Planning policy - section 3F: research

5. Proposal 1: A Minimum LZCGT Contribution Standard

5.1 Key Points

Proposed methodology to identify the level of CO₂ emission savings that would be reasonable to expect from the use of LZCGT in new buildings.

• The methodology is based on the consideration of what would be an appropriate LZCGT contribution to the energy demand in new domestic buildings under different CO₂ emission reduction standards.
• The data used in these deliberations was based on the predicted energy demand for dwellings ranging in size from 25m² to 300m² calculated using formula prescribed in SAP 2012 (BRE, 2014).
• Three different scenarios were developed representing three different fabric energy efficiency standards: 45kWh/m².annum, 30kWh/m².annum and 15kWh/m².annum. These scenarios provided a reasonable approximation of past, present/near future and future fabric energy efficiency contexts.
• Modelling was based on the domestic sector.
• It is important to recognise that what is judged reasonable, must be considered within the broader context of achieving CO₂ emission reductions in the most impactful, efficient and cost-effective way.
  i. It should not undermine a fabric first approach. This approach seeks to reduce CO₂ emissions by reducing overall energy demand and has numerous long-term economic, social and environmental benefits (IEA, 2019).
  ii. It should not inhibit the potential to develop innovative passive design approaches.
  iii. It should not restrict the ability to deliver essential infrastructure such as affordable and social housing in a cost effective way.
• Taking these factors into consideration the level of LZCGT contribution to CO₂ emission reduction that could be reasonably sought by the current Section 3F policy must by necessity be a minimum standard rather than an aspirational one.

Recommendation of the proportions of emission savings which may reasonably be achieved through the use of LZCGT applied to new buildings over the next 10 year period.

• Four different metrics are currently used to define the contribution of LZCGT to CO₂ emission reduction in Section 3F policies (Scottish Government, 2019c).
  i. An absolute percentage CO₂ emission reduction relative to the 2007 baseline established by Scottish Building Standard 6.1.
  ii. A percentage of the percentage CO₂ emission reduction sought through Scottish Building Standard 6.1 relative to the 2007 baseline.
  iii. Avoidance of a percentage of projected CO₂ emissions as calculated by SAP/SBEM.
  iv. Avoidance of a percentage of projected energy consumption as calculated by SAP/SBEM.
• Each metric has its own merits.
  i. Metric ii. is most frequently used
  ii. Metrics i. and ii. are quite abstract and difficult for architects to relate to in terms of the design process, but relate directly to governmental CO₂ emission reduction targets.
  iii. Metrics iii. and iv. are roughly analogous, relate more directly to the Section 3F policy definition, and are more meaningful and tangible from a designer's perspective.
  iv. It is useful for all stakeholders to be able to calculate each of these metrics and understand the relationship between them.
• On balance, we recommend that in Section 3F policy the minimum LZCGT contribution be defined in terms of a percentage of the percentage CO₂ emission reduction sought through Scottish Building Standard 6.1. because:
  i. It avoids potential conflict between Planning and Building Standards.
  ii. It simplifies Section 3F policy, by allowing the LZCGT contribution to be defined as a constant and perpetual percentage that will automatically deliver increases in real terms with every improvement of Standard 6.1 (Table 6).
  iii. It provides regulatory certainty going forward (Schwartz et al., 2020)
  iv. It ensures changes that occur in Building Standards 6.1 are reflected immediately, proportionately and automatically in Planning.
  v. It allows differences in the CO₂ emission reduction sought for domestic and non-domestic buildings at building standards, to be simply and automatically reflected in the LZCGT contribution sought at Planning.
• It is vital that it is understood that any attempt to increase the level of LZCGT contribution either disproportionately or at more frequent intervals than the changes to overall CO₂ emission reduction sought by Building Standard 6.1, will undermine a fabric first approach and the potential to develop innovative passive design approaches. It should therefore be avoided.
• We recommend that the LZCGT contribution to CO₂ emission reductions be defined as 12% of the percentage CO₂ emission reduction sought through Scottish Building Standard 6.1 (Table 6).

• This is a minimum standard and does not preclude designers of new buildings from using a higher percentage of LZCGT to meet their Target Emission Rate (TER) if they wish to do so.

Table 6: Minimum LZCGT Contribution under different Scottish Building Standard 6.1 contexts.

R%	A %	C %	E %
0% CO2 Reduction	0 %	12.0 %	0 %
30% CO2 Reduction	3.6 %	12.0 %	4.9 %
45% CO2 Reduction	5.4 %	12.0 %	8.9 %
60% CO2 Reduction	7.2 %	12.0 %	15.3 %
75% CO2 Reduction	9.0 %	12.0 %	26.5 %
90% CO2 Reduction	10.8 %	12.0 %	51.9 %
100% CO2 Reduction	12.0 %	12.0 %	100 %

Where:

R% = The percentage CO₂ emission reduction sought by Scottish Building Standard 6.1 relative to the 2007 baseline.

A% = The percentage LZCGT contribution defined as an absolute percentage CO₂ emission reduction relative to the 2007 baseline.

C% = The percentage LZCGT contribution defined as a percentage of the percentage CO₂ emission reduction sought by Scottish Building Standard 6.1.

E% = The percentage LZCGT contribution defined as avoidance of a percentage of projected CO₂ emissions as calculated by SAP/SBEM.

Proposed calculation methodology for use by development management officers to understand whether the specified LZCGT contribution has been reached

• Not all LZCGT have zero carbon emissions. Therefore the simplest way to accurately calculate if the specified LZCGT contribution has been met is to run two separate SAP/SBEM calculations; one for the building as designed with the proposed LZCGT and another with the proposed LZCGT removed and replaced with pre-defined conventional systems. The CO₂ emission rates generated by these two SAP/SBEM calculations are then substituted into a formula to calculate the LZCGT contribution.
• We would recommend that the presumption of what replaces the LZCGT in this second SAP/SBEM calculation is consistent for all buildings; although the appropriateness of this needs further consideration by Building Standards.
• The formula to calculate the LZCGT contribution as a percentage of the percentage CO₂ emission reduction sought by Scottish Building Standard 6.1 is given by:

Where:

R% = Percentage CO₂ emission reduction sought by Standard 6.1

TER = Target Emission Rate

DER = Dwelling Emission Rate with LZCGT

DERNT = Dwelling Emission Rate with no LZCGT

Other Recommendations

• It is essential that going forward planning and building standards work in collaboration to manage issues related to climate change mitigation and CO₂ emission reduction. In respect to Section 3F policy:
• i. Conflict between planning and building standard approaches and legislation need to be resolved to provide clarity of intent and prevent confusion amongst applicants.
• ii. The current implementation and enforcement issues identified by planning authorities in relation to Section 3F policy need to be addressed.
• iii. It was strongly suggested by survey respondents that the LZCGT target contribution be written into Building Standards to resolve i) and ii).
• iv. Inter-departmental working practices at the level of local government need to be developed to support shared objectives, with feedforward and feedback mechanisms to monitor and improve collaborative practices.

5.2 Approach Rationale

5.2.1 What is Reasonable?

It is important to recognise that what is judged a reasonable LZCGT contribution to CO₂ emission reduction, must be considered within the broader context of achieving CO₂ emission reductions in the most impactful, efficient and cost-effective way, whilst simultaneously minimising adverse effects on other wider societal goals. Of particular concern is the potential negative impact on:

i. Fabric First Approaches:

The level of LZCGT contribution should not be so onerous that it undermines the ability of architects to adopt the high fabric energy efficiency standards of a fabric first approach. This approach seeks to reduce CO₂ emissions by reducing overall energy demand and is considered by most of the literature to be the most cost-effective long-term strategy to reduce CO₂ emissions and has numerous other economic, social and environmental benefits (Payne et al., 2015; Grove-Smith et al., 2018; IEA, 2019).

ii. Innovative Passive Design Approaches:

The level should similarly not inhibit established passive design approaches such as Passivhaus, nor should it unduly inhibit the potential to develop further innovative passive design responses (IBO, 2009; Cotterell and Dadeby, 2013; Eberle and Aicher, 2016).

iii. Developer Costs:

As most LZCGTs currently have additional cost implications beyond those that would be incurred to achieve the same CO₂ emission reduction through energy efficiency measures; the level of contribution should not be so onerous that developers might be prompted to take legal action against planning decisions made on this issue.

iv. Tackling the Housing Crisis:

The level of LZCGT contribution should not be so high that it restricts the ability to deliver essential infrastructure such as affordable and social housing in a cost effective way.

v. Tackling Energy Poverty:

The financial benefits of reduced energy consumption due to increased fabric energy efficiency are felt directly by the occupant through reduced energy bills; but financial benefits related to LZCGT may accrue to landlords and are not always passed on to tenants.

