Dynamic Simulation Modelling

Dynamic Simulation Modelling (DSM) is the advanced alternative to SBEM for demonstrating Part L compliance on non-domestic buildings. Where SBEM uses a simplified monthly calculation, DSM tools perform hourly or sub-hourly thermal simulation - modelling heat transfer, solar gains, airflow, and system behaviour in far greater detail (AD L2 2026; NCM 2021, Appendix C, para 209).

When to use DSM instead of SBEM

SBEM is suitable for most straightforward non-domestic buildings – standard offices, retail units, schools with conventional HVAC. DSM becomes necessary or advantageous when the building's design goes beyond what SBEM's monthly method can represent (NCM NCM 2021, Appendix C, para 209):

• Complex HVAC systems – displacement ventilation, chilled beams, mixed-mode ventilation, bespoke configurations not well represented by SBEM's system type list
• Advanced daylighting strategies – light shelves, atria, complex glazing configurations where accurate daylight modelling significantly reduces artificial lighting energy
• Thermal mass effects – exposed concrete, night cooling strategies, phase-change materials where heat storage and release over 24-hour cycles affect energy demand
• Complex building forms – atria, double-skin facades, unusual geometries that are difficult to represent as rectangular blocks in iSBEM
• Natural ventilation design – wind-driven and buoyancy-driven airflow strategies with automatic window controls
• Performance beyond minimum compliance – where the client or design team wants to demonstrate energy performance beyond the Part L target

What DSM models that SBEM cannot

The fundamental difference is temporal resolution. SBEM calculates energy balances on a monthly basis using quasi-steady-state methods based on EN ISO 13790. DSM tools calculate on an hourly or sub-hourly basis, capturing dynamic effects that monthly averaging obscures (SBEM Technical Manual v6.1.e, Section 4.1):

Approved DSM software

DSM tools must be approved under the Secretary of State's Notice of Approval, published on GOV.UK. Only approved software can produce valid BRUKL compliance reports. The current approved tools include (AD L2 2026, Section on DSM; NCM 2021, Appendix C, para 209):

The full list of approved software is maintained by MHCLG and published on GOV.UK. Only the specific version listed in the Notice of Approval is valid – updates and patches must themselves be approved.

Level 5 NDEA requirement

DSM assessments must be carried out by a Level 5 Non-Domestic Energy Assessor (NDEA) registered with an accreditation scheme. This is the highest NDEA qualification level, reflecting the technical complexity of dynamic simulation work:

• Level 3 – simple buildings with standard HVAC (iSBEM only)
• Level 4 – complex buildings using SBEM/iSBEM
• Level 5 – complex buildings using Dynamic Simulation Modelling

A Level 5 NDEA needs competence not only in building physics and Part L regulations, but also in the specific DSM software being used, 3D geometry construction, HVAC system modelling, daylighting analysis, and results interpretation. The additional expertise requirement is one reason DSM carries higher professional fees than SBEM work.

Cost and time implications

DSM assessments typically cost significantly more than SBEM assessments and take longer to complete. The key cost drivers are:

• Software licence fees – commercial DSM tools carry annual licence costs (typically several thousand pounds), whereas iSBEM is free
• Modelling time – building a detailed 3D model with accurate geometry, system definitions, and control strategies takes considerably longer than an iSBEM zone-by-zone entry
• Specialist expertise – Level 5 NDEAs command higher fees than Level 3 or 4 assessors
• Iteration cycles – design teams often use DSM for multiple option comparisons and design optimisation, adding to the total modelling effort

For straightforward buildings – standard offices, retail units, schools with conventional HVAC – SBEM is almost always the more cost-effective route. DSM becomes worthwhile when the building design genuinely cannot be represented in SBEM, or when the additional modelling detail can demonstrate compliance that SBEM would miss.

When SBEM and DSM results differ

Because the tools use different calculation engines, they can produce different BER/BPER values for the same building. However (NCM NCM 2021, Appendix C, para 209):

• The TER/TPER should be very similar because the notional building is well-defined and simple
• The BER/BPER may differ because DSMs capture effects that SBEM's monthly method cannot – thermal mass reducing cooling demand, complex shading, detailed system behaviour
• Neither tool is inherently more “generous” – DSMs may show benefits SBEM misses (thermal mass, daylighting credit) but may also reveal problems SBEM averages away (peak overheating, zone-level inefficiencies)
• A building that fails in SBEM may pass in DSM (or vice versa), depending on the design features

This makes the choice of tool a strategic decision. If a building is marginal in SBEM and has features that DSM models more favourably (good thermal mass, effective daylighting, natural ventilation), a DSM assessment may unlock compliance. Conversely, DSM may expose problems in buildings that look acceptable under SBEM's simplified treatment.

Practical guidance for project teams

1. Start with the question: does this building have features that SBEM cannot adequately represent? If the answer is no, use SBEM.
2. Brief the DSM assessor early. DSM modelling takes time – engage the Level 5 NDEA at RIBA Stage 2 or 3, not at Stage 4 when the design is fixed.
3. Provide complete information. DSM requires detailed geometry (3D drawings), full M&E specifications, HVAC control strategies, and glazing performance data.
4. Use DSM for design optimisation, not just compliance. The hourly results allow the design team to understand energy behaviour, identify peak demands, and test alternative strategies.
5. Budget appropriately. DSM fees reflect the additional skill and time required. Cutting the modelling budget leads to simplified models that lose the advantages of dynamic simulation.