to the 2015 compliant base case.

Table 1.10a: Building Standards 2021 specification options – fabric improved/advanced cases
  2015 compliant case 2021 improved case 2021 advanced case Source/Rationale
External Wall U-value 0.17 0.15 0.13 0.17 is 25th percentile in EPC database; 0.15 appears a reasonable improvement and is around 15th percentile. Whilst 0.13 appears a step up in cost and is below 10th percentile, there are policy drivers for improving fabric further. For information, 0.13 is included in the Wales 2020 proposed notional.
Corridor Wall U-value 0.17 0.15 0.13 Scottish Government do not wish to introduce differentiation by wall type here.
Party Wall U-value 0.0 As per 2015
Floor U-value 0.15 0.12 0.10 0.15 is 50th percentile in EPC database; 0.12 is 25th percentile and reasonable reduction; looking at lower values 0.10 is below 10th percentile but there are policy drivers for improving fabric further. Costs appear to increase significantly at 0.09 so this was seen as a limit.
Roof U-value 0.11 0.09 0.09 0.11 is 50th percentile in EPC database; 0.09 appears a reasonable step down and is around 10th percentile.
Window U-value 1.4 1.2 0.8 Client confirmed use of 1.2 U-value as a step down from 2015. 1.3/1.2 is Wales/ England 2020 proposed notional (1.2 possible limit for double glazing). 0.8 is triple glazing typical value, and pushes fabric further in line with policy drivers, though current high costs of triple glazing and limits on supply are noted.
Window g-value 0.63 0.63 0.57 Linked to glazing type
Door U-value 1.4 1.2 1.0 Low cost with 1.2 seen as a reasonable step down from 2015. Note 1.0 U-value included in England/Wales notional.
y-value 0.08 0.06 0.04 Similar to England/Wales notional with the improved value seen as a possible interim step. Low cost.
Thermal Mass Parameter As actual As per 2015

NotesUnits for U-values and y-values are W/m2K.

1.10.3 2021 modelling specifications – ventilation

158. The modelled ventilation specifications are set out in Table 1.10b, where they are compared to the 2015 compliant base case.

Table 1.10b: Building Standards 2021 specification options – ventilation improved/advanced cases
Element 2015 compliant case 2021 improved case 2021 advanced case Source/Rationale
Ventilation type Intermittent extract fans with trickle vents Intermittent extract fans with trickle vents MVHR High savings for MVHR and reflective of a move towards a passive house approach, but note constraints. Improved case unchanged from 2015. MEV not proposed as gives relatively minor benefit.
Air permeability rate (m³/m².h@50Pa) 5.0 5.0 3.0 Linked to ventilation system type

NotesThe MVHR product used in the modelling is the Vent Axia Sentinel Kinetic Advance S (BRE SAP Product Characteristics Database reference 500477). The system is also assumed to be located exclusively in the heated envelope, with rigid insulated ductwork, and to be installed under an approved scheme, reflecting good practice.

1.10.4 2021 modelling specifications – heating

159. The modelled heating specifications are set out in Table 1.10c, where they are compared to the 2015 compliant gas base case.

160. The gas and ASHP cases reflect heating systems commonly/relatively commonly currently specified in new homes, with the ASHP case also reflecting future policy aims. Other heating options were considered but ruled out for the modelling following consideration of the evidence and discussion with Scottish Government. For example, a gas heating with solar thermal option was excluded given the constraints outlined in section 1.9; in particular as it would be required to be combined with solar PV to achieve more stretching targets, which would form a high capital cost approach compared to installing a larger PV array.