That the determined level of LZCGT contribution to CO₂ emission reduction policy must by necessity be a minimum standard rather than an aspirational one, is not to suggest that the proportion should not be meaningful. A minimum standard can still be impactful. It has been shown that by making a small amount of LZCGT mandatory, architects and developers may be prompted to add additional capacity or adopt other technologies they had not previously considered.

This has been the experience in Switzerland where legislation requiring only 10W/m² on-site renewable energy generation in new buildings has had a much larger impact because architects have generally avoided the tokenism of one PV panel and have instead provided substantial PV arrays (Schwarz et al., 2020). However there is also a note of caution in relation to this policy. Using PV is the most cost effective way to meet this target, therefore this policy has become by default technologically specific. It is considered preferable for policy robustness, that targets are neither too onerous nor written in a way that limits technological choice either directly or indirectly (Schwarz et al., 2020). These are fundamental principles that should be borne in mind when determining the level at which to set the LZCGT contribution.

It is also recognised that the relationship between Section 3F Policy and the Scottish Building Standards needs to be carefully defined and managed so that existing legislative conflicts are not exacerbated by setting a mandatory level of LZCGT contribution to CO₂ emission reduction that undermines the established whole building approach taken by Building Standards. This would also suggest that a minimum standard rather than an aspirational one is the most appropriate approach.

Whilst accepting that LZCGT have a role to play in reducing CO₂ emissions; Section 6: Energy of the Scottish Building Standards clearly promotes the view that the emphasis should be on reducing the overall energy consumption of buildings, and that this is best achieved through a balanced and holistic approach to building design, in which energy demand is limited by addressing both the performance of the building fabric and fixed building services (Scottish Government, 2019a). Exactly how a designer meets the Target Emission Rate (TER) set by the Standard Assessment Procedure (SAP) is completely at their discretion. There is a requirement to assess the feasibility of using LZCGT and the use of renewables is encouraged for both domestic and non-domestic buildings, but there is no compulsion to employ them. Indeed it is explicitly stated that in respect to high-efficiency alternative systems:

'Whilst new buildings do not have to incorporate such technologies, the challenging standards set under Standard 6.1 (carbon dioxide emissions) do mean that they are a more common part of design solutions in energy efficient, low carbon buildings.'
(Annex 6.C.1: Scottish Government, 2019a).

At the same time, we also recognised that Section 7: Sustainability of the Scottish Building Standards does include optional higher Sustainability Levels 'Bronze Active' and 'Silver Active' which accredit the use of LZCGT, but do not stipulate a specific contribution level (Scottish Government, 2019a; Scottish Government, 2019b). Although these are not mandatory requirements the intent to support Section 3F is clearly stated:

'This level is primarily to assist local authorities to meet their obligations under Section 72 of the Climate Change (Scotland) Act 2009 by identifying the use of LZCGT. In this respect, LZCGTs include: wind turbines, water turbines, heat pumps (all varieties), solar thermal panels, photovoltaic panels, combined heat and power units (fired by low emission sources), fuel cells, biomass boilers/stoves and biogas.'
(7.1.3 and 7.1.5: Scottish Government, 2019a).

The only place in the Scottish Building Standards where the contribution of LZCGT is quantified is in optional higher Sustainability Level Aspect 3 in relation to Energy for Water Heating; where it is stated that a proportion of the annual energy demand for water heating should be from heat recovery and/or renewable sources with little or no associated fuel costs (e.g. solar thermal water heating and associated storage or heat recovery from greywater). The LZCGT contributions are defined respectively as:

Domestic Buildings:
Silver Aspect 3: 5% of the dwelling's annual energy demand for water heating
Gold Aspect 3: 50% of the dwelling's annual energy demand for water heating
(7.1.4 and 7.1.6: Scottish Government, 2019a)

Non-Domestic Buildings: (school buildings containing classrooms only)
Silver Aspect 3: 10% of the building's annual energy demand for water heating
Gold Aspect 3: 50% of the building's annual energy demand for water heating
(7.1.5 and 7.1.8: Scottish Government, 2019b)

It should also be noted that in dwellings, energy for water heating is currently calculated relative to occupancy and because many large dwellings have a relative small occupancy and large space heat demand, the hot water demand is proportionally less significant (Burford et al., 2019). The way energy for water heating is calculated is currently being reviewed in the proposed SAP 10. These measures do however give some indication of what is considered a reasonable LZCGT contribution to CO₂ emission reduction from the perspective of building standards, because each Sustainability Level is a package of measures and also includes under Aspect 1 the envisaged corresponding CO₂ emission reduction standard.

Domestic Buildings:
Silver: Aspect 1: 45% CO₂ emission reduction relative to 2007 Standard (Current)
Gold: Aspect 1: 60% CO₂ emission reduction relative to 2007 Standard
(7.1.4 and 7.1.6: Scottish Government, 2019a)

Non-Domestic Buildings: (school buildings containing classrooms only)
Silver: Aspect 1: 60% CO₂ emission reduction relative to 2007 Standard (Current)
Gold: Aspect 1: 75% CO₂ emission reduction relative to 2007 Standard
(7.1.5 and 7.1.8: Scottish Government, 2019b)

In conclusion, we consider a reasonable and appropriate level of LZCGT contribution to CO₂ emission reduction should reflect the following:

i. It should be a reasonable minimum expectation in respect to changing overall CO₂ emission reduction standards.

ii. It should be readily achievable by all buildings whether in an urban or rural context.

iii. The cost implications should not be overly onerous and should represent long-term value for money for stakeholders (Gürtler et al., 2019; Schwarz et al., 2020).

iv. The level should be low enough that is does not dis-incentivise a fabric first approach or innovative passive design responses to CO₂ emission reduction.

v. It should not restrict the designer to only one viable option in choice of LZCGT, either directly or indirectly (Schwarz et al., 2020).

vi. It should not interfere in the ability to deliver wider sociological goals or the ability to provide essential infrastructure such as affordable housing in a cost effective way.

5.2.2 Methodological Approach

To calculate what would represent a reasonable contribution LZCGT could be expected to deliver in respect to CO₂ emission reduction, one must first quantify the impact the use of LZCGT has on energy consumption in the context of new domestic buildings. This approach took into account an appreciation of the real-world limitations that might be encountered, and assumptions of the extent to which LZCGTs might be reasonably expected to deliver a rising proportion of the annual energy demand under tightening CO₂ emission reduction standards. The choice to base modelling on the domestic sector was taken for several reasons:

i. Domestic buildings are the most numerous building type dealt with by planning authorities.

ii. The domestic sector consumes a much larger proportion of energy for space heating, hot water and lighting than other sectors. According to UK national statistics; in 2018 the domestic sector consumed 35,399ktoe (thousand tonnes of oil equivalent) for space heating, hot water and lighting; whilst the industrial and service sectors only used 2,104ktoe and 12,692ktoe respectively (BEIS, 2019a).

iii. When compared to domestic buildings, non-domestic buildings have a fairly diverse range of energy consumption patterns contingent to their functional needs. It was therefore considered more reliable to determine a reasonable LZCGT contribution for the domestic sector and extrapolate to the non-domestic, rather than vice versa.

iv. The conviction that the LZCGT contribution level recommended should not impact negatively on the ability to deliver essential domestic infrastructure such as affordable or social housing in a cost effective way, established this as a context that was essential to model.

