Energy Performance of Buildings (Scotland) Regulations 2025: stock model research

6 HEM Results

This section is intended to provide some explanation of the various metrics that have been calculated using the calculation methodology described in the previous section.

6.1 HEM wrapper outputs

The table below outlines the ratings and metrics produced by the EPC wrapper through the analysis, noting that at time of writing the wrapper and the outputs being produced is still under development and being finalised.

Metric

Heat Retention Rating

Units

kWh/m²/year

Description

Represents annual space heating and cooling demand.

Metric

Energy Use Intensity

Units

kWh/m²/year

Description

Represents the total energy used by the property, including self-generation. Includes unregulated energy uses such as appliances.

Metric

Energy Cost Rating

Units

£/m²/year

Description

Represents the cost of the regulated energy use of the property.

Metric

SAP 10.2 Energy Cost Rating

Units

Dimensionless

Description

Follows the same methodology and tariffs used to calculate the RdSAP 10.2 SAP rating, but using fuel consumption as calculated by HEM.

Metric

RdSAP 10 Energy Cost Rating

Units

Dimensionless

Description

As above, but following the RdSAP 10 methodology, and using RdSAP 10 fuel tariffs, for comparison with RdSAP metrics.

Metric

Space Heating Efficiency Sub-Rating

Units

%

Description

The percentage efficiency of the heating system. Used to derive a band from A (best) to G (worst).

Metric

Space Heating Cost Sub-Rating

Units

£/kWh heat delivered

Description

The cost per unit of heat delivered by the heating system. Used to derive a 0-100 rating, and a band from A (best) to G (worst).

Metric

Space Heating Emissions Sub-Rating

Units

kg CO2e/kWh heat delivered

Description

The Scope 1 CO2e emissions per unit of heat delivered by the heating system. Used to derive a 0-100 rating, and a band from A (best) to G (worst).

Metric

Space Heating Combined Sub-Rating

Units

Dimensionless

Description

A combination of the cost and emissions ratings, weighted as 70% of the emissions rating and 30% of the cost rating. Used to derive a band from A (best) to G (worst).

6.2 RdSAP outputs

The Cotality engine has been used to produce RdSAP 9.94 and RdSAP 10 outputs. Since September 2019, RdSAP 9.94 was the calculation methodology that was approved by all devolved administrations. It was in use over much of the period of policies such as the UK Government’s Minimum Energy Efficiency Standards (MEES). Since 15 June 2025, RdSAP 10 has been the approved methodology for EPCs and has replaced RdSAP 9.94. Both versions are available in the accompanying results spreadsheet.

6.3 Heat Retention Rating

This analysis aims to find a threshold that maximises equivalence between the current SAP rating bands and proposed HRR bands, thereby minimising disruption for homeowners, landlords, and policymakers. In line with this aim this section investigates these thresholds and in particular a threshold HRR level between HRR bands C and D that will result in at least 80% of properties with SAP rating band C when calculated with RdSAP 9.94 remaining in HRR band C or better.

As noted in the methodology, we were able to calculate a HRR for the representative three-year SHCS dataset. The results in this section have been weighted appropriately and therefore may be taken as representative of Scotland’s housing stock.

Outlined in Table 3 below are the percentages of properties that are currently rated SAP C from both the SHCS dataset and Cotality’s assessments of SAP rating, that would retain a C or better rating if the HRR boundary was drawn at specific kWh/m²/year thresholds. Green highlighted results represent boundaries where over 80% of properties rated SAP C retain a C and above HRR rating.

With the threshold between C/D set at 159/160 kWh/m²/year, the remaining bands were then proposed by Scottish Government. Based on these proposed thresholds for the remaining bands, Table 4 below presents the estimated number of homes per HRR band in Scotland based on the analysis dataset.

Given the <= 159 kWh/m2/year C threshold, Tables 5 shows that the proportion of stock in band C or better, and in D or worse, remains approximately the same for both SAP 9.94 and HEM ratings.

By comparing the percentage of total homes in each HRR band against the proportions in the equivalent SAP band, we see that the overall proportion of properties in each band is slightly smoothed, leading to more properties in the higher proposed HEM bands A and B, and fewer properties in C. This will likely improve the granularity within each band and allow for more nuanced comparison of properties within a band than under SAP. This comparison is presented in Figure 2 overleaf.

6.3.1 Analysis of EPC band movement with the 159 kWh/m²/year threshold

Using the bands proposed in Section 6.3, we also looked at what proportion of properties in each band shifted bands. We analysed for the shifts in both the original bands, the spread of new rating based on original band, and for the new band, the proportion of each new band that had previously been from the same or different bands.

First, we have displayed in Table 6 below the proportion of new HRR bands into which each of the original SAP band properties fall. The top row of the table shows what percentage of SAP 9.94 A properties will fall into the HEM HRR bands A, B, C, and so on.

From this table we see, as expected, a broad trend along the diagonal. This shows that the bulk of properties don’t change their score drastically between bands. There are a few points of note that diverge from the general trend slightly. The proportion of SAP E rated properties now calculated as HRR G is about three times that of those dropping from E to F. These will likely be properties with very poor fabric, but with a boost to their SAP rating from having an efficient, relatively cheap to run heating system, or a PV system.

Another area of note is that the majority of SAP A rated properties shift to a B, and over 90 are no longer A. This is because all of these properties have a PV system that is beneficial to the SAP rating but has no impact on the HRR score. As these properties have good or fair fabric still, but do not benefit from their PV, they slip noticeably in the new proposed HRR bands.

By contrast, a large proportion of properties currently rated B under SAP 9.94 jump up to an A. These are properties that have good fabric performance but don’t achieve an A under SAP, likely due to smaller or no PV systems, or slightly less efficient heating systems.

To further analyse the shifts in score, we also mapped based on the makeup of each of the new bands to see what proportion came from previously lower and higher bands. This is presented in Table 7 below.

The flattening effect mentioned earlier can be clearly seen in the proportion of A and B; and E, F and G properties; as rated by their proposed HRR band. 81% of all the HRR B rated properties were rated SAP C, and likewise for both E and F over 60% of these were previously D rated.

Tables 8A-B: proportions of each property type that achieve an HRR band of C or better, compared to their rating under RdSAP 9.94

Looking into the proportions of property types that meet the HRR threshold of <= 159 kWh/m²/year for a C or better rating, as with SAP, we find that flats are over-represented as they generally have a lower heat demand relative to houses.

6.3.2 Alternative rationale for Heat Retention Rating banding

An alternative to maximising equivalence for C or better properties could be considered, that maintains a similar proportionality between bands as to the current RdSAP 9.94 scores. These thresholds are defined in Table 9.