Publication - Research and analysis

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

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

Building regulations - new non-domestic buildings - modelling of proposed energy improvements: research report
Task 2: Develop Improved Notional Building Specifications

Task 2: Develop Improved Notional Building Specifications

1.6 Review of current notional buildings for design optimisation

72. We have considered how the current notional building may not encourage design optimisation of the actual building to reduce energy consumption and/or carbon emissions. The main area identified is where the notional building specification defaults to that of the actual building. This occurs both for the built form and fuel choice of the notional building. There is no benefit in optimising design for either factor.

73. The discussion below relates to the issue of built form. The lack of encouragement of low carbon heating fuels is discussed in Section 1.8.

74. Our view is that to introduce an incentive for built form would be complicated to implement successfully and would be likely to have unintended consequences. Significant further research would be necessary to assess and develop such an approach.

75. The optimal built form of a building is a complex function of many independent and widely varying parameters including:

  • Activity:
    • Occupancy density;
    • Occupied hours/variation profiles;
    • Lux levels;
    • Heating and cooling setpoints.
  • Climate;
  • Servicing strategy;
  • Heating and cooling fuel;
  • Fuel factors;
  • Efficiency of lighting and HVAC plant;
  • Fabric performance:
    • U-values;
    • Thermal bridges.

76. Such complexity would make it difficult to implement, say, a fixed built form in the notional building that is reasonable and fair across the many types of non-domestic buildings. The fixed built form could be varied by building type and usage, but without further research, there may will need to be a significant number of built forms such that a reasonable design is included in the notional building in all cases. Some argue for an absolute performance standard as an alternative (e.g. a fixed energy demand in terms of kwh/m2) to encourage a more energy efficient built form and, again, the complexity is tailoring any absolute standard such that it represents an equitable challenge across different building types (assuming that is Scottish Government’s intent).

77. There is also the potential for unintended consequences when introducing an incentive to encourage improved building form. For example:

  • Building location: Building form is strongly influenced by a constrained site; this is commonly a key factor for city-centre sites. If a new requirement was to be introduced it may effectively discourage the use of a particularly constrained city-centre site and thus encourage developed at out-of-town locations where increased use of private transport may be required.
  • Process requirements: The form of many buildings is strongly influenced by their operational requirements, (e.g. hospitals, schools and factories). If a new requirement ran counter to these requirements, then the building may need to be larger than might otherwise be necessary and include redundant zones within the building. This would increase the cost and embodied energy impact of the building as well as being likely to increase its operational energy demands. This effect is true for all building types to some extent because building form influences the amount of circulation space needed.

78. It is also noted that the national calculation methodology (NCM) ignores or simplifies several ways in which building form will impact on real energy use and environmental impact of the building. This limits the ability that design optimisation is influenced by the notional building and may result in sub-optimal design. For example:

  • Vertical transportation: Taller buildings are likely to have greater energy demands for lifts and escalators which are not currently included in the NCM.
  • Thermal bridging: DSM compliance models do not normally include calculated thermal bridging values but rather simply apply a 10% allowance to all U-values. SBEM does account for this in more detail but it remains unusual for assessors to be provided with accurately calculated Ψ-values.
  • Pumping energy: Build form will influence the length complexity of heating and cooling pipework and thus affect the pumping energy. DHW and CWS circulation and pressurisation systems will be similarly affected and strongly influenced by building height.

1.7 Identification of potential improved specifications

79. To provide an evidence base for potential improvements to the future notional building specification, the following steps have been taken to help ensure that decisions are well-supported by evidence and relevant to new buildings in Scotland.

  • Review of EPC database;
  • Review of England Part L proposals (Welsh proposals are still in development at the time of the analysis and are not included here);
  • Review UK Cost Optimal Report 2018;
  • Review of consultation responses;
  • Review of research informing 2015 standards.

80. The principal source for this review is the EPC database. The analysis in Task 1 shows the distribution of buildings currently being constructed to different specifications. Specifications that would be representative of good practice could be taken as those around 75% of the distribution (i.e. only 25% of buildings have a better specification). Best practice has been taken to be those specifications around 90% of the distribution (i.e. only 10% of buildings have a better specification).

81. The other sources identified above have been reviewed to confirm the conclusions of the review of the EPC database and/or provide additional insights.

82. The analysis undertaken to support the proposed changes to Part L in England took account of detailed feedback from industry representatives on the feasibility of achieving different component values, taking into account factors such as capital cost, cost-effectiveness, market availability and performance gap issues. This analysis also reviewed potential near-term future heating sources considering standards beyond 2020.

83. The UK Cost Optimal Report 2018 provides the second cost optimal assessment of energy performance requirements for the United Kingdom as required by the European Energy Performance of Buildings Directive. Analysis of Sections 11 and 12 of this reveal tipping points in the cost effectiveness of improvements to building fabric, services and renewables for different building types. This has been drawn upon to help select preferred standards for analysis.

84. We reviewed information from the consultation responses provided to the Scottish Government’s 2018 Scottish Building Regulations: Review of Energy Standards: ‘Call for Evidence’. This provides evidence on approaches to meeting 2015 standards.[5] We identified no relevant evidence that differed or added to learning from the EPC database analysis.

85. We reviewed information from the research informing the 2015 standards. This provides evidence on approaches to meeting 2015 standards.[6] We identified no relevant evidence that differed or added to learning from the EPC database analysis.

1.7.1 Fabric

86. Table 20 compares the 2015 Section 6 Notional fabric performance parameters with those from the other sources described above. The EPC database does not include U-values for individual building elements (wall, roof etc.) but rather includes the whole-building average U-value. The average U-value is a function of each individual element U-value and their relative areas; most importantly the glazed area varies significantly between different buildings and will be a strong influence on the whole-building U-value.

87. Table 21 shows U-values for each individual building element calculated on the basis of the percentage improvement in whole-building U-value; this is effectively assuming that the glazed area is the same as the Notional in all cases; consequently, this table should be used with caution but is helpful in sense-checking the other sources.

