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

6. Modelling approach

6.1 Archetype/geometry

The modelling has been undertaken using the archetypes described in Section 3. The dwelling information is presented in Table 1 of this report.

6.2 Specification

The modelling has been undertaken using the archetypes described in Section 4. A summary of the applied input values is presented in Appendix B.

6.3 Modelling packages

6.3.1 SAP 10.2

The latest SAP.exe, produced and shared by BRE for use by SG, was used to manually run each input file through SAP 10.2 via a command prompt interface, with results extracted using an automated Python script process developed by AECOM and BRE. The data used to inform the modelling was obtained from the archetype and specification data outlined above and followed the latest SAP conventions (version 10.0 incorporating amendments made in 27/01/2023 and 14/07/2023).

Models were run for the UK average climate data (as is the approach for Building Regulations calculations). This represents the weather conditions (ambient temperature, wind speeds and solar irradiance levels) for the East Pennines region. It is possible to run the SAP 10.2 calculations using the West of Scotland region; however, this would not represent the outputs expected for current compliance calculations.

6.3.2 PHPP v10.6

Passivhaus Planning Package v10.6 has been used produce energy demands and consumption rates. PHPP v10.6 is an excel based design tool produced by the Passivhaus Institute, in which all Passivhaus certified buildings must be modelled. In line with PHPP modelling conventions, the geometry was adjusted to external dimensions and greater detail was added to the window inputs based on the archetype drawings. The fabric and services inputs were modelled in line with HEM inputs; however, some conversion was required to align the inputs with the PHPP required units.

The typical PHPP convention for a block of flats would be to model the entire building. However, to align with the SAP and HEM modelling, PHPP models were created for each individual flat, including a top, mid and ground floor. Results have been aggregated to represent a whole block in the reporting below.

PHPP v10.6 Variants was used to model all scenarios. PHPP v10.6 Variants is an additional PHPP worksheet that allows simultaneous modelling of multiple design options.

The model was run with the UK Zone 15 -West of Scotland region, which applies the Glasgow Airport climate dataset, to best represent the average weather conditions for Scotland.

6.3.3 HEM v0.33 FHSv0.24

The Home Energy Model requires significantly more input information to support the modelling of a typical year of consumption. As this model required the most detailed information, inputs were derived for this model and then translated (i.e. simplified) for input into the other modelling packages. The key area where more input detail was required relate to the heating and hot water systems.

Data on the equivalent area of ventilators was required; this was aligned with requirements of Section 3.14 of the technical handbook to achieve the minimum ventilation levels.

The HEM also requires detailed information on the inverter characteristics for a given PV array. In all archetypes it was assumed that the PV array was connected to a string inverter which had sufficient DC-AC capacity to satisfy the maximum array output power. This avoided the instances of inverter clipping which would not be accounted for in the other models.

The model was run with the CIBSE Test Reference Year (TRY) for the Glasgow region in 2020, under the ‘High’ warming scenario with 50^th percentile likelihood to best represent the average weather conditions for Scotland.

6.3.4 Summary of model differences

A technical summary of the methodological differences between SAP, PHPP and HEM has been provided in Appendix C.

6.4 Limitations

Following discussions between AECOM and SG on the 18/10/2024, SG advised that the HEM v0.33 FHSv0.24 should be used for the purposes of calculating energy consumption rates that are to be compared with PHPP and SAP 10.2. Although the version of HEM applied is not the final version that is to be adopted alongside the updates to the Domestic Technical Handbook, it has been used here to demonstrate the potential differences in the impact of the scenarios being considered by the SG for future updates to the Section 6 notional performance specification.

Where AECOM has modelled identical dwelling information in each of the three modelling packages, there have been several areas which cannot be replicated identically, this has frequently been down to the level of input information required in each model. For example, SAP 10.2 does not account for nearby shading impacts on opening. As the SAP 10.2 cannot accept geometric data on shading structures such as reveals or balconies, it applies a static default assumption for all windows. Both PHPP and HEM have been modelled with the actual geometric data to give the closest representation of the actual dwelling in a real-world scenario.

Each modelling package required a unique format of weather data to be applied. A single consistent weather file could not be obtained for modelling in each package. As such, most aligned weather files have been used from each package; however, given the large range in variability of weather data these similar files still have significant differences. Despite this, each model can be considered as appropriate for a dwelling in the Glasgow region of Scotland.

In the case of PHPP, the modelling conventions have not been fully applied for this study. This is to allow for the comparison of single dwellings within a communal block. The typical PHPP convention would be model the entire building; however, PHPP models have been undertaken for single flats within the building to estimate the dwelling level energy consumption rates. As such, aspects such as building level distribution system losses, additional heat loss from any communal areas and the accounting of unheated spaces within the building have all been excluded. This is considered closer to the SAP and HEM modelling conventions. When considering the delivered energy rate from PHPP, the self-usage rates for solar energy generation have been postprocessed. This is due to PHPP not calculating the self-usage rates within the model. Instead, the predicted self-utilisation rate for the PV generation has been taken from the equivalent HEM model and applied to the PHPP model outputs when calculating delivered energy.