To determine a reasonable level of LZCGT contribution the predicted energy demand for new dwellings ranging in size from 25m² to 300m² was calculated using formulae prescribed in the Standard Assessment Procedure (SAP 2012) and three different fabric energy efficiency scenarios (BRE, 2014). These scenarios were developed to provide a reasonable approximation of past (2012), present/near future (2020-2021) and future (2024-2050) fabric energy efficiency contexts, and allow a better understanding of how tightening CO₂ emission reduction standards could potentially impact on the contribution and cost-effectiveness of LZCGT. The fabric energy efficiency standards chosen were 45kWh/m².annum, 30kWh/m².annum and 15kWh/m².annum respectively. Although theoretically fabric energy efficiencies greater than 15kWh/m².annum are possible; on balance this was considered a practical upper limit in respect to future scenarios. This is the level of fabric energy efficiency prescribed for Passivhaus and given the right impetus it is readily achievable using prefabricated timber frame construction in Scotland (Kinghorn Housing Association, 2010; Paul Heat Recovery Scotland, 2020a).

The data generated by this means was used to calculate and graph the potential impact of using LZCGT to either replace specific proportions of space and/or water heating demand or to generate electricity to offset demand. The implications of these measures were considered both in terms of the energy generated (kWh/annum) and the proportion of the overall annual energy demand this represented. By considering the variety of measures that might be capable of delivering these results and bearing in mind real world limitations it was possible to determine what might be a reasonable LZCGT contribution to include in Section 3F policy. Whilst making this determination particular emphasis was placed on ensuring that potential LZCGT contribution levels would not be overly onerous for dwellings within the size range of 45m² to 100m², which was identified as the critical range likely to contain the majority of social and affordable housing (Appendix E; Joyce, 2011; Onyango et al., 2016; Scottish Government, 2017b, pp. 18-19). This critical sub-group of dwellings became our limiting context against which final judgements of reasonableness were primarily made.

This LZCGT contribution was subsequently calculated and expressed in three ways (Table 6):

i. An absolute percentage CO₂ emission reduction relative to the 2007 baseline established by Scottish Building Standard 6.1.

ii. A percentage of the percentage CO₂ emission reduction sought through Scottish Building Standard 6.1 relative to the 2007 baseline.

iii. Avoidance of a percentage of projected CO₂ emissions as calculated by SAP/SBEM. This is roughly analogous to avoidance of a percentage of projected energy consumption.

The third metric is roughly analogous to the one used throughout this methodology, is the most meaningful and tangible from a designer's perspective, and relates directly to the Section 3F policy definition. However it is recognised that the second metric has distinct advantages in terms of policy definition because:

i. It avoids potential conflict between Planning and Building Standards.

ii. It simplifies Section 3F policy, by allowing the LZCGT contribution to be defined as a single constant percentage applied to both domestic and non-domestic buildings that will automatically deliver increases in real terms with every improvement of Building Standard 6.1 (Table 6).

iii. It provides regulatory certainty moving forwards.

iv. It ensures changes that occur in Building Standards 6.1 are reflected immediately, proportionately and automatically in Planning.

v. It allows differences in the CO₂ emission reduction sought for domestic and non-domestic buildings at building standards, to be simply and automatically reflected in the LZCGT contribution sought at Planning (Scottish Government, 2019a; Scottish Government, 2019b).

5.3 Proposed Methodology: Identifying a reasonable level for LZCGT contribution to CO₂ emission reduction in new buildings

5.3.1 Methodological Steps

1 Calculate and graph the annual energy demand of dwellings sized 25m² to 300m², under three different fabric energy efficiency scenarios, using:

i. Space heat demands

Scenario 1: 45kWh/m².annum (Past 2012),

Scenario 2: 30kWh/m².annum (Present/Near Future 2020 to 2021)

Scenario 3: 15kWh/m².annum (Future 2024 to 2050)

ii. The hot water demand calculation method outlined in SAP 2012 (BRE, 2014).

iii. The electricity for lighting demand calculation method outlined in SAP 2012 (BRE, 2014). Assuming 75% fixed low energy lighting outlets (Scottish Government, 2015, p85).

iv. Electricity consumed by pumps and fans associated with building services.

Scenarios 1 & 2: 75kWh/annum for general building services.

Scenario 3: 75kWh/annum plus electricity to operate an MVHR unit. This was calculated using the method outlined in SAP 2012 (BRE, 2014).

2 With respect to each scenario; calculate and graph relative to dwelling size, the potential contribution to annual energy demand represented by:

i. Replacing different proportions of the space heat demand and/or hot water demand with LZCGT

ii. Generating electricity with LZCGT (PV panels) to offset annual energy demand.

3 With respect to each scenario; calculate and graph relative to dwelling size, each of these strategies as a percentage of the annual energy demand.

4 Recognise that energy use and CO₂ emissions in buildings are analogous systems, and mathematically define the relationship between these elements, so that the percentage LZCGT contributes to annual energy demand can be expressed in terms of CO₂ emission reduction. Potential CO₂ metrics definitions to include:

i. An absolute percentage CO₂ emission reduction relative to the 2007 baseline established by Scottish Building Standard 6.1.

ii. A percentage of the percentage CO₂ emission reduction sought through Scottish Building Standard 6.1 relative to the 2007 baseline.

iii. Avoidance of a percentage of projected CO₂ emissions as calculated by SAP/SBEM.

5 Utilizing the information modelled for each scenario; determine what would represent a reasonable minimum LZCGT contribution to annual energy demand under different CO₂ emission reduction standards. Criteria for judging a reasonable LZCGT level include:

i. It is readily achievable by all buildings whether in an urban or rural context.

ii. It is low enough that is does not dis-incentivise a fabric first approach.

iii. The cost implications should not be overly onerous and should represent long-term value for money.

iv. It does not restrict the designer to only one viable option in choice of LZCGT, either directly or indirectly.

v. It should not interfere in the ability deliver wider sociological goals or provide essential infrastructure such as affordable housing in a cost effective way. Dwellings between 45m² – 100m² were used as the critical limiting context.

5.3.2 STEP 1: Calculate Annual Energy Demand

Calculate and graph the annual energy demand of dwellings sized 25m² to 300m², under three different fabric energy efficiency scenarios.

Occupancy: N

In domestic buildings, occupancy is fundamental to the calculation of the heat demand for domestic hot water [SAP Box No. 64] and the electric demand for lighting [SAP box No. 232]. For dwellings the UK Government's Standard Assessment Procedure SAP 2012 calculates an assumed occupancy, N [SAP Box No. 42] for a proposed dwelling relative to its total floor area TFA [SAP Box No. 4] by the formula:

If TFA > 13.9,
N = 1 + 1.76 x [1 - exp(-0.000349 x (TFA -13.9)²)] + 0.0013 x (TFA -13.9)
If TFA < 13.9,
N = 1
Formula 1 (BRE, 2014)

This formula is based on empirical evidence of average occupancy of dwellings in England (BRE, 2008). Figure 5 illustrates this relationship and indicates that from approximately 90m² the average occupancy pattern of dwellings begins to change and eventually saturates at close to 3 occupants in larger homes.

Annual Space Heating & Space Cooling Demand: SHD

Three scenarios were developed for this study, each representing a level of fabric energy efficiency that might be reasonably expected and achievable by a moderate sized dwelling built under the Scottish Building Standards. These scenarios were developed to provide a better appreciation of past (2012), present (2020 to 2021) and future (2024 to 2050) fabric energy efficiency contexts, and allow a more accurate model of how tightening CO₂ emission reduction standards could potentially impact on the contribution and cost-effectiveness of LZCGT (Appendix E). The annual space heating & space cooling demands suggested for each scenario are:

Scenario 1: 45kWh/m².annum
Past
2012

Scenario 2: 30kWh/m².annum
Present/Near Future
2020 - 2021

Scenario 3: 15kWh/m².annum
Future
2024 - 2050

Using these values for each scenario, the annual space heating & space cooling demand (kWh/annum) was calculated for dwellings ranging in size from 25m² to 300m² using Formula 2. These are graphically represented in Appendix B: Figures S1.1, S2.1 and S3.1. Most domestic buildings do not require space cooling.