Table 20: Comparison of 2015 Section 6 Notional fabric performance parameters with those from other sources
Variable Sec 6 Notional EPC Database Part L for England 2020 Proposal Cost Optimal Report
MV & AC NV Data Peak
(Mode)
Side-lit Top-lit Side-lit Top-lit
Heated & Naturally Ventilated Heated & Cooled or Heated & Mechanically Ventilated 75th percentile 90th percentile 75th percentile 90th percentile MV & AC NV Option 1 Option 2
Wall U-Value (W/m²K) 0.23 0.20 EPC Database does not provide this information directly, but an approximation can be inferred from the whole-building average U-value, see Error! Not a valid result for table.. 0.26 0.18 0.26 0.30 0.21
Roof U-Value (W/m²K) 0.18 0.16 0.18 0.15 0.18 0.25 0.20
Floor U-Value (W/m²K) 0.22 0.20 0.22 0.15 0.22 0.25 0.20
Window U-Value (W/m²K) 1.80 1.60 1.60 1.40 - 1.80 1.40
G-value 60% 50% 40% 29% 40% Undefined
Light transmittance 71% 71% 71% 60% 71%
Rooflight[7] U-Value (W/m²K) 1.80 1.80 1.80 1.50 1.80
G-value 52% 52% 40% 29% 40%
Light transmittance 57% 57% 71% 60% 71%
Air Tightness 3, 5 or 7 4.4 3.5 4.7 3.9 4.5 4.5 5 3 5 7 5
Average U-value change from Notional 0% -27% -37% -34% -41% -20% -31% +16% -13% +16% NA NA
Table 21: Fabric performance parameters derived from percentage improvement in whole-building U-values in EPC database
Variable EPC database
MV & AC NV Peak
75th percentile 90th percentile 75th percentile 90th percentile MV & AC NV
Wall U-Value (W/m²K) 0.15 0.13 0.13 0.12 0.16 0.14
Roof U-Value (W/m²K) 0.12 0.10 0.11 0.09 0.13 0.11
Floor U-Value (W/m²K) 0.15 0.13 0.13 0.12 0.16 0.14
Window U-Value (W/m²K) 1.17 1.01 1.06 0.94 1.28 1.10
Rooflight U-Value (W/m²K) 1.31 1.13 1.19 1.06 1.44 1.24

88. Table 22 shows proposed fabric standards for modelling analyses to support proposed changes to Section 6.

89. Previous work has identified tipping points in the costs and performance of building fabric elements. Key amongst these is the difference between double glazing and triple glazing. Double glazing can achieve U-values lower than 1.4W/m² (in the vertical plane) however below this U-value triple glazing is typically a more cost-effective option. However once triple glazing is adopted there is a strong argument for specifying a significantly improved U-value between 0.9 and 0.7W/m²K. On this basis the proposed window U-values set out for analysis in Table 22 are 1.6, 1.4 and 0.9W/m²K. Achieving lower U-values for rooflights is more challenging so the high option has been omitted for rooflights.

90. The current Section 6 Notional specification uses different U-values for naturally and mechanically ventilated buildings; mechanically ventilated buildings receive the lower/more challenging U-value standards. It is understood that there is a desire to simplify and improve the specification of the Notional building. Therefore the “Low” option set out in Table 22 adopts the current mechanically ventilated fabric standard and applies it to all ventilation strategies.

91. Reviewing Table 20 it can be seen that the opaque U-values identified in the cost optimal report are equal to or higher than the Section 6 2015 standards for mechanically ventilated buildings, however Option 2 from the Part L 2020 consultation for England is a significant improvement on this. Therefore, this Option 2 specification has been selected as the “medium” option for analysis as shown in Table 22. The specification draws on previous unpublished analysis which identified a tipping point for masonry wall construction. Between U-values of 0.18 and 0.15W/m²K the cavity in a typical masonry wall build-up drops below 40mm consequently requiring measures to mitigate the transfer of moisture.

92. Table 21 should be used with caution (see Paragraph 86), however the figures shown implies that many Scottish buildings may be achieving U-values significantly lower than those proposed for Option 2 under the English Part L 2020 consultation (U-values in the range of 0.15 to 0.09W/m²K). Given the relative uncertainty around the data in Table 21, U-values have been selected for the “high” option based on the 75th percentile rather than the 90th percentile; these are listed in Table 22.

93. The current 2015 Section 6 standard for air tightness is a function of the building size and activity type (side-lit or top-lit) varying between 3 and 7m³/m²/hr @50Pa. It is understood that there is a desire to simplify the specification of the Notional building so it is proposed that a single air tightness value should be used. Reviewing Table 20 it can be seen that The cost optimal report identified values of 5 and 3m³/m²/hr @50Pa whilst the Part L for England Consultation is proposing 5 or 3m³/m²/hr @50Pa. The Scottish EPC database records the air permeabilities achieved in completed buildings, this shows that 25% of floor area in new building achieved air tightness ratings of 4.4 or lower for mechanically ventilated buildings and 4.7 or lower for naturally ventilated buildings. Table 22 proposes values of 5, 4 and 3m³/m²/hr @50Pa for the “high”, “medium” and “low” options. Section 1.5.2.1 shows that less than 5% of non-domestic floor area achieves an air tightness of less than 3m³/m²/hr @50Pa. Reducing air-tightness below this value generally achieves only a small improvement in the modelled building performance. However, the method by which air tightness is modelled in SBEM and DSMs is approximated, it is therefore possible that real-world building performance is more sensitive to this parameter than modelling suggests.

Table 22: Proposed Low, Medium and High fabric specifications for modelling
Variable Sec 6 Notional Suggested Options
Heated & Naturally Ventilated Heated & Cooled or Heated & Mechanically Ventilated Low Medium High
Wall U-Value (W/m²K) 0.23 0.20 0.20 0.18 0.15
Roof U-Value (W/m²K) 0.18 0.16 0.16 0.15 0.11
Floor U-Value (W/m²K) 0.22 0.20 0.20 0.15 0.13
Window U-Value (W/m²K) 1.80 1.60 1.60 1.40 0.90
G-value 60% 50% 50% 29% 29%
Light Transmittance 71% 71% 71% 60% 60%
Rooflight U-Value (W/m²K) 1.80 1.80 1.80 1.50
G-value 52% 52% 52% 29%
Light Transmittance 57% 57% 57% 60%
Air Tightness 3, 5 or 7 5 4 3

94. There is a synergy between high fabric performance and the use of heat pumps. Heat pumps operate more efficiently when providing heat at lower temperatures; the low heating demands that enhanced fabric achieves facilitates the use of low temperature heating systems. Section 1.8 discusses the possibility of using gas boilers and PV to achieve similar CO2 and primary energy performance to an ASHP system; this challenge is made easier when enhanced fabric is used.

