iSBEM Updates for 2026

The 2026 edition of iSBEM is expected to be the most significant update to the government's free SBEM interface since the introduction of primary energy as a compliance metric in v6.x (Part L 2021). Aligned with the Future Buildings Standard and AD L2 2026, it is expected to introduce updated carbon and primary energy factors, new HVAC system types, expanded renewable energy inputs, and changes to reflect the 2026 NCM Modelling Guide.

Heat pump seasonal efficiency tool

A key expected feature is an integrated tool for calculating heat pump seasonal efficiency. In the current iSBEM v6.1.e, assessors enter heating system seasonal efficiency as a ratio in the Building Services form (iSBEM User Guide v6.1.e, Compliance volume, HVAC Systems tab). The 2026 edition is expected to replace this with a more detailed calculator that derives SCOP (seasonal coefficient of performance) and SEER (seasonal energy efficiency ratio) from climate data and part-load performance curves.

The tool is expected to account for:

• Part-load performance curves - how the heat pump performs at different fractions of its rated capacity
• Climate-adjusted operating hours - the proportion of annual hours at each outdoor temperature bin, using standard NCM weather data
• Defrost losses - energy consumed by defrost cycles on air-source heat pumps at low ambient temperatures
• Back-up heating contribution - where a secondary heat source (typically electric resistance) supplements the heat pump below a bivalent point. The current iSBEM already supports bivalent systems via the Bi-valent Systems sub-tab (iSBEM User Guide v6.1.e, Basics volume, Building Services form)

Multiple lighting inputs per zone

The current iSBEM v6.1.e allows a single lighting specification per zone - assessors select a lamp type or enter a luminous efficacy and light output ratio (iSBEM User Guide v6.1.e, Basics volume, Lighting sub-tab). The 2026 edition is expected to permit multiple lighting types within each zone, enabling more accurate representation of:

• Task lighting at different efficacy to ambient lighting
• Emergency lighting with parasitic loads modelled separately
• Display lighting alongside general lighting in retail zones (the current v6.1.e already supports display lighting as a separate input for retail activities)
• Separately controlled circuits within one space (e.g., perimeter daylight-dimmed vs core time-switched)

This would be a significant improvement for buildings with high-performance primary lighting and lower-efficacy secondary lighting. Under the current single-input approach, the blended average penalises the overall zone result. With multiple inputs, each lighting type would be modelled on its own merits, and the calculation would reflect the actual installed mix.

Under the current NCM 2021, the notional building assumes 95 luminaire lumens per circuit-watt with occupancy sensing (manual-on-auto-off) and photoelectric daylight dimming in all appropriate zones (NCM 2021, paras 71 and 73–76). The multiple-input feature would make it easier to demonstrate that the actual building meets or exceeds this benchmark.

Updated carbon and primary energy factors

The 2026 edition will use updated fuel emission factors and primary energy factors reflecting the continued decarbonisation of the electricity grid. The Part L 2021 NCM already uses monthly grid electricity CO₂ factors ranging from 0.111 to 0.163 kgCO₂/kWh (NCM 2021, Table 30). The 2026 NCM is expected to reduce these further. By contrast, natural gas remains at 0.210 kgCO₂/kWh (NCM 2021, Table 29).

Gas and oil factors are not expected to change significantly because fossil fuels do not benefit from grid decarbonisation. The net effect is that electric heating systems (especially heat pumps) will perform even better in the 2026 calculation, while gas boilers will be further penalised. This updated factor set is a primary mechanism driving the shift to low-carbon heating under the FBS.

New HVAC system types

The 2026 NCM activity and system databases are expected to include new and updated HVAC system types reflecting current technology. The current iSBEM v6.1.e already supports a wide range of system types including VRF/VRV, split/multi-split DX, chilled beams, and bivalent heating (iSBEM User Guide v6.1.e, Basics volume, Building Services form). Expected additions include:

• Ambient loop heat pump systems – communal ambient loops with individual zone heat pumps, increasingly common in mixed-use developments
• Heat network connections – standardised inputs for buildings connected to district heating/cooling networks, with a compliance route based on the network's declared carbon intensity
• VRF with heat recovery – improved modelling of simultaneous heating and cooling in variable refrigerant flow systems, crediting the energy recovered when one zone needs heating while another needs cooling
• Hybrid heat pump systems – heat pump with gas boiler backup, modelled with a defined heat pump fraction that reflects the proportion of annual heating delivered by each source

These additions mean that fewer projects need to resort to Dynamic Simulation Modelling purely because iSBEM cannot represent their HVAC configuration.

Expanded renewable energy inputs

The renewables section is expected to be substantially enhanced. The current iSBEM v6.1.e already supports PV, solar thermal, wind generators, and CHP (iSBEM User Guide v6.1.e, Basics volume, Building Services form). Expected additions include:

• Battery storage – new inputs for on-site electricity storage, allowing assessors to model how stored PV generation offsets grid demand during non-generation hours
• Solar PV improvements – more granular overshading categories, support for building-integrated PV (BIPV), and east/west split array modelling
• Updated CHP parameters – revised default efficiencies and load profiles reflecting current combined heat and power technology

Lifts, escalators, and moving footways

Following the amended definition of fixed building services in SI 2026/335, the 2026 iSBEM is expected to include inputs for vertical transportation for the first time:

• Lifts – number, speed, usage profile, and whether regenerative drives are fitted
• Escalators and moving footways – length, usage hours, and standby mode capability

These are new regulated energy loads that were not captured in previous editions. For buildings with significant vertical transportation – large retail, multi-storey offices, transport hubs – this can materially affect the BER and BPER results.

Fabric and thermal bridging improvements

The 2026 iSBEM is expected to include an improved thermal bridging interface. The current iSBEM v6.1.e handles thermal bridges at three levels - global defaults, zone-level overrides, and individual envelope/window psi-values (iSBEM User Guide v6.1.e, Basics volume, Thermal Bridges sub-tab). The notional building fabric values will also be updated to align with the FBS specification - the 2026 NCM will define tighter U-values that set a higher bar for compliance targets.

Using calculated psi-values rather than the conservative defaults (which are degraded by the greater of 0.04 W/mK or 50% from BRE IP 1/06 values per NCM 2021, Table 11) can significantly reduce fabric heat loss - often the difference between a marginal pass and a comfortable compliance margin.

Workflow guidance for transitioning assessors

If you are transitioning from the Part L 2021 version of iSBEM (v6.1.e) to the 2026 edition, the key points to bear in mind are:

1. Check the version number - ensure you are running the FBS-aligned version, not the Part L 2021 v6.1.e. Opening an older project file in the new version may require re-entering some data; always re-run the calculation after upgrading (iSBEM User Guide v6.1.e, Compliance volume, Common errors).
2. Update your construction libraries - the NCM database (construction library, activity database) is updated independently of the software. Ensure both are current from uk-ncm.org.uk.
3. Use any new heat pump tool - if the 2026 edition includes an integrated seasonal efficiency calculator, use it rather than manually entering efficiency values.
4. Model lighting properly - take advantage of any new multiple lighting inputs per zone rather than calculating blended averages.
5. Expect tighter targets - the updated notional building specification and carbon factors mean buildings that passed under Part L 2021 may no longer comply. Review the BRUKL under FBS guidance for practical detail on how the compliance bar has moved.
6. Account for lifts and escalators - these are expected to be regulated energy loads under the FBS. Gather lift specifications (speed, drive type, usage) before starting the model.
7. Save frequently - iSBEM saves projects as .nct files and has no auto-save. Use descriptive file names including project name, version, and date (iSBEM User Guide v6.1.e, Basics volume, General form).