A BB101-2018 Assessment is a formal, performance-led evaluation of ventilation, indoor air quality and thermal comfort in school buildings, carried out in accordance with Building Bulletin 101: Guidelines on Ventilation, Thermal Comfort and Indoor Air Quality in Schools (2018 update). For education-sector projects, compliance with BB101 is not optional; it forms a critical part of DfE, ESFA and local authority technical submission packs and directly influences whether a school design is accepted at planning and technical review stage.
Unlike lighter-touch compliance routes, a BB101 assessment must be grounded in dynamic thermal modelling, typically carried out using software such as Design Builder or IES-VE. The modelling environment must simulate occupied hours, internal gains, DSY1 2020 weather data and adaptive comfort thresholds to determine whether spaces, primarily classrooms and teaching areas, meet EN15251 adaptive criteria, from which BB101 derives its summer performance benchmarks.
BB101-2018 Assessment and the Shift Toward Adaptive Performance Modelling
BB101:2018 marks a departure from static overheating thresholds used in previous design guidance. Earlier standards relied on fixed temperature limits, such as a 28°C threshold for classroom overheating. The updated methodology now aligns with the CIBSE TM52 adaptive comfort model, where acceptable temperature limits rise and fall according to the running mean outdoor temperature (Trm).
This is significant for designers. Instead of asking, "Does the classroom exceed 28°C?", BB101 asks, "Does the operative temperature (Top) exceed the adaptive Tmax limit derived from Trm, and for how long?" Design Builder, when configured correctly, calculates Trm, Top, Hours of Exceedance (He), Daily Weighted Exceedance (We), and Tmax/Tupp values for each teaching zone. These metrics mirror the EN15251 adaptive methodology and enable a direct assessment against BB101’s three overheating criteria.
Modelling Requirements: DSY1 Weather Files and Occupied Hour Simulation
To reflect realistic teaching environments, BB101 requires modelling to be conducted using the Design Summer Year (DSY1 2020, 50th percentile) weather file for the appropriate region. This differs from earlier guidance that permitted Test Reference Year (TRY) data, which underrepresented peak heat scenarios.
Models must also apply occupied school hours (typically 09:00–16:00 with a defined lunch break) and include internal gains from occupants, small power, lighting and teaching equipment loads, consistent with DfE design expectations. At Energy Digest, we configure all BB101 simulations using correctly indexed school occupancy schedules, ensuring that CO₂ and overheating performance metrics represent a credible operational scenario suitable for DfE review.
Ventilation and CO₂ Control: Natural, Mechanical and Hybrid Strategies
The BB101 framework establishes differentiated CO₂ thresholds depending on ventilation strategy:
Mechanical or hybrid ventilation mode
<1000ppm daily average, with peaks not exceeding 1500ppm for more than 20 minutes
Natural ventilation mode
<1500ppm daily average, with short-term tolerance up to 2000ppm
For Design Builder and IES users, this means that CO₂ concentration must be monitored dynamically at seated head height through the occupied day. BB101 is explicit that openable windows alone are not sufficient to ensure CO₂ control in practical spaces such as laboratories and technology classrooms. Where natural ventilation is adopted, free area validation and mixing strategy analysis must be conducted to determine whether draught mitigation and adequate dilution can be achieved without resorting to full mechanical intervention.
Cold Draught Mitigation and Winter Mode Performance
A BB101-2018 Assessment does not treat ventilation solely as a summer condition issue. A compliant strategy must also avoid cold draught discomfort during winter, particularly when outdoor temperatures fall below 5°C. This is where mixing ventilation strategy becomes essential. Dynamic simulation can be used to validate whether incoming fresh air achieves suitable temperature lift before reaching the occupied zone.
At Energy Digest, we develop models that simulate mixing plumes, airflow free area, and supply air temperature trajectories, ensuring that fresh air introduced via high- or mid-level openings arrives at the occupied zone no more than 5K below the maintained internal temperature, in line with BB101 guidance.
Thermal Comfort Categories and Activity-Specific Compliance
BB101 adopts the Category I–IV framework based on EN15251, linking acceptable comfort bands to activity type and building classification. For example:
| Space Type | New Build Required Category | Refurbishment Minimum Standard |
|---|---|---|
| Classrooms, exam halls, general teaching spaces | Category II | Category III (IV only if Category III cannot be achieved) |
| Sports halls used for exams | Category II with reduced Hours of Exceedance threshold (18h) | N/A |
| Special needs and medical teaching spaces | Category I | Category I always |
A BB101 assessment must demonstrate that each relevant zone is meeting the correct adaptive comfort limits and CO₂ ventilation standard, not just the building as a whole. In practical delivery, this means zonal reporting, not just a summary. This is why Design Builder zone-by-zone weather file integration and CO₂ overlay logic is crucial to meeting assessment expectations.
From Methodology to Deliverable: Why Design Teams Commission BB101-2018 Assessment
Too often, BB101 modelling is left until later design stages, by which point glazing proportions, window types and mechanical allocations are fixed. Introducing cold-draught mitigation or improved ventilation control logic late in the process often leads to expensive redesign or justification diagrams added under pressure. Engaging BB101 compliance modelling at RIBA Stage 2–3 enables architectural teams to adapt passive ventilation strategy, free area calculation and internal gain profiles before technical fixing.
Energy Digest: BB101-2018 Assessment Modelling and Submissions
At Energy Digest, we will prepare a full BB101-2018 Assessment using Design Builder, with:
- DSY1 2020 adaptive overheating simulation using EN15251 logic
- Zone-by-zone CO₂ monitoring outputs aligned with BB101 ppm thresholds
- Ventilation free-area analysis and draught mitigation strategy justification
- Thermal comfort category narrative aligned to DfE/ESFA documentation standards
- Design Builder export diagrams ready for submission in technical appendices
We act not just as modellers, but as technical partners to design teams, helping architects and M&E consultants shape a compliant strategy before planning or ESFA gateway submission.