1.7.2 Services

95. Table 23 compares the 2015 Section 6 Notional building service performance parameters with those from the other sources described above. The EPC database does not include data for lighting efficacies or automatic controls; neither does it include information on ventilation performance parameters. However, it does contain data on the efficiency of the heating and cooling sources used.

96. Table 24 shows proposed “Low”, “Medium” and “High” building service standards for modelling analyses to support proposed changes to Section 6.

1.7.2.1 Space Heating

1.7.2.1.1 Gas Boiler

97. Gas boiler technology is mature and not expected to see significant improvements in the next five years. The market for manufacture installation and maintenance of gas boilers is well established and competitive. The gas boiler efficiencies from the EPC database show that 25% of buildings include boilers with seasonal efficiencies of 97% or higher and 10% achieve 98% or higher. These are efficiencies are at the upper end of what is achievable with current technology. It is possible that some lodged EPCs may be incorrectly based on net rather than gross efficiencies; this would artificially inflate the reported efficiencies in the database. All the other sources shown in Table 23 suggest gas boiler efficiencies significantly lower than this, ranging between 91% and 93%; this is a very narrow range. The current 2015 Section 6 Notional building uses 91% which is at the bottom of the narrow range suggested by the other analyses; it is therefore proposed that 93% is considered for the Section 6 2020 analysis.

1.7.2.1.2 Radiant Heating

98. For naturally ventilated top-lit spaces (such as distribution warehouses) radiant gas heaters are the dominant heating type. This technology is mature and not expected to see significant improvements in performance in the next five years. The market for manufacture, installation and maintenance of gas radiant heaters is well established and competitive. The gas radiant heater efficiencies from the EPC database show that 25% of buildings include gas radiant heaters with seasonal efficiencies of 92% or higher and 10% achieve 93% or higher. These are efficiencies are at the upper end of what is achievable with current technology. Both the Part L for England proposals and the Cost Optimal Report suggest values of 86%. The current 2015 Section 6 Notional building also uses 86% which is less efficient than around 80% of radiant heating systems recorded in the EPC database. It is therefore proposed that 92% is considered for the Section 6 2020 analysis.

1.7.2.1.3 Air Source Heat Pumps (ASHPs)

99. ASHPs have represented a small part of the heating market for several decades; recent increased concern about climate change has been a strong driver for the increased use of this technology and have encouraged improvements in efficiency and reductions in capital and maintenance costs. Many global manufacturers are actively developing improved products using refrigerants with reduced Global Warming Potential (GWP). Table 23 shows that the current 2015 Section 6 Notional efficiency for an ASHP is much lower than those recorded in the EPC database and the English Part L analysis. On this basis Table 24 shows the proposed “Low”, “Medium” and “High” options which are aligned to the 50th, 75th and 90th percentiles from the EPC database.

100. ASHPs can be used for both space heating and hot water. The efficiency of a heat pump is higher when supplying lower temperatures. A common temperature regime for a wet heating system is 80⁰C flow and 60⁰C return; very few heat pumps can achieve flow temperatures above 70⁰C so a lower temperature system is required. To improve the heat pump efficiency, it is common to design heating system with flow temperatures around 50⁰C or even lower.

101. Figure 16 shows four curves illustrating the relationship between heating flow temperature and Seasonal Coefficient of Performance (SCoP); the green and blue curves show the idealised theoretical relationship based on two different Carnot efficiencies, and the purple and orange curves show the calculated relationship for two real heat pumps based on detailed performance data supplied by manufacturers and the TRY weather for Glasgow. It can be seen that the performance of real ASHPs is similar to, but differs slightly from, the idealised performance.

102. The current 2015 Section 6 performance requirements (as well as the EU ErP and several other standards) for ASHPs are based on the products’ efficiency when measured in accordance with EN 14511. This standard makes standardised assumptions about the ambient air temperature (i.e. the temperature of the source from which the heat pump extracts heat) and the heating system flow temperatures. These standard assumptions mean that the “official” efficiency will differ from that calculated using project-specific values such as the location weather data and system flow temperature. EN 14511 requires that performance is measured at a minimum of one of four heating system flow temperatures (35⁰C, 45⁰C, 55⁰C and 65⁰C). Figure 16 shows several thousand reported efficiencies from the Eurovent database measured at each of these temperatures; it can be seen that these broadly align with the curves described above.

103. Figure 16 shows three horizontal black dotted lines corresponding to the 50th, 75th and 90th percentiles reported in the EPC database (see Section 1.5.3.1.2). Comparing these dotted-lines with the Eurovent data (and assuming that modellers are inputting the appropriate values in the EPC models) it can be inferred that many ASHP heating systems are being designed with flow temperatures below 55⁰C. The 50th and 75th percentile values appear to only be achieved by system operating at 45⁰C or lower, whilst the 90th percentile is only achieved by systems at 35⁰C. As the EPC database does not report heating system flow temperature it is not possible to validate this.

Figure 16: Relationship between Heating system flow temperature and SCoP.
Table illustrating the relationship between Heating system flow temperature and seasonal coefficient of performance in the temperature range of 35 ºC to 70 ºC. This illustrates that performance reduces in a relatively linear way as the flow temperature is increased.
1.7.2.2 Domestic Hot Water (DHW)

104. The efficiency of gas boilers and other combustion-base heat generators supplying DHW systems is generally similar to that of the same or equivalent heat generators providing space heating. However, the efficiency of equivalent heat pumps serving these two load types varies more significantly. This section considers what efficiency to include in the notional buildings for ASHPs serving DHW systems.