The Good and Best Practice specifications make improvements to the fabric energy efficiency, ventilation system and DHW system, which are expected to lower the dwelling Space Heat Demand. The DE and DHN technology routes are not expected to be overly sensitive as the system capacity is not expected to significantly impact operational efficiency (i.e. a lower heat demand will require fewer emitters, but the heat generator efficiency will be broadly unchanged). In the case of the ASHP, it is understood that operating efficiency is sensitivity to system size. As such, Good and Best Practice specifications may reduce the heating demand below the efficient operating range of the heat pump. (i.e. result in greater heat pump cycling and lower SCOPs). Due to the complexity of resizing the heating systems, the ASHP and emitter circuit properties have not been changed based on specification level. Therefore, there may be further CAPEX and CIA benefits to those reported in section 5 & 6, which if accounted for would improve the benefits of ASHP twinned with the Good and Best Practice specifications.

6.5 Cost information

Currie and Brown has provided cost rates for the specification options presented in Section 4. As the HEM modelling has the most detail, this has been used as the basis for costing. CAPEX rates have been obtained from robust sources and exclude VAT. Where based on nationally recognised, rates a regional factor specific for Scotland has been applied. Assumptions for site specific costs, such as labour, have been adjusted to represent the typical hourly rates for Scotland.

The rates provided represent the central cost information for developments in the central belt sources.

Regulated fuel costs have been calculated using the model energy consumption rates. The fuel consumption has been multiplied by the relevant domestic central estimate cost rate for 2025, sourced from the HM Treasury Green Book (Tables 4-8). The regulated fuel costs consist of the variable fuel tariff only and excluded the standing changes associated with the fuel mix for each heating technology. As such, the additional operational costs associated with having a metered DHN connection in addition to the electrical connection has not been captured. In the absence of Green Book rates, other sources have been used. The price of heat from the BAU DHN case has been assumed as £0.12/kWh of delivered energy. This is anticipated to be a reasonable 2025 cost rate for a network with 90% ASHP generated heat. The SAP 10.2 cost of heat given in Table 12 of the SAP document (£0.0444/kWh) is not considered to be realistic. The SAP 10.2 rate for exported electricity (£0.0559/kWh) has been applied. Current electrical export rates may be higher than this; however, the availability of such tariffs may vary across Scotland. As such, the SAP 10.2 figure have been used to represent a possible average.

For the costing of DHN, an assumption has been made for the typical dwelling connection charge. This connection charge may be highly variable and may not be cost optimal when compared to in-situ heating solutions. It is anticipated that connections to DHNs for new dwellings will be driven by wider regulatory requirements regardless of the cost effectiveness. It is likely that, in most cases, the connection charge would be equal to the counterfactual cost of the dwelling providing its own heat source.

6.6 Cost Impact Assessment

Quantification of the national costs and benefits of adopting different performance standards has been calculated in line with the Green Book supplementary guidance: valuation of energy use and greenhouse gas emissions for appraisal. The following information sources were used to conduct the analysis:

• Model period – new home construction for 10 years from 2026 with 60 years of operational, maintenance and component replacement impacts after construction.
• Build rate and housing mix – based on average (central assumption), maximum (high assumption) and minimum (low assumption) number of completions between 2020-2023 by calendar year.
• Housing types – Detached, End-Terrace, Mid-Terrace and low-rise Block of Flats were assessed. The archetypes represent 93% of new build dwellings based on EPC completions data in the following proportions: Detached (31%), End-Terrace (23%), Mid-Terrace (8%) and low-rise Block of Flats (31%), with the remaining 7% of development being bungalows, maisonettes, duplexes or other types of dwelling.
• For each archetype, Good and Best Practice specifications were applied for heating provided by ASHP, DE or DHN. The heating technology split has also been determined based on EPC completions data, appropriately allocating alternative heating technologies for those which do not already demonstrate compliance with the New Build Heat Standard. The proportion of heating type varies between the archetypes, with the overall split as follows: ASHP (67.7%), DE (14.5%), and DHN (17.8%).
• Energy consumption, specifications and quantities for each policy option – energy consumption and generation by type (electricity and heat) as calculated using HEMv0.33FHSv0.24, PHPPv10.6 or SAP10.2 modelling methodologies.
• Specifications and quantities for each policy option – elemental construction details for building envelop, services and performance (e.g. airtightness levels) with elemental quantities for each building type and specification option
• Construction costs in 2025 and over time – capital costs based on analysis by Currie & Brown’s cost consultants and reflecting cost levels in Scotland (central belt) in Q4 2024. Projected changes in the real costs of different technologies are estimated using projections published by the International Energy Agency and International Renewable Energy Agency.
• Maintenance and replacement costs – based on component lifespans published by CIBSE and BCIS with maintenance costs estimated for those items varying between energy options only.
• Energy prices – based on retail energy prices and variable cost projections published by DESNZ in 2024. Costs of heat supply estimated at 12p per kWh and then adjusted based on changes in costs of gas and electricity based on a heat network supply mix of 90% ASHP and 10% gas boilers.
• Carbon emissions from energy use – emission factors and projections published by DESNZ in 2024. Emissions from heat networks based on a supply mix of 90% ASHP and 10% gas boiler.
• Value of carbon emissions and air quality impacts – values and projections published by DESNZ in 2024
• Rate of transition from current to future policy scenarios – assumed 20% per year uptake of new policy standards in each year following introduction of new regulations.

A bespoke cost benefit analysis model was applied to estimate the capital, operational and replacement costs of homes built to differing standards over a 10-year policy assessment period and for 60 years of use from the year of construction. The model calculates energy costs and associated carbon emissions, emission costs and costs associated with impacts on air quality. The net costs and benefits of a specific policy option are estimated by comparison to a counterfactual scenario that assumes no change in current regulations. Two options for changes to regulations were assessed based on migration to either a good or a best practice standard.