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

1. Executive Summary

AECOM and Currie and Brown have undertaken a technical, capital expenditure and a cost impact assessment on a range of options for new build dwelling notional performance standards to inform Scottish Government (SG) updates to Section 6 of the Building Standards Domestic Technical Handbook.

The technical analysis has been undertaken to consider the implications of modelling methodology, dwelling specification and heating technology upon key regulated metrics and costs to both the developer and occupier, to represent the emerging building stock in Scotland. The modelling packages, heating technologies and performance specifications applied include:

• Modelling Packages: Standard Assessment Procedure (SAP) 10.2, Home Energy Model (HEM) v0.33 with Future Homes Standard Wrapper (FHS) v0.24, Passivhaus Planning Package (PHPP) v10.6
• Heating Technologies: Air source heat pump (ASHP), Direct electric heating (DE), District heating network connection (DHN)
• Performance Specifications: Business as Usual (BAU) performance specification, Good practice performance specification, Best practice performance specification

The capital expenditure (CAPEX) has been estimated from the design information of each option considered by the technical analysis. The estimates represent the typical rates for Scotland and have been used to inform cost effectiveness of specification changes by comparing the CAPEX against the improvement on key and supporting metrics for regulations calculations.

The national costs and benefits of adopting the different performance standards has been calculated and 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 findings of this work have been summarised below:

In response to Alex Rowley MSP's Proposed Domestic Building Environmental Standards (Scotland) Bill, in December 2022, the then Minister for Zero Carbon Buildings, Active Travel and Tenants' Rights confirmed that the Scottish Government would make legislation by December 2024 to deliver ‘a Scottish equivalent to the Passivhaus standard.’

When referencing the Passivhaus standard, the Space Heat Demand is most often considered. However, this forms part of a range of targets which are required to achieve certification. This includes Primary Energy or Primary Energy Renewable (kWh/m²/yr), a representation like the Dwelling Delivered Energy Rate (DDER) (kWh/m²/yr) applied by current regulations. Furthermore, absolute targets are not used by Section 6 of the building regulations; instead, targets calculated from a notional dwelling performance standard are used to assess compliance.

The results highlight the need to consider multiple compliance metrics to ensure that new build dwellings have a balanced specification which reduces fuel consumption and is cost efficient both to construct and operate.

When considering Space Heat Demand alone, the DE technology route appears comparative to the ASHP and DHN technology routes; however, has a significantly different DDER and Regulated Fuel Cost. It was also noted that tariff variability, which may be challenging to account for in Building Regulations compliance calculations, could significantly impact actual dwelling fuel bills. If a target SH demand is applied in combination with a target delivered energy rate (TDER), it is expected that mitigation of high regulated fuel costs is possible. This will essentially eliminate the option to apply DE as a heating technology within a dwelling with a high SH demand. However, it was found that applying a target SH demand for dwellings with an ASHP may not be required to achieve a low DDER.

The ASHP was shown to consistently result in the lowest DDER across the archetypes modelled. The DE and DHN were shown to result in a similar DDER across the archetypes modelled; however, the DE had a higher associated regulated fuel cost.

ASHP BAU has the lowest CAPEX for the houses. This relationship was not observed for the block of flats where the ASHP was the highest for the BAU specification.

Good and Best Practice specifications achieve improvements to both SH demand and DDER for all archetypes. The Best Practice specification was also found to achieve the SH demand target of 15kWh/m²/yr in PHPP for half of the archetypes assessed (representing circa 40% of the new build housing stock). The CAPEX effectiveness of the Good Practice option was found to be highest; however, it does not achieve the aspiration of aligning with the SH standard from Passivhaus.

The Cost Impact Assessment indicates that neither the good nor best practice standards deliver a net benefit in comparison to the current regulations when modelled with HEM or SAP. Using the PHPP energy model, the good practice standard achieves a £1million net benefit but there is no net benefit for the best practice standard. The carbon and air quality benefits of the considered options are exclusively those linked to energy efficiency measures or additional renewable energy generation. Because the energy being saved is already low carbon the value of these savings is proportionately smaller than might be the case where the policy options entail a switch in technology to heat pumps or other low carbon sources.

SG are encouraged to consider the findings of this study and the conclusions drawn when appraising the final options for updating Section 6 of the Domestic Technical Handbook.