105. Domestic Hot Water (DHW) systems are often designed to heat water to 60⁰C to mitigate legionella risks associated with storing hot water at lower temperatures. DHW systems which heat water instantaneously upon demand can avoid this because the warmed water is not stored for long periods. Heat pump DHW systems generally include water storage and so are often designed to heat water to 60⁰C, and this relatively high temperature results in a reduced heat pump efficiency. It is possible to design heat pump DHW systems which heat water instantaneously however this approach is not widely adopted. Therefore, for the purposes of developing a widely accepted heat pump DHW solution for the notional building, it is proposed that when the notional building uses heat pumps, there should be separate heat pumps for space heating and DHW and the respective efficiencies of these two heat pumps reflect the different temperatures at which they are likely to operate. EN 14511 and the related standards do not include generation of DHW and the current Section 6 Non-Domestic Building Service Compliance Guide does not appear to specify a test method but simply sets a minimum standard of 2.0. This implies that it is up to the design team to determine the efficiency by whatever means they choose.

106. The SCoP of an ASHP providing space heating is weighted towards the unit’s performance in cooler weather when the heating demand is higher. DHW demand is typically required all year round and so the average air temperature when the unit is running is higher for DHW than for space heating. The efficiency of ASHPs providing DHW is therefore increased by the average air temperature and reduced by the need to provide water at 60⁰C. Using the same calculation method as was used to produce the orange and purple curves in Figure 16 above, it has been found that the net impact of these two effects is approximately 10%, i.e. the average efficiency of an ASHP providing space heating at 55⁰C in Glasgow can be approximately 10% higher than that of the same unit being used to provide DHW. However, if the space heating system has a lower flow temperature then the effect of the higher temperature requirements of DHW become dominant so the difference between DHW and space heating efficiency will increase. As is described in paragraph 103, it can be inferred that the majority of ASHP heating systems in the EPC database are using flow temperatures below 55⁰C. A reduction factor can be applied to the ASHP DHW efficiency used in the notional building to reflect this.

107. Figure 17 shows the approximate relationship between DHW and space heating SCoP for a widely-used heat pump from a leading manufacturer, based on a range of assumed space heating efficiencies.

  • If we assume that the 50th percentile in Figure 16 (SCOP = 3.44) is likely to represent systems operating at 45⁰C the equivalent DHW SCoP may be assumed to be approximately 73% of this value (i.e. 2.5).
  • Similarly if we assume that the 75th and 90th percentiles are more representative of heating systems operating at 35⁰C then the adjustment is around 62%, so the 75th percentile value for space heating (SCOP=4.0) is adjusted an equivalent DHW SCoP of 2.5 and the 90th percentile for space heating (SCOP = 4.35) is adjusted to an equivalent DHW SCoP of 2.7.

108. These three adjusted values for the 50th, 75th and 90th percentiles respectively are proposed for the “Low”, “Medium” and “High” options for ASHPs serving DHW systems.

Figure 17: Indicative relationship between ASHP SCoP for space heating and DHW provision across a range of space heating flow temperatures
Table illustrating the ratio of domestic hot water SCoP to space heating SCoP as heating system flow temperature increases between flow temperatures of 25 ºC to 70 ºC. This illustrates that the relative performance for domestic hot water is lower than that for space heating as the flow temperature reduces.
1.7.2.3 Cooling

109. Demand for cooling has increased significantly over the last two decades driven by higher comfort expectations of building occupants, reduced costs and economic forces encouraging the development of deep-plan mechanically ventilated buildings. Being closely related to ASHPs, chiller technology is also undergoing similar developments to improve efficiency and reduce the GWP of refrigerants used. The current SEER for the 2015 Section 6 Notional building is 4.5; the English Part L consultation proposes a higher value of 5.5. The EPC database shows that the 75th and 90th percentiles are significantly higher at 6.4 and 7.1. On this basis the proposed “Low”, “Medium” and “High” options are 5.5, 6.4 and 7.1 respectively.

1.7.2.4 Lighting

110. The efficacy of luminaires in the 2015 notional building is 60llm/cW for naturally ventilated buildings and 65llm/cW where mechanical ventilation or cooling is included. Over the last few years the rapid development of LED technology has driven a large improvement in the efficiency of installed lighting systems. LED technology is still developing rapidly and is predicted to continue to do so for many years. The English Part L consultation proposes an efficacy of 95llm/cW to reflect LED technology; this same value was identified in the cost optimal report. Even in the few months since these analyses LEDs have continued to improve, so it is proposed that the should be towards the upper end of what is widely available today, we believe this to be in the region of 125llm/cW. AECOM’s lighting specialists advise that market analysts widely expect efficacies in the range 200-270llm/cW by 2025. It may therefore be worth considering whether there is a mechanism by which lighting efficacy (and other notional building performance parameters) can be improved in the period between the principal Section 6 updates. The English Part L analysis identified that LEDs have effectively removed the justification for a difference in efficacy between general and most display lighting. However, the improvement in the efficacy of some types of reflector lamp lag behind that of other lighting types. Nevertheless, it is proposed that the notional efficacy of general and display lighting values should be aligned.

1.7.2.5 Ventilation

111. Where the 2015 Section 6 Notional building uses mechanical ventilation with supply and extract, it has a heat recovery efficiency of 70% and fans are controlled based on gas-sensors linked to inverter drives. This heat recovery efficiency is now superseded by the European Eco-Design Directive which stipulates that such heat recovery should have a minimum efficiency of 73%[8]. Heat recovery systems can achieve efficiencies in excess of 90%, however the size and cost of systems that can achieve this are large. The English Part L 2020 consultation proposes an efficiency of 76%, therefore this figure is proposed in Table 24. Gas-sensors with inverter speed control is the most efficient option currently available in SBEM so this solution is applied across all three options.