Table 1.10c: Building Standards 2021 specification options – heating gas/ ASHP cases
Element 2015 compliant (gas) case 2021 gas cases (improved and advanced) 2021 ASHP cases (improved and advanced) Source/Rationale
Space Heating Source Condensing gas boiler Condensing gas boiler ASHP ASHP high savings and reflects solution looking to 2024
Emitters Radiators (standard size) Radiators (large) Radiators (large) Future-proofing measure in gas case
Efficiencies 89.0% (SEDBUK) 89.5% (SEDBUK) Around 250% (SPF as modelled in SAP) Heat pump efficiency significant improvement on 2015 notional. Gas boiler and heat pump efficiency as per England/Wales Part L 2020 proposals/analysis
Flow temperatures >55°C 55°C 45°C Future-proofing measure
Controls Time and temperature zone control, interlock, ErP Class V controls, delayed start Time and temperature zone control, interlock, ErP Class V controls, delayed start Time and temperature zone control As per 2015
Flue Gas Heat Recovery None None n/a As per 2015
Pump details 2013 or later, in heated space 2013 or later, in heated space n/a As per 2015
Flue type Balanced, fan-assisted Balanced, fan-assisted n/a As per 2015
Boiler type Detached: 18kW system/regular
Semi/Mid/Flat: 24kW combi
Regular for houses, combi for flats n/a Requested by Scottish Government to align with 2015 notional and reflect desire for storage for flexibility in the future – though combis currently considered more common in semi-detached/mid-terrace houses
Domestic Hot Water Source As for space heating As for space heating As for space heating As per 2015. BRE confirmed previously that SAP modelled heat pump efficiency would take into account top-up for water heating
Hot water cylinder size (where applicable) 200l for detached house only 200l for detached house 150l for semi-detached and mid-terrace houses 180l (integral) Gas boiler hot water cylinder sizing based on Hot Water Association calculator. ASHP sizing taken from heat pump product modelled.
Hot water cylinder declared loss factor (where applicable) 1.89 kWh/day 1.65 kWh/day for detached 1.39kWh/day for semi-detached and mid-terrace houses 1.35 kWh/day Tightened to match requirement in England/Wales for gas boiler. ASHP declared loss factor based on heat pump model used.
Primary circuit loss assumptions (where applicable) Cylinder thermostat, separate timer, fully insulated primary pipework As per 2015
Shower flow rate (l/min) 8 As per 2015
Waste Water Heat Recovery None Yes, efficiency 55%, utilisation factor 0.98, waste water factor 0.9, connected to all showers None Cost-effectiveness. Efficiency is improvement on 2015 notional, same level proposed for Wales Part L 2020, and allows for some flexibility in flats across blocks where shower trays could be used on ground floor. Shower connection assumption as per England and Wales Part L 2020 proposals.
Secondary heating None As per 2015
Electricity tariff Standard As per 2015

NotesThe ASHP product used in the modelling is the 5kW Panasonic Aquarea High Performance (BRE SAP Product Characteristics Database reference 103455, intended for highly energy efficient homes). The product was selected as it provided good heating efficiencies as modelled in SAP for the design heat losses of the 2021 dwelling types.

Further work may be needed to assess how the efficiency might be specified in the notional building and to consider the range of heat pumps which could achieve similar performance; this is a complicated task as heat pump efficiencies vary with plant size ratio as modelled in SAP (i.e. ratio of maximum output to dwelling design heat loss). It is recommended that this is discussed with BRE as the contractor responsible for SAP, the Product Characteristics Database, and SAP's heat pump calculation engine.

1.10.5 2021 modelling specifications – PV

161. The modelled PV specifications are set out in Table 1.10d and compared to the 2015 compliant gas base case. The areas proposed for the gas 2021 cases are based on the findings from the existing evidence that it is more cost-effective to maximise the PV array size. An area equivalent to 40% of the building foundation area has been modelled. This has also been proposed in other UK administrations as representing a reasonable maximum to allow for some flexibility and to avoid significant roof redesign (e.g. a change to mono-pitch roofs).

162. The ASHP 2021 cases exclude PV to reflect the significant improvement in performance which will be achieved through installing ASHP alone and to provide closer parity with the gas cases. In addition, if a single notional building was set, including PV in the ASHP case would be expected to exclude fossil fuels from compliance which is contrary to the aims expressed by the Scottish Government for the 2021 standards revision.

163. Another key decision where PV is included was whether or not to include battery storage. Previous work has suggested that this adds significant capital and replacement costs which will not be outweighed by the benefits of increased use of generated electricity on site as captured in the current modelling and cost-benefit assessment. Battery storage has therefore not been included in the specifications. However it is understood that Scottish Government policy objectives are strongly supportive of storage, and that there are wider benefits which may not be captured in the current analysis, such as reduced impacts on the electricity grid, avoidance of the need for export limiters as a temporary measure, and (where and when this is possible) load-shifting with time of use tariffs. Indeed, the Scottish Government has suggested that benefit may be assigned to onsite generation only where it can be used onsite (immediately or via storage). It is suggested that the promotion of battery storage is primarily a policy decision for the Scottish Government, which could be informed in part by the information presented above.