SHD = TFA (m²) x Annual Space Heating/Cooling Demand (kWh/m².annum)

Formula 2

Annual Hot Water Demand: HWD

In SAP 2012 annual average hot water usage in litres per day [SAP box No. 43] is calculated relative to assumed occupancy N [SAP Box No. 42] using the formula:

Daily domestic hot water usage (litres) V_d,average = (25 x N) + 36
Formula 3 (BRE, 2014)

Over the course of the year the energy content of the hot water used [SAP Box No. 45] can be calculated in kWh/annum using the formula:

Formula 4 (BRE, 2014)

Where:

n_annual = number of days in a year

= 365
ΔT_annual = annual average temperature rise of hot water drawn off

= 37.0

In reality the energy consumed heating domestic hot water will be greater than this, due to specific equipment efficiencies, transmission losses or storage heat losses. SAP 2012 calculates these losses and apportions a proportion of them to dwelling heat gains. For the purposes of this calculation we will consider the annual energy content of domestic hot water a fair approximation of the annual hot water demand. For each scenario, the annual hot water demand (kWh/annum) was calculated for dwellings ranging in size from 25m² to 300m² using Formulae 3 and 4. These are graphically represented in Appendix B: Figures S1.1, S2.1 and S3.1.

Annual Lighting Demand: LD

SAP 2012 calculates the annual energy use for lighting in a dwelling using the formula:

E_L = E_B x C₁ x C₂

Formula 5 (BRE, 2014)

Where:

Assuming C₁ = 0.625 (75% low energy lighting outlets) and C₂ = 0.96; for each scenario, the annual energy demand for lighting (kWh/annum) was calculated for dwellings ranging in size from 25m² to 300m² using Formula 5. These are graphically represented in Appendix B: Figures S1.1, S2.1 and S3.1.

Annual Electricity Demand for Pumps and Fans: P&F

The annual electricity demand for pumps and fans (P&F) has been assumed to be 75kWh/annum in Scenarios 1 and 2, no matter the size of dwelling. In SAP 2012, this represents 45kWh/annum for a gas boiler flue fan and 30kWh/annum for a central heating pump for radiators or underfloor heating (BRE, 2014).

However, Scenario 3 has been defined with a very high fabric energy efficiency and air-tightness on a par with a Passivhaus, it is therefore expected that an MVHR unit will be necessary to maintain good indoor air quality. Consequently, in addition to the 75kWh/annum assumed for Scenario 1 and 2; Scenario 3 also includes an allowance for electricity consumed by an MVHR unit.

With reference to SAP 2012, the electricity used to operate an MVHR unit is given by Formula 6.

E_MVHR = IUF x SFP x 2.44 x n_mech x V

Formula 6 (BRE, 2014)

Where:

IUF = Applicable in-use factor

SFP = Specific fan power

n_mech = Throughput of the MVHR system

= 0.5 air changes per hour (ach)

V = Volume of the building (m³)

= TFA x H

TFA = Total floor area [SAP box No. 4]

H = Room height (assumed as 2.4m)

To provide a realistic estimate of the actual electricity consumed in operating an MVHR unit in various sized dwellings, data was drawn from the product characterisation database with respect to a Paul Novus 300 (BRE, 2016a). This is the type of highly-efficient MVHR unit that would be specified in a Passivhaus context. The assumptions made in the calculation of Formula 6 for Scenario 3 are set out in Table 7. Realistically these will vary in different dwellings.

Table 7: Specific fan power ( SFP) and in-use factors ( IUF) for a Paul Novus 300 MVHR unit, with respect to different built contexts (Scenario 3) ( BRE, 2016a).

Dwelling Size	Description	SFP	IUF (Rigid Duct)
TFA ≤ 100m2	Kitchen + 2 Wet Rooms	0.66	1.4
100m2 < TFA ≤ 200m2	Kitchen + 3 Wet Rooms	0.73	1.4
TFA > 200m2	Kitchen + 4 Wet Rooms	0.83	1.4

The values calculated for each Scenario are graphically represented in Appendix B: Figures S1.1, S2.1 and S3.1.

Annual Energy Demand: AED

Combining these four elements, the total annual energy demand (for space heating, space cooling, hot water, lighting and attendant pumps and fans) was calculated for dwellings ranging in size from 25m² to 300m² in each scenario, using Formula 7. These are graphically represented in Appendix B: Figures S1.1, S2.1 and S3.1.

AED = SHD + HWD + LD + P&F

Formula 7

In relation to the SAP 2012 document this would be represented by:

AED = [SAP Box 98] + [SAP Box 107] + [SAP Box 64] + [SAP Box 232] + [SAP Box 231]

Formula 8

Figure 6 depicts the changes in the annual energy demand relative to dwelling size under each fabric efficiency scenario. This graph aptly illustrates the impact tackling fabric efficiency has on reducing the annual energy demand and CO₂ emissions from a building. This in turn reduces the scale and cost of the heating plant or LZCGT required to meet this annual energy demand.

It should also be noted that whilst LZCGT may last for 15 – 20 years before needing replacement, the fabric will perform throughout the lifetime of the building and is much more expensive to upgrade at a later date than during the construction phase (CCC, 2019d, p14). If the fabric is not to a good standard from the start, the building will tend to underperform throughout its life (Urge-Vorsatz et al., 2013; Beradi, 2016).

5.3.3 STEP 2: Calculate Potential LZCGT Contributions to Annual Energy Demand.

With respect to each scenario; calculate and graph relative to dwelling size, the potential contribution to annual energy demand represented by:

i. Replacing different proportions of the space heat demand and/or hot water demand with LZCGT; and

ii. Generating electricity with LZCGT (PV panels) to offset annual energy demand.

Replacing a Proportion of Space Heat Demand and/or Hot Water Demand with LZCGT

For each scenario, the potential contribution of LZCGT in kWh/annum was calculated for each of the following intervention strategies for dwellings ranging in size from 25m² to 300m². Single system intervention strategies are graphically represented in Appendix B: Figures S1.2, S2.2 and S3.2; Dual system intervention strategies in Appendix B: Figures S1.3, S2.3 and S3.3.

Single System Intervention Strategy:

i. 10% of the hot water demand

ii. 20% of the hot water demand

iii. 50% of the hot water demand

iv. 66.6% of the hot water demand

v. 10% of the space heat demand

vi. 20% of the space heat demand

vii. 50% of the space heat demand

viii. 66.6% of the space heat demand

Dual System Intervention Strategy:

i. 10% of the hot water demand + 10% of space heat demand

ii. 10% of the hot water demand + 20% of space heat demand

iii. 10% of the hot water demand + 50% of space heat demand

iv. 20% of the hot water demand + 10% of space heat demand

v. 20% of the hot water demand + 20% of space heat demand

vi. 20% of the hot water demand + 50% of space heat demand

vii. 50% of the hot water demand + 10% of space heat demand

viii. 50% of the hot water demand + 20% of space heat demand

ix. 50% of the hot water demand + 50% of space heat demand

x. 66.6% of the hot water demand + 66.6% of space heat demand

Generating Electricity with LZCGT (PV panels) to Offset AED

In the UK, 1 kWp of PV will typically produce 720 to 940kWh/annum, depending on factors such as the latitude and prevailing microclimate of the site, as well as the orientation, inclination and over shading of the installation (BRE, 2014). SAP 2012 calculates the electricity produced by a PV module in kWh/year using the formula:

E_PV = 0.8 x kWp x S x Z_PV

Formula 9 (BRE, 2014)

Where:

For the purpose of this study, we will presume the modelled PV units benefit from the same conditions used to calculate the contribution of PV units to the Target Emission Rate (TER) in SAP calculations. This is defined as: the region is 'UK average', orientation 'SW', pitch '30°' and over shading 'none or very little' (Scottish Government, 2019a). Using these assumptions, Formula 9 calculated the electricity produced by 1kWp of PV as 823kWh/annum.