Table 23: Comparison of 2015 Section 6 Notional building service performance parameters with those from other sources
Variable Sec 6 Notional EPC Database Part L for England[9] 2020 Proposal Cost Optimal Report
MV & AC NV Data Peak Side-lit Top-lit Side-lit Top-lit
Heated & Naturally Ventilated Heated & Cooled or Heated & Mechanically Ventilated 75th percentile 90th percentile 75th percentile 90th percentile MV & AC NV Option 1 Option 2
Heating & Cooling Gas boiler 91% 97% 98% 97% 98% 95% 95% 86% NA 91% NA
ASHP 1.75 4.00 4.35 4.00 4.35 3.25 3.25 3.2 NA
Radiant gas heater (top-lit) 86% NA NA 92% 93% NA 95% NA 86% NA 86%
Cooling SEER 4.50 6.40 7.10 NA NA 6.50 NA 5.5 3.60
Domestic Hot Water Gas Boiler 91% 97% 98% 97% 98% 95% 95% 93% 91%
ASHP 1.75 2.50 2.70 2.50 2.70 2.40 2.40 3.55 NA
Lighting & Ventilation Lighting luminaire (llm/cW) 60 65 EPC Database does not provide this information. 95 95 75
Daylight lighting control Single zone daylight dimming Single zone daylight dimming Yes but undefined
Occupancy Lighting Control Manual on auto off Auto on auto off Yes but undefined
Parasitic Power 0.3W/m² or 3% for daylight 0.3W/m² for occupancy 0.1W/m² Undefined
Display Lighting (llm/cW) 22 95 Undefined
Display Lighting Control none time switch Undefined
Ventilation Heat Recovery 70% 76% Undefined
Demand Control Ventilation gas-sensors, inverters gas-sensors, inverters Undefined
Table 24: Proposed Low Medium and High building service specifications for modelling
Variable Sec 6 Notional Suggested Options
Heated & Naturally Ventilated Heated & Cooled or Heated & Mechanically Ventilated Low Medium High
Heating & Cooling Gas Boiler 91% 93%
ASHP 1.75 3.44 4.00 4.35
Cooling SEER 4.50 5.50 6.40 7.10
Domestic Hot Water Gas Boiler 91% 93%
ASHP 1.75 2.50 2.50 2.70
Lighting & Ventilation Lighting Luminaire (llm/cW) 60 65 125
Daylight Lighting Control Single zone daylight dimming Single zone daylight dimming
Occupancy Lighting Control Manual on auto off Manual on auto off
Parasitic Power 0.3W/m² or 3% for daylight 0.3W/m² for occupancy 0.1W/m²
Display Lighting (llm/cW) 22 125
Display Lighting Control none time switch
Ventilation Heat Recovery 70% 76%
Demand Control Ventilation gas-sensors, inverters gas-sensors, inverters

1.7.3 On-site Generation

112. Table 25 compares the 2015 Section 6< Notional photovoltaic (PV) array size with those from the other sources described above. Table 26 shows proposed “Low”, “Medium” and “High” photovoltaic arrays sizes for modelling analyses to support proposed changes to Section 6.

113. The 2015 Section 6 notional building has a PV array sized to the lesser of 4.5% of the GIA and 50% of the roof area. The cost optimal report effectively identified the optimal size of PV array as being as large as possible however the modelled options limited this to 40% of roof area[10]. The Part L consultation proposed 40% of roof area for top-lit spaces and 20% for side-lit spaces on the basis that side-lit spaces often have a greater proportion of the roof area occupied by plant. Analysis of the EPC database shows that PV is more widely used for naturally ventilated building than mechanically ventilated/cooled buildings. The approximate percentages of GIA and roof area have been derived from the EPC database by comparing the PV outputs with those of the notional building; this approach suggests that 25% of naturally ventilated buildings have a PV array which is larger than either 2.6% of the GIA or 29% of the roof area whilst 10% of these buildings have PV areas greater than 13% of the GIA or 145% of the roof. This last figure implies that either some of the PV is ground-mounted or the buildings are tall and so the percentage of GIA is the limiting factor.

114. To determine how much PV the notional building should have it is necessary to decide whether the aim is to encourage developers to put as much PV on roofs as is practical or to link the incentive to the size and energy use of the building. If the aim is to encourage the use of on-site generation then linking this to roof area is desirable; if the aim is to link this to size and energy use, then GIA is a helpful metric. The proposed approach is to develop a notional building specification which includes PV when gas is the heating and DHW fuel but to not include PV when ASHPs are used for heating and DHW. PV is included for gas heating only to broadly equalise the level of challenge associated with achieving compliance whether gas heating or an ASHP is used; this is discussed from Paragraph 155.

Table 25: Comparison of 2015 Section 6 Notional photovoltaic array sizes with those from other sources
Variable Sec 6 Notional EPC Database Part L for England[11] 2020 Proposal Cost Optimal Report
MV & AC NV Side-lit Top-lit Side-lit Top-lit
Heated & Naturally Ventilated Heated & Cooled or Heated & Mechanically Ventilated 75th percentile 90th percentile 75th percentile 90th percentile Option 1 Option 2
PV Area: Lesser of: % of GIA 4.5% 0% 6.3% 2.6% 13.0% NA NA NA
% of roof area 50% 0% 70% 29% 145% 20% 40% 40%

115. Note that the values in Table 26 are based on the same PV output as is currently used for the notional building under Section 6 2015 (i.e. 120kWh/m²) which is equivalent to a nominal efficiency of approximately 16%.

Table 26: Proposed Low Medium and High onsite generation specifications for modelling
Variable Sec 6 Notional Suggested Options
Heated & Naturally Ventilated Heated & Cooled or Heated & Mechanically Ventilated Low Medium High
PV Area: Lesser of: % of GIA 4.5% 6.5% 13.0%
% of roof area 50.0% 50.0% 50.0%

1.7.4 Trends in Heating Fuel Selection

116. The EPC database has been analysed to assess recent trends in heating fuel selection for new buildings. This analysis is intended to inform the choice of heating fuel and technology in the proposed notional specification by assessing the current uptake of different technologies. Figure 18 shows the quarterly variation in floor area served by each heating fuel. This is based on the EPC model inputs for each HVAC system in each building, rather than using the single heating fuel for the whole building which is reported on the EPC. This approach has been taken to capture the effect of many buildings having multiple heating fuels serving different areas of the building. For example, a building may have gas boilers serving most of the floor area but use ASHPs to provide heating (and cooling) to a small number of rooms.

117. Figure 18 shows the following key trends over the period of the analysis:

  • Use of natural gas declines from 2013 to 2015 and then remains fairly constant.
  • Use of ASHPs increases from 2013 to 2015 and then decreases.
  • Use of oil is low from 2013 to 2015 and then increases significantly.
  • Biomass is used for around 40% of floor area in Q1 of 2013 and 25% in Q1 of 2014. In general there is a downward trend in biomass use across the whole period.

