This article covers new school buildings in England. BB101 also covers refurbishment, ventilation and air quality; those are mentioned only where they affect overheating.
Which test applies to your project
BB101 is the guidance that Approved Document F points to for ventilating schools. Its overheating section isn't a Building Regulations requirement: BB101 lists it as a recommended design standard, and Approved Document F makes no provision for overheating. In practice, it is set by the client's brief, the local authority, or the DfE.
| BB101 (2018) | DfE Technical Manual (January 2026) | |
|---|---|---|
| Typical use | Local authority, academy trust and independent school projects that specify BB101 | Projects delivered under the DfE's employer's requirements |
| Method | CIBSE TM52 adaptive comfort, dynamic thermal model | CIBSE TM52 adaptive comfort, dynamic thermal model |
| Must pass | Criterion 1 (40 hours) | Criterion 1 (40 hours) and one of Criterion 2 or 3 |
| Weather file | CIBSE DSY1 2020, 50th percentile | CIBSE DSY1 50th percentile Low Emission 2080 |
| Future test | None | Must be able to adapt to pass with the High Emission 2080 file without changing the superstructure |
| Internal mechanically ventilated rooms | Same adaptive criteria | Operative temperature above 25°C for no more than 3% of occupied hours |
If the brief says "BB101" and nothing else, confirm in writing which version and weather file the client expects. A model run to DSY1 2020 won't satisfy a DfE reviewer.
The three criteria
All three use ΔT, the difference between the room's operative temperature and its maximum acceptable temperature, rounded to the nearest whole degree:
- The comfort temperature is Tcomf = 0.33 Trm + 18.8, where Trm is the running mean of the outdoor temperature.
- The maximum acceptable temperature adds the category range. For Category II (normal expectation, used for teaching spaces), that range is 3 K, so Tmax = 0.33 Trm + 21.8.
| Criterion | What it measures | Limit (BB101) |
|---|---|---|
| 1. Hours of exceedance | Hours with ΔT of 1 K or more, 1 May to 30 September | No more than 40 hours |
| 2. Daily weighted exceedance | Hours above Tmax on one day, weighted by how far above | No more than 6 on any day |
| 3. Upper limit temperature | The highest ΔT at any occupied time | ΔT must not exceed 4 K |
The occupied period is 09:00 to 16:00, Monday to Friday, with the summer holiday modelled as a normal term. A lunch break from 12:00 to 13:00 with no internal gains may be allowed for in classrooms (BB101). The DfE Technical Manual makes the one-hour lunch break standard.
The 40-hour limit replaces the 3% of occupied hours that TM52 uses for other buildings. For sports halls used for exams, BB101 shortens the period to 1 May to 8 July and the limit to 18 hours.
Comfort categories by space
| Space | BB101, new build | DfE Technical Manual |
|---|---|---|
| Teaching and learning, halls, drama, dance, exams | II | II (all teaching, practical, ICT, study, libraries, halls, admin) |
| Offices | II | II (included in admin and staff areas) |
| Sports halls (not used for exams) | III | III (with dance and activity studios) |
| Atria, circulation, reception | III | Not listed |
| Pupils with complex health needs, SEND | I | I |
Sources: BB101 Table 7.8; DfE Technical Manual Table 19.
What the DfE adds on top
The Technical Manual also sets design rules that affect the model:
- External shading. Horizontal brise soleil on every window facing south, south-east or south-west (within 45° of due south).
- No credit for trees. Planting around the building is encouraged, but can't be included in the final overheating model.
- Hot surfaces. No dark-coloured materials or synthetic surfaces within 10 m of the building, and the colour of roof finishes, louvres and paving must be accounted for in the model.
- Openable windows in every perimeter space, even where noise means the ventilation design can't rely on them.
- Occupancy at design numbers, from the schedule of spaces, with a sensible heat gain of at least 70 W and a latent gain of at least 55 W, higher where activity is higher (CIBSE Guide A, Table 6.3). Equipment gains such as cookers in food rooms must be included.
- Fans (ceiling or wall) are acceptable to reduce overheating, subject to a risk assessment and the school's agreement.
Ventilation and air quality limits
BB101 also sets CO₂ limits for teaching spaces, which affect window and ventilation sizing in the same model:
| Ventilation mode | Daily average CO₂ in occupied hours | Maximum |
|---|---|---|
| Mechanical, or hybrid in mechanical mode | Below 1,000 ppm | No more than 1,500 ppm for more than 20 consecutive minutes a day |
| Natural, or hybrid in natural mode | Below 1,500 ppm | No more than 2,000 ppm for more than 20 consecutive minutes a day |
As a starting point before modelling, BB101 suggests openable windows or vents with an effective area of at least 3% of the floor area, which can mean a physical opening of around 5%. The model then confirms whether that's enough.
Usual reasons a classroom fails
- Large south- or west-facing glazing without external shading.
- Single-sided ventilation with restricted window openings, often because of security or noise.
- High-level openings missing, so hot air builds up under the ceiling. BB101 also limits radiant asymmetry from hot ceilings in single-storey teaching spaces to 5 K.
- ICT and equipment gains underestimated, or occupancy below the design number.
- Low thermal mass and no night cooling.
- A model run with the wrong weather file for the brief.
BB101 says mechanical cooling should not normally be needed in general teaching spaces with ICT gains below 15 W/m², or 25 W/m² in practical spaces. Where cooling is used, it should be justified on heat load and energy grounds.
Before the overheating model: a checklist
- Confirm which test the client requires: BB101 2018, the DfE Technical Manual, or a project-specific brief.
- Confirm the weather file in writing.
- Get the schedule of spaces with design occupancy for every room.
- Get the ICT and equipment gains, including cookers and kilns.
- Get window types, opening angles, restrictors and effective areas.
- Agree the ventilation strategy for each space type: natural, hybrid or mechanical.
- Check external shading on south, south-east and south-west elevations.
- Model every occupied space, and report Criteria 1, 2 and 3 for each.
- Run the CO₂ check in the same model.
- Report any failures with the mitigation options tested.
More on the service: BB101 overheating assessments for schools and DfE energy assessments for schools.