The use of photovoltaics has several physical limitations. In the UK, the optimum panel performance is achieved when they are installed at an angle of 30 to 40 degrees, orientated towards the south and not over shadowed. Installation on a south-facing roof is therefore ideal. The size and weight of the PV array and the availability of appropriate roof space are the prime limiting factors in their use. PV panels can be installed at less than optimum angles and orientations but their performance will suffer. Installed horizontally they will still deliver approximately 90% of the optimum performance, but as they will not be able to self-clean efficiently at this angle the output will drop further if they are allowed to become dirty (Free Solar Panels, 2011; Eco Home Essentials, 2020).

Four differently sized PV arrays were modelled for this study (Table 8) and the potential contribution in KWh/annum of each graphically represented in Appendix B: Figures S1.4, S2.4 and S3.4. It should be noted that because of the practical limitations of PV their use should be considered early in the design process. Even with good design orientation, larger systems might not be appropriate for small dwellings and flats because of physical size limitations.

Table 8: Modelled PV arrays, expected size constraints and output in kWh calculated from formula 9 using assumptions outlined above (Clissitt, 2020).

Modelled PV Arrays:
System Rating	kWp	1	2	3	4
Number of Panels (approx.)		4	8	12	16
Area of Array (approx.)	m2	8	16	24	32
Output	kWh/annum	823	1646	2469	3292

5.3.4 STEP 3: Calculate Potential LZCGT Contributions as a Percentage of Annual Energy Demand.

For each scenario the potential contributions of LZCGT in each intervention strategy identified in Step 2 was calculated as a percentage of the annual energy demand for dwellings ranging in size from 25m² to 300m². Single system intervention strategies are graphically represented relative to dwelling size in Appendix B: Figures S1.5, S2.5 and S3.5; Dual system intervention strategies in Appendix B: Figures S1.6, S2.6 and S3.6; and PV intervention strategies in Appendix B: Figures S1.7, S2.7 and S3.7.

For each scenario the percentages calculated for dwellings of a size critical for affordable housing i.e. between 45m² and 100 m² are tabulated for clearer understanding. These are found in Appendix B: Table S1.1, S2.1 and S3.1.

5.3.5 STEP 4: Define the mathematical relationship between Energy Use and CO₂ Emissions in buildings

Mathematically define the relationship between energy use and CO₂ emissions in buildings so that the percentage LZCGT contributes to annual energy demand can be expressed in terms of a CO₂ emission reduction.

Analogous Systems

To facilitate a better understanding of the relationship between energy use in buildings and CO₂ emissions, and a familiarity with the terms used when calculating CO₂ emissions using the UK's Standard Assessment Procedure (SAP) these have been expressed diagrammatically in Figure 7. With reference to this figure; if we assume that the LZCGT contribution on the Energy side of the diagram has little or no associated operational CO₂ emissions (most renewables and heat recovery systems), or if there are sizable CO₂ emissions associated with the LZCGT (e.g. from the electricity consumed in operating a heat pump) this quantity is adjusted appropriately to take this into account; then we could consider Energy and CO₂ Emissions as analogous.

From that we can deduce that:

Formula 10

Where, and in relation to further discussions:

AED = Annual Energy Demand
LZCGT = LZCGT Energy Contribution
OER = Original Emission Rate (0% reduction relative to 2007 Baseline)
TER = Target Emission Rate (calculated by SAP 2012)
DER = Dwelling Emission Rate of the building as designed (calculated by SAP 2012).
Fabric energy efficiency (FEE), equipment efficiency (EE) & LZCGT are all used to achieve this DER
DERNT = Dwelling Emission Rate calculated with LZCGT removed and replaced with predefined conventional energy systems (calculated by SAP 2012)
R% = Statutory required CO₂ emission reduction sought by Building Standard 6.1 defined as a percentage relative to the 2007 baseline standard.
This is currently:
A 45% CO₂ emission reduction for Domestic Buildings
A 60% CO₂ emission reduction for Non-Domestic Buildings

Figure 8 expands on the analogous relationship defined in Figure 7, and depicts in the form of a straight line graph the potential change in the relationship between these elements as the country moves in time from the baseline of the 2007 standard (100% emissions / 0% reduction) towards zero carbon buildings (0% emissions / 100% reduction). In Figures 7 and 8 the space between the lines representing DER and DERNT denotes the contribution made to CO₂ emission reduction by LZCGT. In both diagrams this is shaded green.

In relation to Figure 8; if we presume that the building meets CO₂ emission standard 6.1, but does not go beyond it, i.e. the Dwelling Emission Rate equates to the Target Emission Rate (DER = TER), and the minimum acceptable amount of LZCGT was used to achieve the Target Emission Rate, then the line denoting DERNT would define the minimum acceptable amount of LZCGT as defined under Section 3F planning policy.

Envisioned as a straight line dissecting the upper portion of Figure 8; this implies that the minimum LZCGT contribution required by Section 3F policy could be written in terms of a fixed proportion of the percentage CO₂ emission reduction required by Scottish Building Standard 6.1. Practically this would have a number of advantages in terms of simplifying Section 3F Policy. Although this proportion would remain unaltered; because the Scottish Building Standards are regularly improved, this would 'ensure that all new buildings avoid a specified and rising proportion of the projected greenhouse gas emissions' in real terms, as required by Section 3F Policy (Scottish Parliament, 2019b).

Three ways of measuring LZCGT contribution to CO₂ emission reduction in buildings have been identified in current Section 3F policies (Scottish Government, 2019c). For clarity we have designated these as A%, C% and E% respectively:

A%	An absolute percentage CO2 emission reduction relative to the 2007 baseline established by Scottish Building Standard 6.1.
C%	A percentage of the percentage CO2 emission reduction sought through Scottish Building Standard 6.1 relative to the 2007 baseline.
E%	Avoidance of a percentage of projected CO2 emissions as calculated by SAP/SBEM.

How these metrics are calculated and their relationship to each other is expressed in Figure 8, and defined mathematically below:

An absolute percentage CO₂ emission reduction relative to the 2007 Standard: A%

With reference to Figures 7 and 8; for any CO₂ emission reduction standard (R%), the absolute percentage LZCGT contributes to CO₂ emission reduction is defined by Formula 11.

Formula 11

To visualise how this proportion increases with changes to building standards over time refer to Figure 8.

A percentage of the percentage CO₂ emission reduction sought by Building Standard 6.1: C%

The percentage CO₂ emission reduction sought by Building Standard 6.1 is defined relative to the 2007 Standard. It is different for domestic and non-domestic buildings. The LZCGT contribution as a percentage of this percentage CO₂ emission reduction is defined by Formula 12.

Formula 12

Figure 8 clearly illustrates the possibility that by using this metric, the LZCGT contribution could be defined as a constant percentage yet still deliver increasing CO₂ emission reductions with every improvement in building standards.

Avoidance of a percentage of projected CO₂ emissions as calculated by SAP/SBEM: E%

This metric is the one most closely aligned to the definition included in Section 3F policy. With reference to Figures 7 and 8, it is defined by Formula 13.

Formula 13

With reference to the analogies drawn between energy use and CO₂ emissions from buildings explored in Figure 7 and Formula 10, and taking into consideration the assumptions we made in drawing those analogies; we can conclude that this metric will deliver a broadly similar percentage to defining the LZCGT contribution as a percentage of annual energy demand. This is not exact. However if an awareness of any associated CO₂ emissions and the carbon factors SAP applies to different fuels and technologies are borne in mind, and some of the LZCGT contribution discounted proportionately, it provides a good approximation. For designers it is the most useful metric because it relates directly to real world variables they can understand and manipulate in the design process.

5.3.6 STEP 5: Determine a Reasonable Minimum LZCGT Contribution.

Overview

This methodology approached identifying what would be a reasonable minimum level of LZCGT contribution to CO₂ emission reduction in new buildings from a pragmatic and practical standpoint. It sought to understand what might be the impact on annual energy demand of utilising LZCGT in new buildings to either replace a proportion of the annual energy demand or generate electricity to offset that demand, and what real-world limitations might be encountered in doing so.