The use of other heating fuels is insignificant in most quarters, with no clear trend being apparent.

Figure 18: Quarterly variation in floor area served by each heating fuel based on lodged EPCs
Graph illustrating the quarterly variation in floor area served by each heating fuel based on lodged EPC data between 2-13 and the start of 2019.  The main trends are commented on in the text immediately prior to the graph.

118. It is noted that the database covers a period in which spans two different version of Section 6, i.e. versions that first came into effect in 2010 and 2015. The analysis of Figure 18 set out above appears to show that post 2015, a greater proportion of new build floor area was served with oil-fired heating whilst a lesser proportion of floor area was heated by ASHPs. In some quarters the new floor area heated by oil is greater than that heated by gas.

1.8 Consideration of low carbon heating and renewable technologies

119. The notional building under the current (2015) version of Section 6 uses the same heating fuel as the actual building in all cases. In the context of the Scottish Government’s ambition to radically reduce the use of fossil fuels in new buildings this approach is to be reviewed. As part of this review the possibility to use a low carbon heat source in all has been considered.

120. As part of the Scottish Governments strategy to decarbonise heat, district heat networks are being encouraged and supported. It is therefore preferred that the changes to Section 6 allow new buildings to connect to both new and existing district heat networks.

121. The above ambitions may be moderated by the desire to avoid the negative impacts of a sudden change to the building servicing strategies implemented in new buildings. Negative impacts could stem from supply change constraints affecting low carbon heat sources, economic impacts of falling demand for fossil-fuelled plant, skills shortages in relation to the design, installation, commissioning and maintenance of low carbon heat sources which operate in significantly different ways to fossil-fuelled plant. Whilst there is a wide range of low-carbon heat sources, all of these require the building location to have certain characteristics, these are summarised in Table 27. To help address these concerns it is proposed that the analysis should identify routes to compliance which use fossil-fuelled systems in combination with onsite generation from renewable sources such as PV.

122. The analysis below explores the relative viability of achieving carbon reductions through the use of low/zero-carbon heat sources compared to using fossil fuelled heating combined with PV. We understand from the client’s brief that the latter should be a reasonable and viable option.

123. Table 27 sets out the limitations and constraints associated with the most widely used low/zero-carbon heat sources. This comparison helps to select those heat sources which may be considered to be viable to the most non-domestic buildings.

Table 27: Summary of site constraints for low-carbon heating sources

Heat Pump - Air-source (ASHP)

Requires access to external air; may be challenging for units within larger buildings. However, this is a relatively unusual barrier and can be overcome through a coordinated design with the larger building.

Heat Pump - Water-source (WSHP)

Requires access to a suitable body of water which is rarely available.

Heat Pump - Ground-source (GSHP)

Requires suitable ground conditions, this can be viewed as a large unquantified project risk until ground investigations can be undertaken. The costs and uncertainty may discourage the use of this technology.

Heat Pump - Sewer-source (SSHP)

Requires access to a suitable sewer and cooperation from sewer owner/operator[12]. Such reliance on a third party may be deemed to be an unacceptable project risk. This technology is still deemed to be relatively innovative.

Biomass boiler

Requires suitable access for fuel delivery and storage. Ongoing maintenance and operational requirements may be deemed to be burdensome for building occupants/owners.

Solar water heating

Generally only suitable for domestic hot water provision although innovative systems can provide space heating as well.

124. Based on the constraints identified in Table 27, it is proposed that, where applicable, the preferred low carbon heat source for the notional building should be an ASHP as this technology has the least challenging constraints and is therefore likely to be technically viable in more cases than the other technologies considered.

125. ASHPs can be used for both space heating and hot water. The efficiency of a heat pump is higher when supplying lower temperatures. A common temperature regime for a wet heating system is 80⁰C flow and 60⁰C return; very few heat pumps can achieve flow temperatures above 70⁰C so a lower temperature system is required. To improve the heat pump efficiency, it is common to design heating system with flow temperatures around 50⁰C or even lower. Some countries require that space heating systems are designed with flow temperatures no greater than a specified threshold, for example in Sweden regulations introduced in 1984 stipulate a maximum temperature of 55⁰C[13]. This requirement was initially indented to simply reduce energy demands but it also facilitated the later retro-fitting of heat pumps[14]. The relationship between heating flow temperature and heat pump efficiency is characterised as a power relationship with no significant step changes and so there is no strong technical justification for a particular heating flow temperature to be selected. The 55⁰C threshold stipulated in Sweden has the advantage that it aligns to the current EN 14511 standard as well as several related standards (see paragraph 102). Figure 16 shows that relatively few ASHP report their performance at the higher temperature of 65⁰C and that, at this temperature, none of these meet the current minimum standard of 2.5. At 55⁰C the majority of ASHP meet the 2.5 requirement and at 45⁰C almost all do.

126. The use of an ASHP will result in lower CO2 and primary energy values than those achieved by a gas boiler system. Using the proposed CO2 and primary energy factors, analysis has been undertaken to assess the amount of PV required to reduce the total CO2 and primary energy values of a building using gas boilers with those of the same building using an ASHP. This analysis has initially been based upon the efficiencies of these technologies in the current 2015 Section 6 notional building (gas boiler 95% efficiency and ASHP SEER of 3.44). The results of this analysis are shown in Table 28 expressed as the area of 20% efficient PV needed per unit area of gross internal floor area (GIA). Single storey buildings with roofs that are close to being flat will have a roof area equal to the GIA, in multi-storey buildings the ratio of roof area to GIA reduces. Table 28 shows that the ratio of PV area to GIA exceeds 1 for CO2 when the heat demand exceeds around 65kWh/m²/yr, for primary energy this threshold is in excess of 145kWh/m²/yr. When this threshold is passed the area of PV needed is greater than can be accommodated on the roof of a single-storey building if the roof is covered completely. When these ratios exceed 0.5 the PV can no longer be accommodated on a two-storey building when the roof is completely covered. The current 2015 Section 6 notional building has PV up to a maximum of 50% of the roof area. It is recognised that in many cases it may not be viable to completely cover the roof area with PV, for example there may be rooflights or roof-top plant that reduces the available area. In the case of roof-top plant it may be possible to put PV on a frame above the plant but this will increase the cost and visibility of the PV. It is also possible to locate PV on areas other than the roof, for example façade-integrated PV or on a canopy over parking areas.

