When DSM Modelling Is Needed for Commercial EPCs — Atriums, Glazed Cores and Better Ratings

DSM is not automatically mandatory, but Building Regulations Part L and the National Calculation Methodology require it where SBEM cannot adequately model the building's physics. The most common triggers are large, unconditioned or partially conditioned atriums, extensive glazed circulation cores, complex shading, natural ventilation with mixed-mode operation, significant thermal mass exposed to the occupied space, and buildings where the actual operating hours or internal gains differ materially from the SBEM defaults.

Atriums are the classic example. A six-storey office with a full-height glazed atrium is not three separate rectangular zones. Solar gain stacks through the atrium, buoyancy-driven airflow moves heat vertically, and the boundary between conditioned and unconditioned space is blurred. SBEM cannot capture that dynamic behaviour, so it tends to overestimate cooling and heating loads. DSM models the atrium as it actually behaves, which usually shows lower annual energy use and a better rating.

Glazed cores create the same problem on a smaller scale. A building with extensive glass entrance halls, lift lobbies and connecting corridors has large solar gains and high heat loss in winter. Without DSM, those spaces are treated as standard conditioned zones, producing a distorted load profile. DSM can represent buffer spaces, vestibules, winter gardens and solar-controlled glazing accurately.

The EPC rating is a comparison between the modelled carbon emissions of the actual building and a hypothetical benchmark building of the same size and type. Any input that overstates real energy consumption pushes the rating down. SBEM's conservative defaults do exactly that in complex buildings: they assume standard glazing, standard HVAC efficiency and standard controls even when the real design is far better.

DSM lets the model reflect what is actually built. High-performance glazing with selective coatings, external shading, daylight-linked lighting controls, exposed thermal mass with night purging, demand-controlled ventilation and efficient heat recovery can all be represented in detail. When the model is set up correctly, the result is a more accurate — and often substantially better — EPC rating.

We have seen ratings improve by one or two full bands when a building moves from SBEM to DSM. The gain is not from bending the rules; it comes from showing that the as-designed building genuinely uses less energy than the conservative default assumptions imply. For landlords and developers facing MEES deadlines or lender requirements, that band improvement can protect asset value and avoid unnecessary retrofit spend.

A DSM model is not just a compliance tool. Because it simulates hour-by-hour behaviour, it reveals how the building will perform in practice. Design teams can test overheating risk, identify glazing or shading problems, size plant more accurately and compare the capital cost of different facade and HVAC strategies. This reduces the risk of oversized chillers, underperforming natural ventilation and tenant comfort complaints after handover.

For developers, the model supports optioneering early in design. For landlords, it provides a baseline for operational energy reporting. For funders and valuers, it demonstrates that the building was designed and tested against a realistic energy model rather than a blunt default.

DSM also produces BRUKL outputs and EPCs that are accepted by building control and lodged on the Non-Domestic Energy Performance Certificate register. The same model can be reused for BREEAM energy assessments, overheating analysis and post-occupancy evaluation, so the investment in modelling pays off across multiple workstreams.

Not every DSM model is better than SBEM by default. The benefit depends on the quality of the inputs and the skill of the modeller. Common errors include using conservative default schedules when the building will operate differently, ignoring exposed thermal mass, failing to model realistic daylight dimming controls, and setting glazing properties that do not match the actual specification.

Another mistake is treating DSM as a bolt-on after the design is fixed. The biggest gains come when the modeller is involved early enough to influence glazing ratios, atrium geometry, shading depth and plant selection. A late-stage DSM study can still improve the EPC, but it cannot redesign the facade.

Finally, some providers run DSM with a deliberately cautious model, which simply reproduces the SBEM result. That defeats the purpose. The value of DSM is that it can show the real, optimised performance of the building — provided the model is built and reviewed by people who understand the physics and the compliance rules.

At Oak Tree Rule, our SBEM and DSM Part L service combines accredited energy assessors with building physics experience. We use DSM where it adds value, not as a default upsell. For every project we first check whether SBEM is sufficient; if the building has atriums, glazed cores, complex shading or high thermal mass, we recommend DSM and explain the likely rating impact before starting work.

We model in IES Virtual Environment and DesignBuilder, produce BRUKL and EPC outputs, and work alongside architects, MEP engineers and contractors to make sure the model reflects the design that is actually going to be built. If the rating is not where it needs to be, we advise on targeted changes — glazing specification, shading, lighting controls, ventilation strategy — rather than expensive bolt-on plant.

If you are designing or refurbishing a building with atriums, glazed cores or unusual geometry, get in touch. A correctly prepared DSM model can give you a much better EPC rating and a building that actually performs as intended.