In order to achieve this objective the predicted annual energy demands for dwellings ranging in size from 25m² to 300m² were modelled in respect to three different fabric energy efficiency scenarios to represent tightening CO₂ emission reduction standards. Whilst these scenarios might be characterised as representing past, present/near future and future contexts, this is not strictly true. Although fabric energy efficiency standards are increasing in response to improved CO₂ emission reduction standards, not all buildings are advancing at the same pace in this respect. When comparing the same graphs developed for different scenarios it is obvious that buildings with lower fabric energy efficiencies must provide proportionately larger scaled LZCGT equipment to offset their larger annual energy demands. This has cost implications, and in some cases limit the technological options available. All the fabric energy efficiency levels modelled are currently achievable within the Scottish context of prefabricated timber frame construction (Kinghorn Housing Association, 2010; Paul Heat Recovery Scotland, 2020a). Further they are considered cost effective by the mid-2020s for brick and block cavity wall construction methods used in other parts of the UK (Currie and Brown, 2019).

When making these judgements as to what level of LZCGT contribution could be reasonably expected in new buildings in different CO₂ emission reduction contexts, the following criteria were considered:

i. It should be a reasonable minimum expectation, in respect to the overall level of CO₂ emission reduction new buildings are expected to achieve due to the changing regulatory climate.

ii. It should be readily achievable by all buildings whether in an urban or rural context.

iii. It should be low enough not to undermine or dis-incentivise a fabric first approach or other innovative passive design responses to CO₂ emission reduction.

iv. It should not be too onerous in terms of cost, and should offer long-term value for money for stakeholders (Gürtler et al., 2019).

v. It should not restrict the architect to only one viable option in choice of LZCGT, either directly or indirectly (Schwarz et al., 2020).

vi. It should not interfere with the ability to deliver wider sociological goals or provide essential infrastructure such as affordable housing in a cost effective way. The delivery of affordable housing was defined as a critical consideration and all judgements focussed on the impact the contribution level would have on this context, specifically dwellings within the size range of 45m² to 100m² (Appendix E).

An initial quantification of what would be a reasonable minimum LZCGT contribution to annual energy demand was achieved by focussing on the timeframe at each end of the path to zero carbon buildings, and considering what would be a reasonable expectation of the role of LZCGT in each of these extremes of circumstance. Then, assuming that the required LZCGT contribution would increase gradually and proportionately to improvements to Building Standard 6.1, as depicted by Figure 8; the LZCGT contribution at intermediary CO₂ emission reduction standards were calculated and explored to ascertain whether these too would be a reasonable ask in those circumstances. Finally it was considered how a 100% emissions reduction standard could be met. Once we were satisfied that our criteria were broadly met at each stage of the journey to zero-carbon buildings, Formulae 11, 12 and 13 were used to calculate and express the LZCGT contribution determined with respect to annual energy demand in terms of CO₂ emission reduction.

To conclude this calculated minimum LZCGT contribution was compared with specific targets and timeframes for CO₂ emission reduction set by the Scottish Government to ascertain whether the target value was consistent with these aspirations (Scottish Government, 2018a, pp. 87-89; Scottish Parliament, 2019a).

Historic Context: 30% CO2 Emissions Reduction Standard

When Section 3F planning policy was first introduced, most early adopters of the policy did not stipulate a specific contribution to be made by the LZCGT. As a result many architects and developers took the easiest and least expensive way to meet this obligation and pass the SAP/SBEM test. Although in some regions where wind power was prevalent dwellings were already moving towards air source heat pumps (ASHP) as the main heating system; in most other areas small PV arrays were incorporated into designs or biomass stoves were employed as secondary heating systems (Onyango et al., 2016). It is a convention in SAP that most secondary heating systems are designated to supply 10% of the space heat demand, irrespective of their frequency of use (BRE, 2014). This was therefore an easy way to improve SAP scores and simultaneously meet Section 3F obligations.

Modelled under Scenario 1; 10% of the space heat demand represents 5.7% to 6.7% of the annual energy demand of dwellings in the critical 45m² to 100m² size range (Appendix B: Table S1.1 and/or Figure S1.5). Modelled with respect to the improved fabric energy efficiency of Scenario 2; the equivalent figures are 4.7% to 5.7% of the annual energy demand of dwellings in this critical range (Appendix B: Table S2.1 and/or Figure S2.5). This suggests that a minimum LZCGT contribution of approximately 5% of annual energy demand was typically being achieved with this strategy.

To determine what other strategies could meet this LZCGT contribution level in the context of Scenario 1, a horizontal line is drawn across Appendix B: Figures S1.5, S1.6, and S1.7 at 5% and the proportion of the curves that lie above this line represent options that would deliver an equal or greater contribution. As these curves are plotted relative to dwelling sizes in the range of 25m² to 300m², it is possible to determine what LZCGT options might be suitable for a dwelling of a specific size (Appendix B: Figure B.1 for further explanation of how to interpret the graphs). This information is also tabulated for the critical 45m² to 100m² dwelling size range (Appendix B: Table S1.1). The process can be repeated for Scenario 2, using Appendix B: Figures S2.5, S2.6, S2.7 and/or Table S2.1.

As it is clear that there were many technological options available to meet this contribution level within the critical dwelling size range; 5% of the annual energy demand was considered a reasonable minimum LZCGT contribution in the historic context of the 30% CO₂ emissions reduction standard.

Future Context: 90% CO₂ Emissions Reduction Standard.

At the opposite end of the spectrum on the journey to zero carbon building at 90% CO₂ emission reduction standard; it is likely that high fabric energy efficiency will be essential and that the main space heat and hot water demands will have to be met by LZCGT.

Let us assume that 100% of the space heat and hot water demands are met by a 300% efficient air source heat pump (ASHP). At this level of efficiency for every 1kWh of electricity the heat pump consumes, it produces 3kWh of heat energy. Or seen from an alternative perspective it could be considered that the ASHP is producing 66.6% of the space heat and hot water demands with little or no associated CO₂ emissions. With reference to Scenario 3, 66.6% of the space heat and hot water demands represent 53.3% to 52.4% of the annual energy demand of dwellings in the critical size range of 45m² to 100 m² (Appendix B: Figure S3.6 and/or Table S3.1).

Whilst at this level of LZCGT contribution the number of technological options becomes limited; by following the procedure described above but using Appendix B: Figures S3.5, S3.6, S3.7 and/or Table S3.1 and drawing a horizontal line across at approximately 52% of annual energy demand some options become apparent (See Appendix B: Figure B.1 for further explanation of how to interpret the graphs). For dwellings within the critical 45m² to 100 m² size range this contribution could be met completely with a 2kWp or 3kWp PV array, or a combination of PV, Solar Thermal or Waste Water Heat Recovery (WWHR). Alternatively, because SAP offers very low carbon factors for biomass and biogas these too could be considered available, if not preferred, options.

Intermediary Contexts: 45%, 60% & 75% CO₂ Emission Reduction Standards.

Having previously determined that the LZCGT contribution as a percentage of annual energy demand is roughly analogous to the LZCGT contribution to CO₂ emission defined by E% (Figure 7, Formulae 10 and 13); we utilized this knowledge to create a straight line graph using the origin point (0% LZCGT contribution), and the two LZCGT contribution levels suggested for 30% CO₂ emission reduction (approx. 5% LZCGT contribution) and 90% CO₂ emission reduction (approx. 52% LZCGT contribution). This is the graph depicted in Figure 8. The values used result in a LZCGT contribution defined as a percentage of the percentage CO₂ emission reduction sought by Scottish Building Standard 6.1, i.e. C% equal to 12%.

Using this graph (Figure 8), LZCGT contributions for 45%, 60% and 75% CO₂ emission reduction standards were calculated (Table 9) and considered as to their appropriateness. All allowed various different LZCGT solutions, and did not appear to be overtly onerous with respect to the CO₂ emission reduction standard to which they were linked.