Table 28: Analysis of Area of PV required to equalise CO2 and primary energy performance of buildings with gas boilers (efficiency=91%) and ASHPs ( SEER=3.44)
Annual Heat Demand
(kWh/m²/yr)
Area of PV required to equalise total CO2
(m²PV/m²GIA)
Area of PV required to equalise total primary energy
(m²PV/m²GIA)
5 0.073 0.027
15 0.220 0.082
25 0.367 0.136
35 0.514 0.191
45 0.661 0.245
55 0.808 0.299
65 0.955 0.354
75 1.102 0.408
85 1.249 0.463
95 1.396 0.517
105 1.543 0.572
115 1.690 0.626
125 1.837 0.681
135 1.984 0.735
145 2.131 0.789

127. Figure 19 shows the distribution of the space heating demand for gas-heated buildings in the EPC database. This shows that the majority of gas-heated buildings have a space heating demand below 40kWh/m²/yr and less than 10% of buildings have a space heating demand greater than 80kWh/m²/yr.

Figure 19: Distribution of space heating demand of gas heated buildings in the EPC database
Graph illustrating the distribution of the space heating demand for gas-heated buildings in the EPC database. This shows that the majority of gas-heated buildings have a space heating demand below 40 kilowatt hours per metre squared per year and less than 10% of buildings have a space heating demand greater than 80 kilowatt hours per metre squared per year.

128. Figure 20 shows the distribution of the total of space heating and DHW demand for gas-heated buildings in the EPC database. This shows that around half of gas-heated buildings have a combined space heating and DHW demand below 80kWh/m²/yr and that the remaining buildings principally have a combined heat demand between 80 and 300kWh/m²/yr.

Figure 20: Distribution of total of space heating and DHW demand of gas heated buildings in the EPC database
Graph illustrating the distribution of the total of space heating and DHW demand for gas-heated buildings in the EPC database for both actual building and notional building. This shows that around half of gas-heated buildings have a combined space heating and DHW demand below 80 kilowatt hours per metre squared per year.

129. This analysis suggests that, with no other improvements, the majority of single-storey buildings with gas heating and PV would be able to improve upon the CO2 performance of a notional building using an ASHP for space heating only. Similarly, the majority of single-storey buildings with gas heating and PV may be able to improve upon the primary energy performance of a notional building using an ASHP for space heating and DHW if the ASHPs providing space heating and DHW can achieve this overall combined efficiency, for buildings where the DHW demand dominates this will be particularly challenging. However as more storeys are added to the building this route to compliance becomes increasingly challenging. Nevertheless, there are many other routes to achieving compliance such as improving energy efficiency and other low carbon technologies.

130. The amount of PV required to comply can be reduced by setting the notional ASHP to a lower SEER. The analysis above was based on the proposed notional ASHP SEER of 3.44 (see Table 8), Table 29 shows this the effect of changing the proposed notional ASHP SEER to 2.0[15]. It can be seen that whilst the amount of PV required to comply for primary energy is approximately halved, that needed to comply for CO2 is only reduced by around 22%; this is caused by the differences in gas and electricity factors for CO2 and primary energy.

Table 29: Analysis of Area of PV required to equalise CO2 and primary energy performance of buildings with gas boilers (efficiency=91%) and ASHPs ( SEER=2.0)
Annual Heat Demand
(kWh/m²/yr)
Area of PV required to equalise total CO2
(m²PV/m²GIA)
Area of PV required to equalise total primary energy
(m²PV/m²GIA)
5 0.060 0.016
15 0.181 0.047
25 0.302 0.078
35 0.423 0.110
45 0.544 0.141
55 0.665 0.172
65 0.786 0.204
75 0.907 0.235
85 1.028 0.266
95 1.149 0.298
105 1.270 0.329
115 1.391 0.360
125 1.512 0.392
135 1.633 0.423
145 1.754 0.454

131. The analysis above has considered the technical viability of using PV to achieve reductions in CO2 and primary energy. The analysis below considers the cost effectiveness of this approach as a means of reducing the carbon emissions of a building[16].

132. The capital cost data and modelling results for the analysis to support the English Part L consultation have been analysed to review the relative cost effectiveness of ASHPs and gas heating and PV. The result of this analysis is summarised in Table 30, this suggests that the capital cost effectiveness of ASHPs is greater than gas heating and PV in buildings with larger heat demands but lower in those buildings with lower heating demands. This is likely to be due to the ASHPs being better utilised in buildings with larger heating demands.

Table 30: Analysis of whether gas heating and PV or ASHP is more cost effective in terms of savings per pound of capital cost for a range of buildings modelled for the English Part L 2020 analysis
More cost-effective technology
(Gas+PV or ASHP)
Office – deep plan, air conditioned Hotel Hospital Secondary School Retail Warehouse Distribution Warehouse
Primary Energy Gas + PV ASHP Gas + PV Gas + PV Gas + PV ASHP
CO2 Gas + PV ASHP ASHP Gas + PV ASHP ASHP

133. The analysis above (paragraphs 126 to 130) compared the technical ability of gas heating with PV against an ASHP and identified that buildings with lower heat demands can more easily use PV to achieve the same CO2 and primary energy savings as an ASHP whilst those with higher heat demands may not have sufficient roof space to achieve this. The results in Table 30 align to this finding by suggesting that ASHPs are a more cost-effective improvement for buildings with higher heat demands (i.e. when they are also more technically viable) and that PV is more cost effective in buildings with lower heat demands (i.e. where PV is a viable means of achieving the same improvements), this is summarised in Table 31.

Table 31: Summary of comparison of PV and ASHP technical viability and cost-effectiveness
Criterion High heat demand Low heat demand
Technical viability Table 28, Table 29, Figure 19 and Figure 20 suggest that ASHP can generally achieve greater CO2 and primary energy savings than PV because PV is limited by the roof area available. Table 28, Table 29, Figure 19 and Figure 20 suggest that PV (combined with gas-boiler) is likely to be able to achieve similar CO2 and primary energy savings to those achieved by an ASHP.
Cost-effectiveness Table 30 suggests that ASHP is generally the more cost effective of these two technologies. Table 30 suggests that PV is generally the more cost effective of these two technologies.