Table 1.10d: Building Standards 2021 specification options – PV
  2015 compliant (gas only) 2021 gas cases (improved and advanced) 2021 ASHP cases (improved and advanced) Source/Rationale
PV calculation PV included, sizes shown in Table 1.10e. 40% building foundation area None See text above – most cost-effective to maximise roof area. ASHP case excludes PV to provide closer parity with gas case.
Battery storage No No No See text above – there are strategic and policy reasons for promoting battery storage but excluded due to high costs and limits on modelled benefits. Alternative mechanisms to incentivise may be required.

NotesThe 2021 proposed calculations take into account significant recent improvements in standard panel performance (6.5 m²/kWp is assumed).

Further consideration may need to be given to high-rise flats, but basing the array size on building foundation area helps to take into account the number of storeys in blocks of flats across which roof-based arrays would need to be shared.

Table 1.10e: Building Standards 2021 specification options – PV array sizes as applied to building sub-types
PV array sizes (kWp) 2015 compliant case (gas only) 2021 cases (gas improved and advanced) 2021 cases (ASHP improved and advanced)
Detached house 1.61 4.33 n/a
Semi-detached house 0.95 2.60
Mid-terrace house 0.95 2.60
Flat 0.85 1.44
Block of flats 10.20 17.25

1.10.6 2021 specifications – consideration of low carbon heating and renewable technologies

164. The choice of low carbon heating and renewable technologies within the 2021 notional building will have the most significant impact on the overall target. Their selection should be considered within the context of Scottish Government policy commitments and objectives, and of the wider impacts such decisions will have (including outside of Building Standards which focus on the individual dwelling level) – section 0 provides a summary. The evidence review findings in sections 1.7.2 and 1.9 are also particularly relevant in highlighting various advantages and disadvantages of different technologies. Some of these were also discussed – alongside others – in a recent Scottish Government report on climate mitigation options for the built environment (Aether, 2017).

165. The Scottish Government has indicated that the 2021 solutions should be achievable with higher carbon fuels, likely with on-site renewable generation. As noted in section 1.6, they also wished to explore the option of moving to a single notional building in 2021 which would not vary by fuel type. Hence, the notional building could be based on one of the following:

  • Option 1: a higher carbon heating system with on-site renewable generation,
  • Option 2: a lower carbon heating system that can be complied with using a higher carbon fuel with on-site renewable generation,
  • Option 3: a notional building with alternative options depending on whether, say, a lower or higher carbon heating system is used in the actual building.

166. Option 1 could be based on a gas heated solution plus PV. Previous work by AECOM has suggested that if such a target was adopted, a solution which specifies a heat pump in practice is likely to form a lower capital cost alternative for compliance, at least for individual houses. This could have the benefit of encouraging ASHP adoption, but could also potentially allow relaxation of other specification elements (e.g. fabric) where ASHP is used, which may not be desirable – though setting robust minimum fabric and services performance values could help to mitigate this risk. Particularly in the context of reduced carbon emission (and primary energy) factors for electricity, it could also be important to ensure that it does not form a backwards step for electric-heated homes for which the target is currently based on an ASHP-heated notional building.

167. As an alternative, Option 2 could be adopted through setting a target based on a heat pump specification. However, if the heat pump is assumed to have an improved efficiency compared to the Scotland 2015 ASHP/electric compliant case (to better reflect performance of heat pumps on the market), AECOM analysis suggested that such an option may preclude compliance using gas heating even where a significant amount of PV is installed (above the 40% building foundation area equivalent array size proposed in the modelling) and high levels of energy efficiency are specified (with carbon targets being particularly challenging if/where these are set).

168. Alternatively, Option 3 could be adopted where the notional building is differentiated by fuel type e.g. a gas heated solution plus PV for most options and an ASHP specification where the actual building has a heat pump (and potentially also where it has a different type of electric heating, i.e. as in the 2015 standards).

169. Further analysis in section 1.11 tests the gas and ASHP comparison in more detail and presents the results of the 2021 modelling (see section 1.11.1 in particular). Following consideration of the above options, and informed by initial modelling results from Task 3, Scottish Government decided that their preference is Option 3. This is similar to the approach adopted in 2015 standards and helps ensure that high standards are achieved for different fuel types. This is therefore reflected in the following analysis.



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