Final Context: Net Zero Carbon - 100% Emission Reduction Standard.

Moving forward towards 100% CO₂ emission reduction; the combination of using a ASHP to provide the majority of the space heat and hot water demand coupled with a photovoltaic array to offset the electricity used to operate the heat pump, lights and other pumps and fans was considered, and appears to be a viable and achievable option. The practical limitations to this approach is the size of the PV array needed to offset the electricity demand and the availability of sufficient roof space orientated in the correct southerly direction to meet these requirements. If this approach is to be taken, or maintained as a future upgrade option, these limiting factors need to be addressed at the design stage and could be highlighted by planning officers.

Considered in respect to Scenario 3; a 45m² dwelling using a 300% efficient ASHP would have an annual energy consumption of 1077 kWh/annum (Appendix B: Table S3.1). If this was coupled with a PV array to offset the remaining energy demands, this array would need to be approximately 1.5kWp or 12m² in size (Table 8). Given the right orientation this is probably achievable for small individual dwellings but may be more problematic in flats. Considering a 100m² dwelling in the same context, to offset remaining energy demands the PV array would have to deliver 1897 kWh/annum, and therefore would need to be sized at approximately 2.5kWp or 20m². Again this is probably achievable given the right building orientation.

Undertaking the same calculations for the other scenarios shows that this option might still be available at the fabric energy efficiency standard depicted by Scenario 2, but becomes increasingly difficult at lower fabric energy standards. This is because the size of the PV array required to offset a larger remaining energy demand might exceed the available roof space.

Comparison with Scottish Government Targets and Timeframes for CO₂ Emission Reduction

The Climate Change Plan (Third Report) sets out several ambitions related to energy efficiency and CO₂ emission reduction in buildings; including the target of 35% of heat in domestic buildings being supplied by low carbon technologies by 2032 (Scottish Government, 2018a, pp. 87-89). Further the Committee on Climate Change recommend that by 2025 at the latest there should be a joint move to both an ultra-high fabric energy efficiency (considered consistent with an annual space heat demand of 15 to 20kWh/m².annum) and low carbon heating (CCC, 2019b, p66; 2019d, pp 14-15).

To determine to what extent the minimum LZCGT contribution calculated through this methodology reflects this aspiration; Scenario 3 was used to calculate 35% of heat demand (SHD + HWD) for dwellings ranging in size from 25m² to 300m², as a percentage of the annual energy demand (AED). The results ranged from 28.5% to 26.6% of AED respectively, and are graphically represented in Figure 9. If we assume, as we did previously, that the LZCGT contribution as a percentage of annual energy demand is analogous to E%; then with reference to Figure 8, if the minimum LZCGT contribution is defined as 12% of the percentage CO₂ emission reduction sought by Standard 6.1 (R%), we can state that for any CO₂ emission reduction sought:

(Formula 14)

DERNT = (100 - R%) + A% = 100 - 0.88 R%

(Formula 15)

(Formula 16)

Substituting for A% (Formula 14) and DERNT (Formula 15) in Formula 16; and rearranging gives Formula 17.

(Formula 17)

This can be used to calculate the CO₂ emission reduction standard (R%) that would need to be sought to meet the 35% of heat demand target if the minimum LZCGT contribution is defined as 12% of that standard. The results for dwellings sized between 25m² and 300m² ranged from 76.8% to 75.1% respectively, and are graphically represented in Figure 9.

These values were also compared to the interim targets and timeframe set out in the Climate Change (Emissions Reduction Targets) (Scotland) Act 2019 for reducing GHG emissions in Scotland (Scottish Parliament, 2019a). These targets and timeframe are graphically represented in Figure 10. Extrapolating from this graph, in 2032 a projected interim target of 78% GHG reduction appears to be anticipated. This corresponds closely to our calculated values of 76.8% to 75.1%. We can therefore conclude that a minimum LZCGT contribution of 12% of the percentage CO₂ emission reduction sought by Standard 6.1 is likely to deliver the target of 35% of heat demand in new domestic buildings by 2032. The timeframe for delivering this target in new building could be accelerated by advancing the CO₂ emission reductions sought by Scottish Building Standard 6.1. Currie and Brown (2019) consider these levels could be achieved easily and cost effectively by employing heat pumps, high levels of fabric energy efficiency and MVHR by the mid 2020's.

5.4 Recommendations and Compliance Targets

Through the methodology described above a reasonable minimum level of LZCGT contribution to CO₂ emission reduction for new buildings was determined. We recommend that this level is set at a universal and perpetual 12% of the percentage of CO₂ emission reduction required by Scottish Building Standard 6.1 relative to the 2007 baseline (Scottish Government, 2019a; Scottish Government, 2019b). This judgement was reached by considering what would be a reasonable LZCGT contribution to annual energy demand in new buildings, against a changing regulatory framework that would tend to simultaneously increase fabric energy efficiency. Taking inspiration from the principles identified by Schwartz et al (2020) in relation to innovative and robust energy policy design; care was taken in making this judgement that certain criteria were met. These were:

i. It should be a reasonable minimum expectation, in respect to the overall level of CO₂ emission reduction new buildings are expected to achieve due to the changing regulatory climate.

ii. It should be readily achievable by all buildings whether in an urban or rural context.

iii. It should be low enough not to undermine or dis-incentivise a fabric first approach or other innovative passive design responses to CO₂ emission reduction.

iv. It should not be too onerous in terms of cost, and should offer long-term value for money for stakeholders.

v. It should not restrict the architect to only one viable option in choice of LZCGT, either directly or indirectly.

vi. It should not interfere with the ability to deliver wider sociological goals or essential infrastructure.

With respect to the final criteria the delivery of affordable and social housing was defined as a critical consideration and limiting parameter. Final judgement was therefore focussed on the impact the contribution level would have on this critical context of dwellings within the size range of 45m² to 100 m² (Appendix E).

Having determined a reasonable LZCGT contribution as a percentage of annual energy demand, it was necessary to express this in terms of CO₂ emissions. Three alternative ways of defining LZCGT contribution relative to CO₂ emissions were identified in current Section 3F policy (Scottish Government, 2019c). These were defined as A%, C% and E% respectively.

A%	An absolute percentage CO2 emission reduction relative to the 2007 baseline established by Scottish Building Standard 6.1.
C%	A percentage of the percentage CO2 emission reduction sought through Scottish Building Standard 6.1 relative to the 2007 baseline.
E%	An avoidance of a percentage of projected CO2 emissions as calculated by SAP/SBEM.

The final metric (E%) most closely aligns to the definition included in Section 3F policy, and equates reasonably well, although not exactly, to the LZCGT contribution defined as a percentage of annual energy demand. Using the formulae developed to define the relationship between these metrics (Formulae 11, 12 and 13), it was possible to express the LZCGT contribution determined as a percentage of annual energy demand in terms of each. These values were calculated for specific CO₂ emission reduction standards and the results set out in Table 9. The recommended LZCGT contributions can also be calculated graphically by reference to Figure 8.

Table 9: Recommended minimum LZCGT Contributions under different Scottish Building Standard 6.1 carbon dioxide emissions contexts.

R%	A %	C %	E %
	Formula 11	Formula 12	Formula 13
0% CO2 Reduction	0 %	12 %	0 %
30% CO2 Reduction	3.6 %	12 %	4.9 %
45% CO2 Reduction	5.4 %	12 %	8.9 %
60% CO2 Reduction	7.2 %	12 %	15.3 %
75% CO2 Reduction	9.0 %	12 %	26.5 %
90% CO2 Reduction	10.8 %	12 %	51.9 %
100% CO2 Reduction	12.0 %	12 %	100 %

R% = the CO₂ emissions reduction sought by Standard 6.1
A% = an absolute percentage CO₂ emissions reduction relative to the 2007 baseline
C% = a percentage of the percentage CO₂ emissions reduction sought by Standard 6.1
E% = Avoidance of a percentage of CO₂ emissions

It is useful for architects and planners to understand the relationship between these metrics. The first two (A% and C%) are quite abstract concepts but useful in government reporting. The final metric (E%) equates reasonably well to the LZCGT contribution defined as a percentage of annual energy demand so is a lot more meaningful and tangible from an architect's or developer's perspective, as it is a real-world meaningful target that can be directly used in planning a building's energy response.