134. The analysis above (paragraphs 126 to 130) has assessed the viability of using gas heating and PV to achieve the same savings as achieved by an ASHP used just for space heating or for space heating and DHW. A key factor in this analysis is the approach that the notional building takes to fuel selection and delivery losses in the DHW system. The English Part L consultation proposes that the DHW system in the notional building be determined by the fuel type and the magnitude of the DHW demand as set out in Table 32. This approach responds to the increase in renewables on the electricity grid and the resultant drop in primary energy and CO2 factors. This change in grid generation mix means that the use of electric point-of-use water heaters is no longer a high-carbon or high-primary-energy solution compared to natural gas. In buildings with low DHW demand the use of a centralised DHW system can result in higher primary energy and CO2 because of the relatively high losses from DHW storage and secondary circulation loops. In buildings with higher DHW demand the relative magnitude of these losses is reduced. Centralised DHW systems also facilitate the use of lower carbon heat sources such as waste heat or bio-fuels.

Table 32: Summary of DHW systems proposed in the Part L 2020 consultation for England
DHW Fuel in the Actual building Notional building DHW fuel for centralised systems DHW system type
High DHW demand activities Low DHW demand activities
Bio-fuel Bio-fuel Centralised Centralised
Dual fuel (mineral + wood) Dual fuel (mineral + wood)
Waste heat Waste heat
Natural gas Natural gas Electric point-of-use
LPG
Fuel oil
Non-electric heat pump
Other fuels whose emission factor > emission factor of natural gas
Electric heat pump Electric heat pump
Electricity (direct) NA Electric point-of-use

135. In light of this analysis it is suggested that the analysis consider the effects of setting the notional building to use an ASHP for space heating. The DHW system could adopt any of the following options:

  • Gas-fired;
  • Point-of-use electric;
  • Hybrid approach proposed for England;
  • Separate heat pumps for space heating and DHW.

136. The proposed approach is set out in Table 34 and Table 35. It is a simplified approach compared to England in that the notional building is based on two fuel types. In doing so, buildings using biomass, dual-fuel and waste heat will find it a little easier to comply as for each of these fuels the primary energy factor is slightly lower, by around 6-8%[17]. However, this is not considered such a large difference that it will drive a significance change in the use of heating fuels, and biomass and dual-fuel boilers are often a few percent less efficient than gas boilers which partly negates the benefit of having a lower primary energy factor. This may be further mitigated by enhancing the backstop limits on fabric and plant efficiency.

1.9 Selection of improved notional buildings

137. Scottish Government requested that up to three options (low/medium/high) be taken forwards for modelling. These should be informed by the findings set out above.

138. The potential specifications set out in Sections 1.7 have been used to create three options for 2021 standards. Table 33 summarises the options shortlisted for modelling and agreed with the client.

Table 33: Low, Medium and High specifications for modelling
Variable Sec 6 Notional Suggested Options
Heated & Naturally Ventilated Heated & Cooled or Heated & Mechanically Ventilated Low Medium High
Wall U-Value (W/m²K) 0.23 0.20 0.20 0.18 0.15
Roof U-Value (W/m²K) 0.18 0.16 0.16 0.15 0.11
Floor U-Value (W/m²K) 0.22 0.20 0.20 0.15 0.13
Window U-Value (W/m²K) 1.80 1.60 1.60 1.40 0.90
G-value 60% 50% 50% 29% 29%
Light Transmittance 71% 71% 71% 60% 60%
Rooflight U-Value (W/m²K) 1.80 1.80 1.80 1.50
G-value 52% 52% 52% 29%
Light Transmittance 57% 57% 57% 60%
Air Tightness 3, 5 or 7 1 5 4 3
Heating & Cooling Gas Boiler 91% 93%
ASHP 1.75 3.44 4.00 4.35
Radiant gas heater (top-lit) 86% 92%
Cooling SEER 4.50 5.50 6.40 7.10
Domestic Hot Water Gas Boiler 91% 93%
ASHP 1.75 2.50 2.50 2.70
Distribution NA (implicitly near point of use) See Table 34 and Table 35
Lighting & Ventilation Lighting Luminaire (llm/cW) 60 65 125
Daylight Lighting Control Single zone daylight dimming Single zone daylight dimming
Occupancy Lighting Control Manual on auto off Manual on auto off
Parasitic Power 0.3W/m² or 3% for daylight; 0.3W/m² for occupancy 0.1W/m²
Display Lighting (llm/cW) 22 125
Display Lighting Control none time switch
Ventilation Heat Recovery 70% 76%
Demand Control Ventilation gas-sensors, inverters gas-sensors, inverters
PV Area 2 (gas notional only), Lesser of: % of GIA 4.5% 6.5% 13.0%
% of roof area 50.0% 50.0% 50.0%

Notes:

1. Function of floor area and activity type (see Table 4 of NCM Modelling Guide).

2. Based on an assumed output of 120kWh/m² (approximately equivalent to a nominal efficiency of 16%).

139. Table 34 shows the proposed space heating fuels for the notional building mapping these against each space heating fuel that the actual building may use. Similarly, Table 35 shows the proposed domestic hot water (DHW) fuels for the notional building. Table 35 also shows the proposed DHW system types (centralised or point of use) that the notional building will use in different circumstances.

Table 34: Proposed space heating fuel for notional building
Actual building space heating fuel Notional building space heating fuel
Bio-fuel Natural gas
Dual fuel (mineral + wood)
Waste heat
Natural gas
LPG
Fuel oil
Non-electric heat pump
Other fuel & supplied heat
Electricity (direct)
Electric (heat pump) Electric (heat pump)
Table 35: Proposed domestic hot water ( DHW) fuel and system type for notional building
Actual building DHW fuel Notional building DHW fuel for centralised systems DHW system type
High DHW demand activities Low DHW demand activities
Bio-fuel Natural gas Centralised Electric point-of-use
Dual fuel (mineral + wood)
Waste heat
Natural gas
LPG
Fuel oil
Non-electric heat pump
Other fuel & supplied heat
Electricity (direct)
Electric (heat pump) Electric (heat pump)

Contact

Email: buildingstandards@gov.scot