However, on balance we recommend that in Section 3F policy the minimum LZCGT contribution be defined in terms of a percentage of the percentage CO₂ emission reduction sought through Scottish Building Standard 6.1 for several reasons:

i. By linking directly to the Building Standard 6.1, it avoids potential conflict between Planning and Building Standards.

ii. It simplifies Section 3F policy, by allowing the LZCGT contribution to be defined as a constant and perpetual percentage that will automatically deliver increases in real terms with every improvement of Standard 6.1 (Table 9, Figure 8).

iii. It provides regulatory certainty going forward (Schwartz et al, 2020)

iv. It ensures changes that occur in Building Standards 6.1 are reflected immediately, proportionately and automatically in Planning.

v. It allows differences in the CO₂ emission reduction sought for domestic and non-domestic buildings at building standards, to be simply and automatically reflected in the LZCGT contribution sought at Planning (Scottish Government, 2019a,b).

Going forward the extent of the LZCGT contributions in real terms will be determined relative to the CO₂ emission reduction set by Scottish Building Standard 6.1 at that time. Defined as a percentage of the percentage CO₂ emission reduction sought by Standard 6.1, the LZCGT contribution will remain constant at 12%. In real terms this will be a specified and rising proportion of the projected CO₂ emissions as required by Section 3F planning policy. It is vital to understand that any attempt to raise the minimum LZCGT contribution either disproportionately or at more frequent intervals than the changes to the overall CO₂ emission reduction sought by Building Standard 6.1, would be counter-productive and to the detriment of fabric energy efficiency (Figure 8). Such a situation should therefore be avoided.

5.5 Proposed Calculation Methodology

5.5.1 Compliance Formulae

The following formulae can be used to ascertain whether the specified LZCGT contribution has been met by a particular planning application. These formulae have be calculated as set out in Section 5.5.3.

• The formula to calculate LZCGT contribution defined as an absolute percentage CO₂ emission reduction relative to the 2007 baseline is given by:

• The formula to calculate the LZCGT contribution defined as a percentage of the percentage CO₂ emission reduction sought by Scottish Building Standard 6.1 is given by:

• The formula to calculate the LZCGT contribution defined as an avoidance of a percentage of projected CO₂ emissions as calculated by SAP/SBEM:

5.5.2 LZCGT Contribution Based on CO₂ Emission Rates (kgCO₂/m²)

Not all LZCGT have zero carbon emissions. Therefore the simplest way to accurately calculate if the specified LZCGT contribution to CO₂ emission reduction has been met by the proposed building is to run two separate SAP/SBEM calculations; one for the building as designed with the proposed LZCGT and another with the proposed LZCGT removed and replaced with pre-defined conventional systems. The CO₂ emission rates generated by these two separate SAP/SBEM calculations are then substituted into a formula to calculate the LZCGT contribution. This second SAP/SBEM calculation is not currently required for Building Standards purposes.

We would recommend that the presumption of what replaces the LZCGT in this second SAP/SBEM calculation is consistent for all buildings; although the appropriateness of this needs further consideration by Building Standards. Reasonable assumptions might be:

i. Renewable heat energy is displacing natural gas.

ii. Renewable electrical energy is displacing grid electricity

Three alternative ways of defining and calculating LZCGT contribution relative to CO₂ emissions are set out below. Each delivers the same result in real terms, and they all comply with the intention of Section 3F policy, i.e.

' . . . to ensure that all new buildings avoid a specified and rising proportion of the projected greenhouse gas emissions from their use, calculated on the basis of the approved design and plans for the specific development, through the installation and operation of low and zero-carbon generating technologies.'

Section 72 of the Climate Change (Scotland) Act, 2009 (Scottish Parliament, 2019b).

Although on balance our recommendation is to define the LZCGT contribution in terms of a percentage of the percentage CO₂ emission reduction sought by Standard 6.1; it will be for the Scottish Government to choose which one will be most useful in their ongoing reporting. As the LZCGT contribution percentages differ greatly between these definitions the need to avoid any ambiguity in use or reporting from various actors is paramount. Absolute clarity as to the definition and calculation methodology is therefore essential.

5.5.3 Compliance Calculations

In determining the compliance formulae the following abbreviations are used:

OER = Original Emission Rate (0% reduction relative to 2007 Baseline)
TER = Target Emission Rate (calculated by SAP 2012)
DER = Dwelling Emission Rate (calculated by SAP 2012)
   As designed FEE, EE & LZCGT used to achieve DER
DERNT = Dwelling Emission Rate calculated with no LZCGT (calculated by SAP 2012)
   As designed FEE & EE with LZCGT replaced with predefined conventional energy systems

All the above are specific to the particular building and are measured in kgCO₂/m²

R% = Statutory required CO₂ emission reduction sought by Scottish Building Standard 6.1 defined as a percentage relative to the 2007 baseline.
This is currently:
A 45% CO₂ emission reduction for Domestic Buildings
A 60% CO₂ emission reduction for Non-Domestic Buildings

The inter-relationships between these elements were defined previously with reference to Figures 7 and 8, and from these definitions we can state:

Reduction in CO₂ Emission Rate due to LZCGT contribution = DERNT - DER

Formula 18

Given that the relationship between OER and TER can be defined as:

Formula 19

LZCGT contribution defined as an absolute percentage CO₂ emission reduction, relative to 2007 baseline.

The percentage of CO₂ emission reductions that can be attributed to the use of LZCGT, calculated relative to a notionally similar dwelling built to the 2007 baseline standard (0% emission reduction) was defined previously as A%:

Formula 11

As the original state is not defined during the SAP/SBEM calculation process, we need to substitute for OER from Formula 19 and simplify:

Formula 20

For domestic buildings; the current CO₂ emissions reduction sought by Standard 6.1 relative to the 2007 standard is 45%. The LZCGT contribution defined as an absolute percentage of CO₂ emission reduction would therefore be defined as:

For non-domestic buildings; the current CO₂ emissions reduction sought by Standard 6.1 relative to the 2007 standard is 60%. The LZCGT contribution defined as an absolute percentage of CO₂ emission reduction would therefore be defined as:

LZCGT contribution defined as a percentage of the percentage CO₂ emission reduction sought by Building Standard 6.1

With reference to Figures 7 and 8, the LZCGT contribution calculated as a percentage of the percentage CO₂ emission reduction sought by Scottish Building Standard 6.1 relative to the 2007 standard (C%) is given by:

Formula 21

Substituting for OER from Formula 19, and simplifying:

Formula 22

Or as defined previously:

Formula 12

For domestic buildings; the current CO₂ emissions reduction sought by Standard 6.1 relative to the 2007 standard is 45%. The LZCGT contribution defined as a percentage of this CO₂ emission reduction standard would therefore be defined as:

For non-domestic buildings; the current CO₂ emissions reduction sought by Standard 6.1 relative to the 2007 standard is 60%. The LZCGT contribution defined as a percentage of this CO₂ emission reduction standard would therefore be defined as:

LZCGT contribution defined as avoidance of a percentage of projected CO₂ emissions as calculated by SAP/SBEM

This metric is the one most closely aligned to the definition included in Section 3F policy. It was previously defined as E%.

Formula 13

5.5.4 Compliance Procedure

Ascertaining whether the specified LZCGT contribution has been met by the proposed building is simply a matter of substituting the relevant information from the two SAP/SBEM calculations into the appropriate formula and comparing the result to the target percentages set out in Table 9 for the relevant regulatory requirement. If the value calculated is greater than or equal to the relevant figure given in Table 9 the proposed building has complied with Section 3F requirements.

A standard Compliance Calculation Spreadsheet which calculates the LZCGT contribution defined as a percentage of the percentage CO₂ emission reduction sought by Building Standard 6.1 is included in Appendix C (Figures C.1 and C.2).